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What conditions are necessary for human pancreatic lipase to activate? Is there an optimal temperature or pH? How quickly does it take effect?
The protein referred to in the question is encoded by gene PNLIP, pancreatic lipase. From this annotation of the protein, I see that there is a signal peptide from amino acids 1 to 16. Thus, this signal peptide must be cleaved before the protein can be active in its digestion of emulsified triacylglyerides.
A paper describes the structural changes induced in human pancreatic lipase by lowering the pH. The secondary structure of the enzyme is stable within a pH range of 3.0 to 6.5. At this pH, a reversible opening of the lid controlling the access to the active site was observed. So, there is another aspect of activation - pH and the ability to open the enzyme lid so that the fat molecule enters the active site.
What conditions are necessary for HPL (human pancreatic lipase) to activate? - Biology
A Lipase is a water-soluble enzyme that catalyzes the hydrolysis of ester bonds in waterinsoluble, lipid substrates. Most lipases act at a specific position on the glycerol backbone of a lipid substrate (A1, A2 or A3). In the example of human pancreatic lipase (HPL), which is the main enzyme responsible for breaking down fats in the human digestive system, a lipase acts convert triglyceride substrates found in oils from food to monoglycerides and free fatty acids. A myriad of other lipase activities exist in nature, especially when the phospholipases and sphingomyelinases are considered.
Lipases are ubiquitous throughout living organisms, and genes encoding lipases are even present in certain viruses. While a diverse array of genetically distinct lipase enzymes are found in nature, and represent distinct types of protein folds  and catalytic mechanisms, most are built on a alpha/beta hydrolase fold (see image below) and employ a chymotrypsin-like hydrolysis mechanism involving a serine nucleophile, an acid residue (usually aspartic acid), and a histidine.
Some lipases work within the interior spaces of living cells to degrade lipids. In the example of lysosomal lipase, the enzyme is confined within an organelle called the lysosome. Other lipase enzymes, such as pancreatic lipases, are found in the spaces outside of cells and have roles in the metabolism, absorption and transport of lipids throughout the body. As biological membranes are integral to living cells and are largely composed of phospholipids, lipases play important roles in cell biology. Furthermore, lipases are involved in diverse biological processes ranging from routine metabolism of dietary triglycerides to cell signaling and inflammation. Several different types of lipases are found in the human body, including pancreatic lipase, hepatic lipase, lysosomal lipase, hepatic lipase, gastric lipase, endothelial lipase, as well as various different phospholipases.
At least three human genetic diseases are caused by mutations in lipase genes. Lipoprotein Lipase Deficiency is caused by mutations in the gene encoding lipoprotein lipase . Cholesteryl Ester Storage Disease (CESD) and Wolman Disease are both caused by mutations in the gene encoding lysosomal lipase, also referred to as lysosomal acid lipase (LAL or LIPA) or acid cholesteryl ester hydrolase .
Below is a computer generated image of a type of pancreatic lipase (PLRP2) from the guinea pig, it is a 3-D model from structure coordinates submitted to the Protein Data Bank from: Withers-Martinez, C., F. Carriere, R. Verger, D. Bourgeois, and C. Cambillau. 1996. A pancreatic lipase with a phospholipase A1 activity: crystal structure of a chimeric pancreatic lipase-related protein 2 from guinea pig. Structure 4:1363-74.
Afonso, C. L., E. R. Tulman, Z. Lu, E. Oma, G. F. Kutish, and D. L. Rock. 1999. The genome of Melanoplus sanguinipes entomopoxvirus. J Virol 73:533-52.
Brady, L., A. M. Brzozowski, Z. S. Derewenda, E. Dodson, G. Dodson, S. Tolley, J. P. Turkenburg, L. Christiansen, B. Huge-Jensen, L. Norskov, and et al. 1990. A serine protease triad forms the catalytic centre of a triacylglycerol lipase. Nature 343:767-70.
Carriere, F., C. Withers-Martinez, H. van Tilbeurgh, A. Roussel, C. Cambillau, and R. Verger. 1998. Structural basis for the substrate selectivity of pancreatic lipases and some related proteins. Biochim Biophys Acta 1376:417-32.
Diaz, B. L., and J. P. Arm. 2003. Phospholipase A(2). Prostaglandins Leukot Essent Fatty Acids 69:87-97.
Egmond, M. R., and C. J. van Bemmel. 1997. Impact of Structural Information on Understanding of Lipolytic Function, p. 119-129, Methods Enzymol, vol. 284.
Gilbert B, Rouis M, Griglio S, de Lumley L, Laplaud P. 2001. Lipoprotein lipase (LPL) deficiency: a new patient homozygote for the preponderant mutation Gly188Glu in the human LPL gene and review of reported mutations: 75 % are clustered in exons 5 and 6. Ann Genet. 44(1):25-32.
Girod, A., C. E. Wobus, Z. Zadori, M. Ried, K. Leike, P. Tijssen, J. A. Kleinschmidt, and M. Hallek. 2002. The VP1 capsid protein of adeno-associated virus type 2 is carrying a phospholipase A2 domain required for virus infectivity. J Gen Virol 83:973-8.
Goni FM, Alonso A. 2002 Sphingomyelinases: enzymology and membrane activity. FEBS Lett. 531(1):38-46
Heikinheimo, P., A. Goldman, C. Jeffries, and D. L. Ollis. 1999. Of barn owls and bankers: a lush variety of alpha/beta hydrolases. Structure Fold Des 7:R141-6.
Lowe, M. E. 1992. The catalytic site residues and interfacial binding of human pancreatic lipase. J Biol Chem 267:17069-73.
Schrag, J. D., and M. Cygler. 1997. Lipases and alpha/beta hydrolase fold. Methods Enzymol 284:85-107.
Spiegel, S., D. Foster, and R. Kolesnick. 1996. Signal transduction through lipid second messengers. Curr Opin Cell Biol 8:159-67.
Svendsen, A. 2000. Lipase protein engineering. Biochim Biophys Acta 1543:223-238.
Tjoelker, L. W., C. Eberhardt, J. Unger, H. L. Trong, G. A. Zimmerman, T. M. McIntyre, D. M. Stafforini, S. M. Prescott, and P. W. Gray. 1995. Plasma platelet-activating factor acetylhydrolase is a secreted phospholipase A2 with a catalytic triad. J Biol Chem 270:25481-7.
Winkler, F. K., A. D'Arcy, and W. Hunziker. 1990. Structure of human pancreatic lipase. Nature 343:771-4.
Withers-Martinez, C., F. Carriere, R. Verger, D. Bourgeois, and C. Cambillau. 1996. A pancreatic lipase with a phospholipase A1 activity: crystal structure of a chimeric pancreatic lipase-related protein 2 from guinea pig. Structure 4:1363-74.
What is a lipase enzyme?
Lipase is an enzyme that breaks down dietary fats into smaller molecules called fatty acids and glycerol. A small amount of lipase, called gastric lipase, is made by cells in your stomach. This enzyme specifically digests butter fat in your food.
One may also ask, how is lipase enzyme produced? The gastric lipase is produced within the stomach and its primary function is to digest fatty acids. The pharyngeal lipase is secreted by the human salivary glands and attacks fatty acids from the moment the food is inside the mouth. The hepatic lipase is a digestive enzyme produced by the liver.
Correspondingly, what does it mean when your lipase is high?
Higher than normal levels of lipase mean that you have a problem with your pancreas. If your blood has 3 to 10 times the normal level of lipase, then it's likely that you have acute pancreatitis. High lipase levels also mean you may have kidney failure, cirrhosis, or a bowel problem.
What level of lipase is dangerous?
A normal lipase level can range from 0-160 U/L depending on the lab. When the pancreas is damaged, these digestive enzymes can be found in the blood at higher levels than normal. Amylase or lipase results more than three times normal levels are likely to mean pancreatitis or damage to your pancreas.
Some lipases work within the interior spaces of living cells to degrade lipids.
- In the example of lysosomal lipase, the enzyme is confined within an organelle called the lysosome.
- Other lipase enzymes, such as pancreatic lipases, are found in the spaces outside of cells and have roles in the metabolism, absorption and transport of lipids throughout the body.
As biological membranes are integral to living cells and are largely composed of phospholipids, lipases play important roles in cell biology.
Furthermore, lipases are involved in diverse biological processes ranging from routine metabolism of dietary triglycerides to cell signaling  and inflammation  .
Another important function of lipases is to help your body package cholesterol for transport in the bloodstream. A specific lipase called LCAT—short for lecithin cholesterol acyltransferase—combines cholesterol with fatty acids, both of which are lipid molecules, or types of fat. The body packages the resulting molecules into transporter particles like LDL and HDL—commonly called bad cholesterol and good cholesterol, respectively—and moves them to or away from the cells.
Pancreas: Structure, Composition and Regulation
In this article we will discuss about:- 1. Structure of Pancreas 2. Composition of Pancreatic Juice 3. Regulation 4. Pancreatic 5. Effects.
Structure of Pancreas:
Pancreas is a dual organ. It has endocrine portion and exocrine portion.
The exocrine part resembles the salivary glands in histology being formed of acini arranged into lobules. Cells of the acini contain numerous mitochondria, a nucleus and granular cytoplasm. The zymogen granules are located more towards the apex and contain enzyme precursors of pancreatic juice. Each acinus drains into a duct.
The cells lining the intra- acinar portion of the duct are called centroacinar cells. The acinar cells secrete various enzymes of the pancreatic juice while the centroacinar and duct cells contribute to the secretion of electrolytes, most important of which is HCO3 – . All the minute ducts from the various lobules unit to form main pancreatic duct of Wirsung. This duct opens into the second part of duodenum along with bile duct.
In most of the normal people, there is also an accessory pancreatic duct of Santorini which opens into the second part of duodenum just above the main duct. In this manner, the exocrine pancreatic secretion drains into the duodenum. Pancreas is supplied by the vagus and sympathetic fibers from celiac ganglion, just like heart, the ganglia for vagus are in the pancreas itself.
Composition of Pancreatic Juice:
Pancreatic juice is a colorless, odorless, highly alkaline fluid of low viscosity, pH = 8- 8.4. Alkalinity is because of HCO3 – .
The important components of pancreatic juice are (Figs 5.17 to 5.19):
ii. Cations: Na + , K + , Mg ++ , Ca ++
Functions of the Individual Components:
Bicarbonates make the pancreatic juice alkaline and alkaline medium is essential for the pancreatic enzymes to perform their digestive functions.
1. The chyme received from the stomach is acidic and acidity is harmful to duodenal mucosa and also does not permit pancreatic and intestinal enzymes to act. HCO3 – neutralizes the acidity rendering the pH of intestinal contents between 7 and 8 which is essential for digestive processes to occur in the small intestine.
2. HCO3 – inactivates pepsin because pepsin being a proteolytic enzyme will digest all pancreatic enzymes.
Normal HCO3 – content of pancreatic juice: 80-120 mEq/liter
Enzymes: Proteolytic Enzymes:
Trypsinogen is the inactive form of trypsin.
It is activated by another enzyme known as enteropeptidase (also called enterokinase) secreted by intestinal mucosa.
It activates trypsinogen to trypsin. Once trypsin is formed, trypsin by itself activates trypsinogen. This type of a reaction is known as an autocatalytic reaction.
Trypsin is a powerful endopeptidase because it acts inside the protein molecule and breaks the peptide bonds adjacent to arginine or lysine thereby breaking larger protein molecules into smaller polypeptides. It requires a pH of 7-8 for its action. Other actions include a weak coagulting action on milk.
All the enzymes of pancreatic juice are secreted into duodenum in the inactive form, if not enzymes themselves will digest the entire pancreas.
Besides this, trypsin has got a specific inhibitor in the pancreas itself.
This is also an endopeptidase. Chymotrypsinogen is activated to chymotrypsin by trypsin.
Chymotrypsin breaks peptide bonds adjacent to aromatic amino acids. The pH required for this action is about 7-8. This enzyme helps to digest large proteins into smaller peptides. Compared to trypsin, it has got more powerful coagulating action on milk.
Some believe that there are 6 different types of chymotrypsinogen and one of these is used to dissolve the lens capsule in the eye to remove cataract.
Procarboxypeptidase A and B:
Both are activated by trypsin into carboxypeptidase A and B, respectively. They are exopeptidases because they cleave or break peptide bonds at the carboxy terminal of the protein.
Carboxypeptidase A, breaks peptide bonds of carboxy terminal attached to branched aliphatic amino acids. Whereas B attacks and breaks peptide bonds of carboxy terminal attached to basic amino acids. These exopeptidases help to form or break individual amino acids from peptides produced by the action of trypsin and chymotrypsin.
Activated to elastase by trypsin. An elastase acts on the protein elastin attacking peptide bonds adjacent to aliphatic amino acids.
It is the most important fat splitting enzyme in the GIT. It acts on emulsified fats, emulsification having been brought about by bile salts in the presence of lecithin and monoglycerides. Bile salts activate pancreatic lipase.
Although from the reaction, it is seen that triglycerides can be broken down by lipase into glycerol and fatty acids, actually since the final two steps are slow, the usual products of lipase action are 2 monoglyceride and fatty acids.
Lipase also requires alkaline pH of 7-8 for the action. Colipase helps to expose the triglyceride molecule which has formed a complex with bile salts. This exposure is necessary for lipase to hydrolyze the triglyceride.
If pancreatic lipase is absent either due to complete destruction of pancreas because of disease or removal of entire pancreas surgically, the digestion and absorption of fats and fat-soluble vitamins is significantly disturbed and more fat is excreted in fecal matter. Normal fat content of feces is up to 5 g/day. If lipase is completely absent, fat content increases to 40-50 g/day. Presence of abnormal amounts of fatty stool is called steatorrhea.
This is activated by trypsin to phospholipase. Phospholipase converts lecithin into lysolecithin by splitting of fatty acid and later can be absorbed.
This enzyme hydrolyses cholesterol ester to yield free cholesterol which is absorbed along with fatty acids.
Actions similar to salivary amylase.
This converts DNA into the respective nucleotide.
This hydrolyses RNA to the respective nucleotide.
Regulation of Pancreatic Secretion:
Pancreatic secretion is regulated by neural and hormonal mechanism of which hormonal mechanisms is more important. Three different phases for pancreatic secretion have been recognized in animals and these are not so well defined in man.
1. Cephalic Phase:
Taste of food stimulates pancreatic secretion. In addition to this, sight or smell also can also stimulate.
2. Gastric Phase:
Presence of food in the stomach by way of mechanical distension and chemical composition stimulates pancreatic secretion.
Distension of stomach causing pancreatic secretion of enzymes is called gastropancreatic reflex.
Cephalic and gastric phases are controlled by vagus. Stimulation of vagus promotes pancreatic secretion which is rich in enzymes, yet pancreatic secretion can go on in the absence of vagus. The action of vagus is through acetylcholine.
3. Intestinal Phase:
Presence of food, HCl, etc., in the small intestine promotes pancreatic secretion through mechanical distension as well as chemical composition.
This very important phase is brought about two hormones, viz.:
b. Cholecystokinin-pancreozymin (CCK-PZ)
This is the first hormone ever to be discovered (and synthesized) in the year 1902 by Bayliss and Starling.
This is a polypeptide hormone having 26 amino acids. This GI hormone is secreted by specialized cells in the mucosa of duodenum and jejunum.
The most important stimulus is the presence of HCl in the duodenum (Fig. 5.20). Whenever pH of chyme in the duodenum falls to less than 4.5, secretin is released into the portal vein and then returns to GIT through the circulatory system.
Actions (functions): Secretin increases the volume of pancreatic juice which is rich in HC03 – and water. Therefore, its main site of action is centroacinar and duct cells of pancreas.
i. It potentiates action of CCK-PZ on pancreas.
ii. Increases secretion of bile from hepatocytes.
iii. Inhibits gastric motility and delays gastric emptying by contraction of pyloric sphincter.
iv. Inhibits gastrin release and gastric secretion.
v. Inhibits gastric motility and delays gastric emptying by contraction of pyloric sphincter.
vi. Releases insulin from beta cells of islets of Langerhans.
Cholecystokinin-pancreozymin (CCK-PZ) initially discovered as the separate hormones: pancreozymin and cholecystokinin, former acting on pancreas and the latter on gallbladder. Subsequently, they were found to be one and the same. This polypeptide GI hormone has got 33 amino acids and is produced from mucosa of duodenum and jejunum.
Stimuli for release of CCK-PZ (Fig. 5.21):
Presence of food in duodenum and jejunum stimulates secretion of this hormone. Although products of protein, fat and carbohydrate digestion all can release CCK-PZ, amino acids form the most powerful stimulus and the next potent stimulus is fatty acids. HCl is a weak stimulus for CCK-PZ.
CCK-PZ acts on pancreatic acini to produce pancreatic juice rich in enzyme content.
While secretin acts through cAMP, CCK-PZ acts not only through cAMP but also increases intracellular Ca ++ concentration in the acini which is necessary for release of pancreatic enzyme.
CCK-PZ promotes contraction of gallbladder. This results in expulsion of bile from the gallbladder into duodenum. An agent that causes contraction of gall­bladder is called cholegogues. CCK-PZ is a very powerful cholegogue.
1. Potentiates action of secretin on pancreas
2. Promotes secretion of bile.
3. Inhibits gastric motility and, therefore, delays gastric emptying.
4. Promotes release of insulin in from the B cells of islets of Langerhans.
5. Promotes pancreatic cell growth.
Pancreatic Function Test:
(Only for exocrine pancreas):
It is now possible to collect pure pancreatic juice right from the pancreatic duct by passing a thin catheter through the mouth into the duodenum with the help of an instrument called duodenoscope.
Pancreatic juice is collected and analyzed for HCO3 – content and trypsin activity. After collecting the morning initial sample, with the individual in fasting condition, an injection of secretin and CCK-PZ is given.
A few minutes after the injection, pancreatic juice is again collected at intervals of 10 minutes and HCO3 – content and trypsin activity are estimated every time. If pancreas is normal, secretin should cause an increase in HCO3 content and PZ should increase in tryspin activity. If pancreas is not functioning normally, both will be reduced.
Effect of Total Pancreatectomy:
This is sometimes done for the carcinoma of the pancreas.
Removal of pancreas leads to following abnormalities:
2. Abnormalities in digestion and absorption of lipids and proteins, but carbohydrates digestion is not affected significantly because salivary amylase and enzymes present in intestinal secretion keep carbohydrate absorption and digestion normal.
Diminished digestion and absorption of lipids causes steatorrhea while impaired digestion and absorption of proteins is reflected by increased nitrogen content of stool which is normally up to 1 g/day.
Some lipases work within the interior spaces of living cells to degrade lipids.
- In the example of lysosomal lipase, the enzyme is confined within an organelle called the lysosome.
- Other lipase enzymes, such as pancreatic lipases, are found in the spaces outside of cells and have roles in the metabolism, absorption and transport of lipids throughout the body.
As biological membranes are integral to living cells and are largely composed of phospholipids, lipases play important roles in cell biology.
Furthermore, lipases are involved in diverse biological processes ranging from routine metabolism of dietary triglycerides to cell signaling ⎙] and inflammation ⎚] .
What are the building blocks of Lipases?
Lipases are enzymes involved in fat digestion. The body includes many subtypes of enzymes, but the term “lipase” usually refers to pancreas lipase. The pancreas is an organ that is located below your stomach. Its role is to break down specific components of dietary fat. Lipase is secreted from the pancreas through a tube that empties into the gastrointestinal tract of the duodenum. Therefore, it acts on the already partially digested food in the stomach.
Lipase, any of a group of fat-splitting enzymes found in the blood, gastric juices, pancreatic secretions, intestinal juices, and adipose tissues. Lipases hydrolyze triglycerides (fats) into their component fatty acid and glycerol molecules.
Initial lipase digestion occurs in the lumen (interior) of the small intestine. Bile salts reduce the surface tension of the fat droplets so that the lipases can attack the triglyceride molecules. The fatty acid and glycerol molecules are then taken up into the epithelial cells that line the intestinal wall, where they are resynthesized into triglycerides for transport to muscles and adipose tissues. At these sites lipases in the bloodstream hydrolyze the triglycerides, and the resulting fatty acids and glycerol are taken up by the cells of these tissues. In the adipose tissues triglycerides are re-formed for storage until the energy needs of the animal increase under conditions of stress or exercise. Lipases in the cells of adipose tissues break down the triglycerides so that fatty acids can reenter the bloodstream for transport to energy-requiring tissues.#
Building Blocks of Lipases-Glycerol and Fats
Lipase is an enzyme that breaks down triglycerides into free fatty acids and glycerol. Lipases are present in pancreatic secretions and are responsible for fat digestion. Lipases are enzymes that play a crucial role in lipid transport. There are many different types of lipases hepatic lipases are in the liver, hormone-sensitive lipases are in adipocytes, lipoprotein lipase is in the vascular endothelial surface, and pancreatic lipase is in the small intestine, each serving individual functions. Hepatic lipase in the liver is responsible for degrading the triglycerides that remain in intermediate density lipoprotein (IDL). Hormone-sensitive lipase is found within fat tissue and is responsible for degrading the triglycerides that are stored within adipocytes. Lipoprotein lipase is found on the vascular endothelial surface and is responsible for degrading triglycerides that circulating from chylomicrons and VLDLs. Pancreatic lipase is found within the small intestine and is responsible for degrading dietary triglycerides.
Hepatic lipase plays a crucial role in the formation and delivery of low-density lipoprotein(LDL). LDL is formed by the modification of intermediate density lipoprotein in the peripheral tissue and liver by hepatic lipase. These LDL particles are taken up, or endocytosed, via receptor-mediated endocytosis by target cell tissue. LDL serves to ultimately transport cholesterol from the liver to peripheral tissue.
A lipase is any enzyme that catalyzes the hydrolysis of fats (lipids).
Lipases perform essential roles in digestion, transport and processing of dietary lipids (e.g. triglycerides, fats, oils) in most, if not all, living organisms. Genes encoding lipases are even present in certain viruses.
Most lipases act at a specific position on the glycerol backbone of a lipid substrate (A1, A2 or A3)(small intestine). For example, human pancreatic lipase (HPL), which is the main enzyme that breaks down dietary fats in the human digestive system, converts triglyceride substrates found in ingested oils to monoglycerides and two fatty acids.
Several other types of lipase activities exist in nature, such as phospholipases and sphingomyelinases however, these are usually treated separately from “conventional” lipases.
Some lipases are expressed and secreted by pathogenic organisms during an infection. In particular, Candida albicans has many different lipases, possibly reflecting broad-lipolytic activity, which may contribute to the persistence and virulence of C. albicans in human tissue.
A diverse array of genetically distinct lipase enzymes are found in nature, and they represent several types of protein folds and catalytic mechanisms. However, most are built on an alpha/beta hydrolase fold and employ a chymotrypsin-like hydrolysis mechanism using a catalytic triad consisting of a serine nucleophile, a histidine base, and an acid residue, usually aspartic acid
Lipase in Human Body
The main lipases of the human digestive system are pancreatic lipase (PL) and pancreatic lipase related protein 2 (PLRP2), which are secreted by the pancreas. Humans also have several related enzymes, including hepatic lipase, endothelial lipase, and lipoprotein lipase.
|bile salt-dependent lipase||bsdl||pancreas, breast milk||aids in the digestion of fats|
|pancreatic lipase||PNLIP||digestive juice||In order to exhibit optimal enzyme activity in the gut lumen, PL requires another protein, colipase, which is also secreted by the pancreas.|
|lysosomal lipase||LIPA||interior space of organelle: lysosome||Also referred to as lysosomal acid lipase (LAL or LIPA) or acid cholesteryl ester hydrolase||Cholesteryl ester storage disease (CESD) and Wolman disease are both caused by mutations in the gene encoding lysosomal lipase.|
|hepatic lipase||LIPC||endothelium||Hepatic lipase acts on the remaining lipids carried on lipoproteins in the blood to regenerate LDL (low density lipoprotein).||–|
|lipoprotein lipase||LPL or “LIPD”||endothelium||Lipoprotein lipase functions in the blood to act on triacylglycerides carried on VLDL (very low density lipoprotein) so that cells can take up the freed fatty acids.||Lipoprotein lipase deficiency is caused by mutations in the gene encoding lipoprotein lipase.|
|gastric lipase||LIPF||digestive juice||Functions in the infant at a near-neutral pH to aid in the digestion of lipids||–|
|pancreatic lipase related protein 2||PNLIPRP2 or “PLRP2” –||digestive juice||–||–|
|pancreatic lipase related protein 1||PNLIPRP1 or “PLRP1”||digestive juice||Pancreatic lipase related protein 1 is very similar to PLRP2 and PL by amino acid sequence (all three genes probably arose via gene duplication of a single ancestral pancreatic lipase gene). However, PLRP1 is devoid of detectable lipase activity and its function remains unknown, even though it is conserved in other mammals.||–|
|lingual lipase||?||saliva||Active at gastric pH levels. Optimum pH is about 3.5-6. Secreted by several of the salivary glands (Ebner’s glands at the back of the tongue (lingua), the sublingual glands, and the parotid glands)||–|
Other lipases include LIPH, LIPI, LIPJ, LIPK, LIPM, LIPN, MGLL, DAGLA, DAGLB, and CEL.
There also are a diverse array of phospholipases, but these are not always classified with the other lipases.
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Enzymes: Meaning, Mechanism, Classification, Factors, and Importance
Let us make an in-depth study of the enzymes. After reading this article you will learn about: 1. Meaning of Enzymes 2. Classification of Enzymes 3. Mechanism of Enzyme Action 4. Factors Affecting Enzyme Action 5. Enzyme Specificity 6. Enzyme Inhibition and 7. Diagnostic Importance of Enzymes.
Meaning of Enzymes:
Enzymes are proteinaceous (and even nucleic acids) biocatalyst which alter (generally enhance) the rate of a reaction.
Free energy of activation and effect of catalysis:
A chemical reaction like substrate to product, will take place when a certain number of substrate molecules at any instant, possess enough energy to attain an activated condition called the “transition state” in which the probability of making or breaking a chemical bond to form the product is very high. “Free energy of activation” is the amount of energy required to bring all the molecules in one gram mole of a substrate at a given temperature to the transition state.
In presence of a catalyst, the substrate combines with it to produce a transient state having a lower energy of activation than that of the substrate alone. This accelerates the reaction. Once the product is formed, the enzyme (catalyst) is free to combine with another molecule of the substrate and repeat the process. Though there is a change in the free energy of activation in presence of an enzyme, the overall free energy change of the reaction remains the same whether the reaction is catalysed by an enzyme or not.
Classification of Enzymes:
Classification of enzymes are based upon:
(2) The presence or absence at a given time,
(3) The regulation of action,
(4) The place of action and
(5) Their clinical importance.
1. Classification Based upon the Reaction Catalysed:
Enzymes are broadly divided into six groups based on the type of reaction catalysed.
Enzymes which bring about oxidation and reduction reactions.
Ex. Pyruvate + NADH—lactate dehydrogenase → Lactate + NAD +
Glutamic acid + NAD—glutamate dehydrogenase → α-ketoglutarate + NH3 + NADH
Enzymes which catalyze transfer of groups from one substrate to another, other than hydrogen. Ex. Transaminase catalyses transfer of amino group from amino acid to a keto acid to form a new keto acid and a new amino acid.
Ex. (α-Ketoglutarate + Alanine—alanine aminotransferase → Glutamate + Pyruvate
Aspartate + α-Ketoglutarate —aspartate aminotransferase Oxaloacetate + Glutamate
Those enzymes which catalyse the breakage of bonds with addition of water (hydrolysis). All the digestive enzymes are hydrolases. Ex. Pepsin, trypsin, amylase, maltase.
Those enzymes which catalyse the breakage of a compound into two substances by mechanism other than addition of water. The resulting product always has a double bond.
Ex. Fructose-1-6-diphosphate—ALDOLASE → Glyceraldehyde-3-phosphate + DHAP
Those enzymes which catalyse the inter-conversion of optical and geometric isomers.
Ex. Glyceraldehyde-3-phosphate—ISOMERASE → Dihydroxyacetone phosphate
These enzymes catalyse union of two compounds. This is always an energy requiring process (active process).
Ex. Pyruvate + CO2 + ATP—pyruvate carboxylase Oxaloacetate + ADP + Pi
2. Classification Based upon the Presence or Absence at a Given Time:
Two types are identified:
(a) Inducible enzymes:
Those enzymes that are synthesized by the cell whenever they are required. Synthesis of these enzymes usually requires an inducer.
Ex. Invertase, HMG-CoA reductase, p-galactosidase and enzymes involved in urea cycle.
(b) Constitutive enzymes:
Enzymes which are constantly present in normal amounts in the body, irrespective of inducers.
3. Classification Based upon the Regulation of Enzyme Action:
They are of two types:
(a) Regulatory enzymes:
The action of these enzymes is regulated depending upon the status of the cell. The action of regulatory enzymes is either increased or decreased by a modulator at a site other than the active site called the “allosteric site”.
Ex. Phosphofructokinase (PFK) and glutamate dehydrogenase.
(b) Non-regulatory enzymes:
The action of these enzymes is not regulated.
Ex. Succinate dehydrogenase.
4. Classification Based upon the Place of Action:
Depending upon the two sites of action, they are divided into—
(a) Intracellular enzymes:
Enzymes that are produced by the cell and act inside the same cell are known as intracellular enzymes.
Ex. All the enzymes of glycolysis and TCA cycle.
(b) Extracellular enzymes:
Enzymes produced by a cell but act outside that cell independent of it. Ex, All the digestive enzymes viz. trypsin, pancreatic lipase etc.
5. Classification Based upon their Clinical Importance:
(a) Functional plasma enzymes:
Enzymes present in the plasma in considerably high concentration and are functional in the plasma due to the presence of their substrate it plasma.
Ex. Serum lipase, blood clotting enzymes.
(b) Non-functional plasma enzymes:
Enzymes present in the plasma in negligible concentration and have no function in the plasma due to the absence of their substrate in it. Non-functional plasma enzymes are of diagnostic importance.
Enzymes are named in 4 digits by the enzyme nomenclature commission, wherein the
1 st digit refers to main classification
2 nd digit refers to sub-classification
3 rd digit refers to sub-sub classification
4 th digit refers to that particular enzyme
Ex. 188.8.131.52 is adenosine triphosphate-creatine phosphotransferase (creatine kinase).
Mechanism of Enzyme Action:
An enzyme (or protein) should be in its native conformation to be biologically active. The three dimensional conformation of enzymes have a particular site where the substrate binds and is acted upon, this site is called the active site.
The active site is earmarked into two specific areas:
(1) Binding site—where the substrate binds and
(2) Catalytic site—where the enzyme catalysis takes place.
The amino acids present at the active site are tyrosine, histidine, cysteine, glutamic acid, aspartic acid, lysine and serine. In aldolase, lysine is present at the active site. In carboxypeptidase, two tyrosine residues are present at the active site. Ribonuclease has two histidines at the active site. Michaelis and Menten established the theory of combination of enzyme with substrate to form the enzyme-substrate complex. According to this, the enzyme combines with the substrate on which it acts to form an enzyme-substrate complex. Then, this enzyme is liberated and the substrate is broken down into the product of the reaction.
E [Enzyme] + S [Substrate] → ES [Enzyme-Substrate complex] → E + Product
The ES complex is also called as ‘Michaelis Menten complex’.
Enzymes accelerate the rate of chemical reaction by four major mechanisms viz.
1. Proximity and Orientation:
The enzyme binds to the substrate in such a way that the susceptible bond is in close proximity to the catalytic group and also precisely oriented to it resulting in the catalysis.
2. Strain and Distortion or Induced Fit Model:
Binding of the substrate induces a conformational change in the enzyme molecule which strains the shape of the active site and also distorts the bounded substrate, thus bringing about the catalysis. The binding of the substrate to the enzyme will bring about a change in the tertiary or quaternary structure of enzyme molecule, which destabilizes the enzyme. In order to attain stability, the enzyme distorts the substrate thereby forming the reaction product.
3. General Acid-Base Catalysis:
The active site of the enzyme has amino acids that are good proton donors or proton acceptors, this result in acid-base catalysis of the substrate.
4. Covalent Catalysis:
Some enzymes react with their substrates to form very unstable, covalently joined enzyme-substrate complexes, which undergo further reaction to form the products.
Factors Affecting Enzyme Action:
The factors influencing the rate of the enzyme catalysed reaction are:
3. Substrate concentration
5. Concentration of products
1. Effect of Temperature:
When all the other parameters are kept constant (i.e. at their optimum level), then the rate of enzyme reaction increases slowly with increase of temperature till it reaches a maximum. Further increase in temperature denatures the protein resulting in decrease in the enzyme action and a further increase in temperature may totally destroy the protein.
The temperature, at which the enzyme activity is maximum, is termed as the optimum temperature. Most of the enzymes are totally inactive at 0° C to 4° C, their activity starts at 10° C and slowly increases reaching its maximum capacity at its optimum temperature. Majority of the enzymes in the human body have their optimum temperatures between 37° C and 40° C.
Beyond this temperature the enzymes become less active and may loose their activity completely at higher temperatures. In fever, rise in temperature increases the metabolic activity due to increase in enzymatic action. Decrease in the temperature leads to hypothermia which is seen in organ transplantation and open heart surgery.
However, life exists in very cold regions and also in hot springs, indicating that the same enzyme that exists in human cell, for instance the enzymes of glycolysis and TCA cycle have their optimum temperatures at extremes of temperatures.
Thus refrigeration bacteria exists with the optimum temperature of its enzymes being near 4°C. Likewise bacteria surviving in hot springs have the enzymes with their optimum temperatures nearing hundred(s) degree Celsius ex. the optimum temperature of Taq polymerase is 72°C.
Vant Hoffs coefficient:
It is the coefficient which explains that for every 10°C rise in temperature the enzyme activity increases 2 fold till the optimum temperature is reached.
2. Effect of pH:
When all the other parameters are kept constant, the velocity of an enzyme catalysed reaction increases till it reaches the optimum pH and then decreases with further increase/decrease in pH. The activity is maximum for most of the enzymes at the biological pH of 7.4. Optimum pH for pepsin is 1.5, acid phosphatase is 4.5 and for alkaline phosphatase it is 9.8.
3. Effect of Substrate Concentration:
When all the other parameters are kept constant including the enzyme concentration, then, as the substrate concentration increases the rate of reaction increases steadily, till the enzyme is saturated with the substrate. At this stage the reaction rate does not increase and remains constant. When a graph is plotted with velocity versus substrate concentration it gives a hyperbolic curve.
This is because, as the concentration of substrate is increased, the substrate molecules combine with all available enzyme molecules at their active sites till no more active sites are available. Thus at this stage, substrate only replenishes the sites when the products are liberated and cannot increase the rate of reaction.
Km is defined as the substrate concentration at which the velocity of the enzyme catalysed reaction is half the maximum velocity.
i. A high Km value indicates weak binding between the enzyme and the substrate.
ii. Low Km indicates strong binding.
Limitations of Michaelis-Menten equation:
i. This equation enables the calculation of approximate value of the maximum velocity and not the accurate value.
ii. It holds good for enzymes which have active site only and not the allosteric site.
iii. It calculates the Km for mono-substrate reactions and not for multi-substrate reactions.
iv. It is used to know the velocity of non-regulatory enzymes but not of regulatory enzymes.
In order to overcome the above limitations a Line weaver-Burke plot is drawn so as to establish a relation between the reciprocals of substrate concentration and velocity.
Line weaver-Burke plot:
Inverting the Michaelis-Menten equation, we get
By this equation we can calculate accurately:
i. The velocity of any enzyme catalysed reaction.
ii. The rate of reaction where more than one substrate is present.
iii. The velocity of all the enzymes.
Regulatory enzymes give a sigmoid curve and non-regulatory enzymes give a hyperbolic curve.
4. Effect of Enzyme Concentration:
As the enzyme concentration increases, the rate of reaction increases steadily in presence of an excess amount of substrate, the other factors being kept constant. A linear curve is produced.
5. Effect of Products:
When the product is more in the reaction mixture, then the rate of reaction decreases due to feedback inhibition.
6. Effect of Light:
The speed of activity of various enzymes changes in different wavelength of light ex. blue light enhances the activity of salivary amylase whereas, U.V. light decreases the velocity.
7. Effect of Ions:
Presence or absence of particular ions enhances or reduces the activity of enzymes ex. Pepsinogen is converted to pepsin in presence of H + ions. Kinases act in presence of Mg +2 ions.
Enzymes are very specific in their reaction. They either act on one particular substrate or catalyse one particular reaction.
Accordingly enzyme specificity is of two types:
1. Reaction Specificity:
These enzymes are specific for the type of reaction they catalyse, irrespective of the substrate on which they act. Thus different enzymes bring about different reactions on the same substrate i.e. enzymes are specific for one particular reaction no matter which substrate it may be ex. amino acids are acted upon both by amino acid oxidase which oxidizes the amino acids to keto acids and decarboxylase that removes carbon dioxide from them.
2. Substrate Specificity:
These enzymes are specific for the substrate upon which they act. This is further classified as follows.
(a) Absolute specificity:
These enzymes are highly specific and act on one particular substrate only and no other substrate. Ex. Urease, catalase, aspartase.
(b) Relative specificity:
These enzymes act on one particular bond. Ex. D-amino acid oxidase.
(c) Group specificity:
These enzymes act on only one particular group.
Is a proteolytic enzyme that acts on peptide bonds contributed by aromatic amino acids like tyrosine, tryptophan and phenylalanine.
Is specific for basic amino acids. Hence it cleaves peptide bonds contributed by lysine and arginine.
Acts on peptide bond near the free amino end.
Specific for free carboxylic group.
Specific for α-1 → 4 glycosidic linkages.
(d) Stereo specificity:
These enzymes act on one particular stereo isomer.
i. Succinate dehydrogenase:
Is specific for the stereo isomer fumarate i.e. cis form of double bond.
Is specific for β glycosidic linkage.
iii. L-amino acid oxidases:
Act on L-amino acids only and not on D-amino acids.
They are non-protein, heat stable, low molecular weight dialyzable organic compounds that are required for the action of enzymes. Generally vitamins act as coenzymes ex. biotin, pyridoxine etc. Enzyme along with a co-enzyme is known as ‘holoenzyme’ and that without a co-enzyme is an ‘apoenzyme’. Apoenzyme (protein) + Co-enzyme (non-protein) → Holoenzyme (active enzyme protein).
Holoenzyme may contain an organic or inorganic compound (metal ions) or both. If organic substances are present with enzymes then they are known as ‘co-enzymes’ and if inorganic substances are acting with the enzymes then they are called as ‘co-factors’ (Mg, Mn, Zn, Co, Se, etc.).
The role of co-enzymes is:
(i) They act as co-substrate or second substrate ex. Pyruvate + NADH → Lactate + NAD + . NADH acts as a coenzyme or second substrate,
(ii) They help in transferring of groups either hydrogen or groups other than hydrogen, and
(iii) Specific activity of a co-enzyme is the number of units of co-enzyme present in one milligram of enzyme protein.
Enzyme unit or activity:
One unit of enzyme activity is the amount of enzyme that converts 1.0 (J.M of the substrate per minute into the products at 25°C.
Specific activity of an enzyme:
It is defined as the number of enzyme units per milligram of the protein.
Enzyme turnover number:
The number of substrate molecules transformed per minute (unit time) by a single enzyme is known as enzyme turnover number. Carbonic anhydrase has the highest turnover number of 36,000,000.
First and second order reaction:
A reaction in which there is only one substrate is termed as 1 st order reaction. A reaction in which two substrates are involved to form a product is termed as 2 nd order reaction, also known as bi-substrate reaction. This involves either single displacement (i.e. both substrates binding to two active sites in the enzyme at the same time) or double displacement (ping-pong displacement, wherein only one substrate binds to the enzyme active site at a given time, once this is released the other substrate binds).
The inactive form of an enzyme is known as zymogen or pro-enzyme. Pepsinogen and trypsinogen are the zymogens of pepsin and trypsin respectively.
Ribonucleic acids that catalyse a reaction similar to that of enzymes are known as ribozymes. These ribozymes help in the processing of the newly transcribed RNA ex. small nuclear RNA (SnRNA) and hetero-nuclear RNA (hnRNA).
Alteration in the enzyme activity by specific substances other than non-specific substances like pH, temperature etc. is called enzyme inhibition.
There are two types of enzyme inhibitions:
1. Irreversible Enzyme Inhibition:
The activity of the enzyme is inhibited by covalent binding of the inhibitor at the active site. The enzyme inhibitor bond cannot be dissociated, so it is permanent and irreversible i.e. it cannot be reversed.
i. Aldolase is inhibited permanently by iodoacetate.
ii. Di-isopropylflurophosphate (DFP), a component of nerve gas, inhibits most of the digestive enzymes permanently in human beings. Hence it is very poisonous.
iii. Para chloromercuric benzoate (PCMB) inhibits the enzymes hexokinase and urease irreversibly.
iv. Organic reagents like alkaloid reagents inhibit enzymes irreversibly.
2. Reversible Enzyme Inhibition:
The inhibitors bind reversibly to the enzyme and so it is not permanent. The inhibition can be reversed by various mechanisms.
(a) Competitive enzyme inhibition:
It is a type of reversible inhibition in which there is competition between substrate and inhibitor for the active site of an enzyme because of the structural similarity. All non-regulatory enzymes show competitive inhibition. Clinically competitive enzyme inhibition is of great importance since most of the drugs act by competitive inhibition.
(i) The enzyme succinate dehydrogenase’s (SDH) substrate is succinic acid and the competitive inhibitors are oxalic acid, mallonic acid and glutaric acid. Among these, mallonic acid is the most potent competitive inhibitor of SDH.
(ii) Folic acid, a vitamin for human beings has para-amino benzoic acid (PABA) as one of its components. Whereas it is not a vitamin for microorganisms i.e., they cannot utilize preformed folic acid from external source, instead they synthesize their own folic acid from aba. Sulpha drugs contain para-amino sulphonate which is structurally similar to PABA and hence competes for the enzyme active site of folic acid synthesis in microorganisms. If excess dose of sulpha drug is given, it results in inhibition of folic acid synthesis thus acting as an antibiotic. Human beings are not affected, because they do not synthesize folic acid.
(iii) Methanol is acted upon by the enzyme alcohol dehydrogenase forming formaldehyde which is highly poisonous. If ethanol if given to methanol intoxicated patients then ethanol competitively binds to alcohol dehydrogenase thereby preventing formation of formaldehyde.
(iv) Allopurinol is the competitive inhibitor of the enzyme xanthine oxidase whose substrate is hypoxanthine. Allopurinol prevents the formation of uric acid by competitive inhibition because it has structural similarity to hypoxanthine. This principle is used in the treatment of gout i.e. abnormal accumulation of uric acid crystals in the joint causing inflammation.
(v) Glaucoma is a condition in which there is accumulation of fluid in the lens resulting in enlargement of eye. This can be treated with ‘acetazolamide’ which inhibits the enzyme carbonic anhydrase competitively. This prevents water formation and subsequent release of more water through the urine.
(b) Non-competitive enzyme inhibition:
It is shown by regulatory enzymes, also called allosteric enzymes.
These are the enzymes that contain a site other than the active site which is called ‘allosteric site’. The action of some enzymes is regulated by ‘effectors’ which can bind reversibly to the enzyme molecule at specific sites other than the substrate binding site called the modulator site or the allosteric site.
There is no competition between substrate and inhibitor for the active site since the inhibitor or modulator binds at the modulator site or allosteric site. If the binding of the effector causes inhibition of the enzyme action then it is called a negative effector and the process is called ‘allosteric inhibition’.
If the enzyme reaction is activated by a modulator then it is called a positive modulator or effector and the process is called ‘allosteric activation’.
Ex. Phosphofructo kinase (PFK) is an allosteric enzyme of the glycolytic pathway.
The positive modulators of this enzyme are AMP and ADP.
The negative modulators of PFK are ATP and citrate.
These are substances (generally proteinacious in nature) that inhibit most of the digestive enzymes, ex. certain roundworms and hookworms survive in the intestine by secreting anti enzymes. Uncooked rice contains certain proteins that act as anti enzymes.
Reversible covalent modification:
Enzyme activity can be regulated by reversible covalent modification.
It is regulated by cyclic inter-conversion of enzyme into two forms:
The inter-conversion is brought about by a ‘converting enzyme’. The process of activation and inactivation of the enzyme is generally brought about by covalent phosphorylation or de-phosphorylation of the target enzyme. For example hormones like epinephrine, glucagon etc. bind to the hormone receptor site on the cell membrane and activate the enzyme adenyl cyclase, which in turn converts ATP to cyclic AMP (cAMP). This cAMP converts inactive protein kinase to active protein kinase (‘a’ form). This protein kinase phosphorylates many enzymes in the cell, some of which become active and yet some others become inactive.
The inactive phosphorylase (‘b’ form) gets converted to active phosphorylase (‘a’ form) upon phosphorylation and affects the breakdown of glycogen to glucose. On the other hand glycogen synthase becomes inactive upon phosphorylation thereby inhibiting the formation of glycogen.
Diagnostic Importance of Enzymes:
Enzymes were classified into two groups based upon their clinical importance as ‘functional plasma enzymes’ i.e., those enzymes present in the plasma in considerably high amounts and are functional in the plasma due to the presence of their substrate in it.
Ex. serum lipase, blood clotting enzymes, and ‘non-­functional plasma enzymes’ i.e., those enzymes that are present in the plasma in negligible amounts and have no function in the plasma due to the absence of their substrate in it. Non-functional plasma enzymes are of diagnostic importance.
The non-functional plasma enzymes are present in higher concentration in tissues and very low concentration in the plasma i.e. in trace amounts, but their concentration in plasma increases immediately following tissue injury or destruction.
If there is tissue damage leading to cell rupture then the enzymes present in that tissue leaks into the blood leading to the increase in the concentration of these enzymes in the plasma. Increase in the level of non-functional plasma enzymes in the blood, indicates the disorder to the tissue where they exist. Different enzymes exist in different tissues in varying levels. Damage to a specific tissue releases a particular enzyme. Therefore estimation of enzymes in the plasma has a diagnostic importance.
The non-functional plasma enzymes include lactate dehydrogenase (LDH), creatine phosphokinase (CPK), alanine amino transferase (ALT) or serum glutamate pyruvate transaminase (SGPT), aspartate transaminase (AST) or serum glutamate oxaloacetate transaminase (SGOT), sorbitol dehydrogenase, alkaline phosphatase, acid phosphatase, amylase, pancreatic lipase etc.
However functional plasma enzymes are already in higher concentration in the plasma, hence their decrease in the concentration in the plasma indicates malfunction of the organ where they are synthesized ex. blood clotting enzymes are synthesized in the liver hence decrease in their concentration indicates liver dysfunction.
Anyway an immediate assessment of the liver function cannot be made by this assessment because by the time the enzyme concentration in the plasma decreases (may take 4 to 5 days), the liver must have regained its normal vitality.
Diagnosis of Myocardial Infarction using Enzyme Assay:
There are three main enzymes that are used in the diagnosis of myocardial infarction (1) Lactate dehydrogenase (LDH) (2) Creatine phosphokinase (CPK)—marker enzyme and (3) Transaminase (AST or SGOT).
(1) Lactate dehydrogenase (LDH):
LDH catalyses the inter conversion of pyruvate to lactate, a very important reaction of anaerobic glycolysis. Glycolysis occurs in each and every cell, in some cells it is always anaerobic (RBC) whereas in others it is aerobic sometimes and anaerobic at some other time (muscle tissue, liver, kidney etc.).
In other words LDH is present in each and every cell of the body. Therefore damage to any of the tissues of the body results in release of LDH into the plasma. Hence it becomes a difficult task to trace out the organ from which it has leaked.
However LDH exists in five isoenzyme forms i.e. multiple forms of the same enzyme (These enzymes bring about the same reaction but exhibit different physical characters like molecular weight, charge, electrophoretic mobility, Km and isoelectric pH). The polypeptides in LDH are designated as ‘H chain’ and ‘M chain’.
All the isoenzyme forms of LDH are tetramer i.e. has four polypeptides in the following combinations:
(e) M4 or LDH5—Liver and skeletal muscle
All these isomers have been successfully separated on Sodium Dodecyl Sulphate Polyacryl Amide Gel Electrophoresis (SDS-PAGE) and their banding pattern from the plasma is established as under—
LDH1 or H4 is predominantly present in the cardiac muscle, whereas the isoenzyme form LDH5 or M4 is more abundant in the skeletal muscle. These two enzymes have different Km values and Km is indirectly proportional to affinity (Km a 1/affinity).
The skeletal muscle enzyme M4 has low Km value for pyruvate and hence greater affinity for pyruvate resulting in high rate of conversion of pyruvate to lactate. The cardiac isoenzyme LDH1 or H4 has high Km value for pyruvate hence lesser affinity for pyruvate, therefore low rate of conversion of pyruvate to lactate. Thus the concentration of H4 or LDH1 isoenzyme form of lactate dehydrogenase increases in the plasma during myocardial infarction. The peak levels of LDH are maintained in the plasma for 6 days following the attack, after which it starts receding in its concentration.
(2) Creatine phosphokinase (CPK):
This is known as the marker enzyme for the diagnosis of myocardial infarction or heart attack, because this is the first enzyme to increase within a short time in the blood plasma following a heart attack. CPK is an enzyme that catalyses the conversion of creatine to creatine phosphate, a high energy compound that works to supply energy during muscle contraction. Therefore this enzyme is present only in a few tissues like the cardiac muscle, skeletal muscle and the brain.
CPK also exists in various isoenzyme forms. It has two polypeptides ‘B’ & ‘M’ that form dimers in the following combinations to give rise to three isoenzymes of CPK.
MB — Predominant in cardiac muscle
MM — Predominant in skeletal muscle
Thus estimation of the isoenzyme MB is indicative of heart attack. CPK maintains a higher concentration in the plasma for 1-2 days. The concentration of CPK after the first attack is 10 times more than the normal and if another attack occurs within a day or two the concentration further increases to 100 fold and a third attack within a short span of time raises the level of CPK to 300 fold which is lethal concentration.
Among the two transaminases, aspartyl transaminase (AST or SGOT) increases in the plasma following an attack and the higher levels are seen in 4 to 5 days following an attack.