Enzymes and isoenzymes

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1 Enzymes and isoenzymes
General properties of the enzymes Coenzymes Zymogens or proenzymes Factors affecting enzyme activity The Michaelis-Menten equation Feedback inhibition Enzyme inhibition Isoenzymes or isozymes Clinical significance of enzymes

2 Enzymes Enzymes are biological catalysts (biocatalysts) which bring about chemical reactions in the living cells. They accelerate the rate of the reaction and usually present in very small amount in various cells. They are organic compounds and number of them have been obtained in crystalline form Today, more than 840 enzymes are known.

3 General properties of the enzymes
They are proteinous in nature. They accelerate the rate of the reaction by: (a) not altering the reaction equilibrium (b) being required in minute quantity (c) being not consumed in the overall reaction. They act as catalysts. They are very specific for their substrates. They possess active sites at which interaction with substrate occurs. They are responsible for lowering activation energy. Some enzymes are regulatory in function.

4 Coenzymes They are stable towards heat.
Generally derived from vitamins. Function as co-substrates They participate in (a) electron transfer reaction such as NAD+ , NADH, FMN, FAD, etc..(b) group transfer reaction such as, CoA, TPP, pyridoxal phosphate. In addition to the protein component, may enzymes require non-protein constituents for their proper functioning. When small organic molecules are bound tightly either by covalent or coordinate bonds they are called as prosthetic group.

5 Zymogens or proenzymes
Active enzyme Activator Zymogen pepsin H+, pepsin Pepsinogen trypsin trypsin, enterokinase Trypsinogen chymotrypsin Chymotrypsinogen carboxypeptidase Procarboxypeptidase elastase Proelastase rennin H+, rennin Prorennin

6 Factors affecting enzyme activity
Major factors responsible to affect the enzmic activity are the following: Concentration of substrate. Concentration of enzyme. Concentration of reaction products. Effect of pH. Effect of temperature. Effect of time.

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11 The Michaelis-Menten equation
The Michaelis constant (Km) corresponds to the substrate concentration that produces half-maximal velocity. Km is independent of enzyme concentration.

12 Derivation of the Michaelis-Menten equation
A mathematical analysis of the kinetics of an enzyme catalyzed reaction was proposed by Michaelis and Menten. K1, K-1 and K2 are rate of constant for various reactions. Reaction is in steady state, with concentration ES constant. Reverse reaction is also negligible. then

13 Enzymes catalyze with two or more substrate
In most enzymatic reactions ,however, two (and some times more) different substrate molecules bind to the enzyme and participate in reaction. 2 products formed together. Products formed one after another.

14 Feedback inhibition or Feedback control
As seen in the diagram the product (P) acts to inhibit some early step in the pathway, but not necessarily the first step in the pathway. By feedback control it is possible not only to inhibit the production of (P), but also to divert the flow of (B) from one pathway to another.

15 Enzyme inhibition The rate of an enzyme catalyzed reaction can be deceased by specific inhibitor (I) compound that combine with the enzyme and prevent normal ES interactions in the active site, thus diminishing the rate of the reaction. Competitive inhibition. Non-competitive inhibition. Mixed inhibition.

16 Isoenzymes or isozymes
The term isoenzymes or isozymes may be define as the enzymes existing in multiple forms having similar catalytic activity but different biochemical and immunologic properties ;they can be separated by electrophoresis. It has been shown that the relative amounts of the isoenzymes of particular enzyme differ in different organs so that in diseases different enzyme patterns are found according to the organ from which they have come. Examples of isoenzymes are: Lactic dehydrogenase (LDH) Alkaline phosphatase (ALP) Creatine phosphokinase or creatine kinase (CPK or CK) Acid phosphatase (ACP) Amylase Cholinesterase Hexokinase Nearly 100 enzymes have been detected to exist as isoenzymes

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