Mechanism of Enzyme Action

An enzyme (E) molecule has a highly specific binding site or active site to which its substrate (S) bind to produce enzyme-substrate complex (ES). The reaction proceeds at the binding site to produce the products (P), which remain associated briefly with enzyme (enzyme-product complex). The product is then liberated and the molecule is then freed in an active state to initiate another round of catalysis. Apparently the affinity of binding site for the product is much lower than that for the substrate.

Enzymes reduce the overall level of activation energy. Even a modest reduction in the value of activation energy leads to very large increases in the reaction rates. The various mechanisms used by enzymes to lower the activation energy are briefly described below :-

• Stabilization of the Transition state of  the substrate reduces the activation energy of the overall reaction (most important feature of the enzymatic catalysis). The binding sites of the enzymes bind more strongly to the substrate molecules in the transition state than to those in the ground or stable state. In addition, when a substrate molecule in ground state binds to the binding site, it is forced into a configuration closer to that of transition state, this lowers the energy needed for reaching the transition state.

• CATALYSIS BY APPROXIMATION is the binding of a substrate to an enzyme by holding it close to another chemical group and in the optimal orientation for the reaction to proceed. In contrast, substrate molecules in the transition state must collide with each other, a random process, for the reaction to occur in uncatalysed systems.

• COVALENT CATALYSIS is the mechanism in which the chemical groups, e.g., amino groups, in the side chains of amino acids present in the active site become covalently bound to a group in the substrate molecule, such substrate molecules are much more reactive than the nonbound molecules. Another substrate may then react with the substrate bound to the enzyme molecule to form the product. The product then becomes free from the enzyme. This strategy of enzyme action is called covalent catalysis.

• REDOX CATALYSIS or ACID-BASE CATALYSIS is the mechanism in which the amino acid chains may act as acids, i.e, donor of protons, or bases, i.e, acceptors of protons, and thereby catalyze the reactions. In addition, electrons may also be transferred during the course of a reaction, such reactions are called redox reactions. The electron transfers generally involve transition metals or coenzymes present as a component of such enzymes. Thus, these mechanism is called acid-base or redox catalysis.

VIDEO OF MECHANISM OF ENZYME ACTION



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