Coupon Accepted Successfully!


Mechanism Of Enzyme Catalysis

  1. Energy of activation is required to sufficiently energize a substrate molecule to reach a transition state in which there is a high probability that a chemical bond will be made or broken to form the product. Enzymes increase the rate of reaction by decreasing the energy of activation.
  2. Specificity
    1. The specificity of an enzyme is determined by the functional groups of the substrate, the functional groups of the enzyme, and the physical proximity of these functional groups.
    2. Two theories have been proposed to explain the specificity of enzyme action.
      1. Lock and key theory. The enzyme active site is complementary in conformation to the substrate, so that enzyme and substrate recognize one another.
      2. Induced-fit theory. The enzyme changes shape on binding substrate, so that the conformation of substrate and enzyme is only complementary after binding. 
  3. Properties of enzymes
    Enzymes are protein catalysts that increase the velocity of a chemical reaction and are not consumed during the reaction they catalyze.

  1. Active sites. Enzymes molecules contain a special pocket or cleft called the active site. The active site contains amino acid side chains that create a three-dimensional surface complementary to the substrate. The active site binds the substrate, forming an enzyme-substrate (ES) complex.
  2. Catalytic efficiency. Most enzyme-catalyzed reactions are highly efficient, proceeding from 103 to 108 times faster than uncatalyzed reactions.
  3. Specificity. Enzymes are highly specific, interacting with one or a few specific substrates and catalyzing only one type of chemical reaction.
  4. Cofactors. Some enzymes associate with a nonprotein cofactor that is needed for enzymatic activity. Holoenzyme refers to the enzyme with its cofactor. Apoenzyme refers to the protein portion of the holoenzyme. In the absence of the appropriate cofactor, the apoenzyme typically does not show biologic activity. A prosthetic group is a tightly bound cofactor that does not dissociate from the enzyme (for example, the biotin (AIIMS Nov 08)  of carboxylases). Term coenzyme is used when cofactor is organic in nature. 
  1. Factors affecting reaction velocity
    1. Substrate concentration                    
    2. Temperature
    3. PH                                   
    4. Enzyme

The effect of these factors on rate of Enzyme Catalyzed reaction resulting into curne of different shapes is as follows:-

  1. Enzyme inhibition
    Enzyme inhibition is of 3 types:

1.  Competitive
2.  Non competitive
3.  Un competitive

The effect of these inhibitions on Km & Vmax is as follows:-




Non Competitive

Un Competitive



No Change



No Change



  1. Enzyme regulation
    There are three ways of enzyme regulations:-
    1. Induction and repression: - Enzyme regulation is under the control of DNA (genes) which makes m-RNA which makes protein which finally regulates enzyme activity.
    2. Covalent modification: In this enzymes one regulated by phosphorylation and dephosphorylation. In certain enzymes, the addition of a phosphate group to a specific amino acid residue [usually serine (Ser), tyrosine (Tyr), or threonine (Thr)] by specific protein kinases dramatically enhances or depresses activity. This is known as phosphorylation. Removal of phosphate group by specific protein phosphatase is known as dephosphorylation.  This dephosphorylation activates some enzymes and inactivates other enzymes.
    3. Allosteric regulation: The activity of enzymes that catalyze key regulatory reactions (committed steps) of metabolic pathways are often subject to allosteric regulation. Their activity can be modulated by the binding of allosteric effectors to a site on the enzyme that is distinct from the active site (i.e., allosteric site). In this regulation there is fine tuning of metabolic pathway.
      1. Effectors are positive if they enhance the rate of a reaction (i.e., activators) and negative if they decrease the rate of reaction (i.e., inhibitors).
      2. Feedback inhibition is negative modulation of the committed step of a metabolic pathway by its end product. This prevents unnecessary production of an excess of end product by shutting down the pathway until more is needed.
Important enzymes regulated by phosphorylation-dephosphorylation
Enzyme Active Inactive
Acetyl CoA carboxylase Dephosphorylated Phosphorylated
Glycogen synthase Dephosphorylated Phosphorylated
Pyruvate dehydrogenase Dephosphorylated Phosphorylated
HMG-CoA-reductase Dephosphorylated Phosphorylated
Pyruvate kinase Dephosphorylated Phosphorylated
Phosphofructokinase-2 Dephosphorylated Phosphorylated
Glycogen phosphorylase Phosphorylated Dephosphorylated
Citrate lyase Phosphorylated Dephosphorylated
Phosphorylase b kinase Phosphorylated Dephosphorylated
HMG - Co reductase kinase Phosphorylated Dephosphorylated
Fructose-2, 6-bisphosphatase Phosphorylated Dephosphorylated


Extra edge
  1. Certain enzymes are synthesized as proenzymes, or zymogens, which are inactive forms of enzymes that become active only after being cleaved at a specific site in their polypeptide chain by specific proteases.
  2. Many digestive enzymes that hydrolyze proteins (i.e., trypsin, pepsin) are synthesized as zymogens in the stomach and pancreas.
  3. Blood clotting is mediated by a series of proteolytic zymogen activities of several serum enzymes.

Test Your Skills Now!
Take a Quiz now
Reviewer Name