In this article we will discuss about the mechanism of enzyme reaction.
In an enzyme-catalysed biochemical reaction, the enzyme molecule binds specifically and reversibly to the substrate molecule resulting in formation and breaking of chemical bonds to produce the product. Thus, there is formation of an unstable enzyme-substrate complex which immediately breaks into the product.
This can be simply represented by the following equation:
The three-dimensional protein molecule of an enzyme possesses specific sites, known as catalytic Sites or active centres, on its surface which bind the substrate molecule. Generally, the number of sites is one per polypeptide chain constituting a protein molecule. The enzyme-protein is much larger in size than the substrate and, therefore, the latter occupies only a small area of the enzyme surface which forms the catalytic site consisting of a few amino acid residues.
Presence of sulfhydryl (-SH) groups in the active centres is characteristic of many enzymes. The relationship between an enzyme and its substrate has been likened to that between a lock and its key. Just as a key fits into a lock and turns the levers to open or close it, so a substrate molecule fits into the active centre of an enzyme molecule to form an enzyme-substrate complex.
The binding causes transformation of the substrate molecule to the product through opening of the bonds and formation of new bonds. As soon as these changes are accomplished, the product is released, because it no longer fits into the active site and the enzyme molecule returns to its original form. It can now bind another substrate molecule at its active centre.
The enzyme reaction can be diagrammatically represented as shown in Fig. 8.24:
As the enzyme molecule remains unchanged after the reaction, a small amount of enzyme can turn over a large amount of substrate to product. However, because all enzyme-catalyzed reactions are theoretically reversible, the forward reaction can continue till the concentration of the product in the enzyme reaction mixture is so high that an equilibrium is attained and, at equilibrium, the rate of forward reaction equals to that of the reverse reaction.
Like chemical catalysts, enzymes accelerate the rate of biochemical reactions by lowering the activation energy. This energy refers to the amount required to bring the reactant molecules to a higher energy level (activated state) where a chemical bond may be formed or broken to yield the product or products. When the reactant molecules reach this activated condition, they are energy-rich and are in a transition state. In enzyme reactions, the enzyme-substrate complex represents the transition state.
The enzyme functions to lower the amount of energy required to bring the substrate to the transition state as shown in Fig. 8.25. An example may be cited to explain the significance of activation energy. A mixture of hydrogen and oxygen will remain unchanged indefinitely, although they have the ability to combine producing water.
An electric spark will bring them to the transition state and they combine with release of energy. Here, the electric spark provides the activation energy. The same reaction is catalysed by an enzyme, called hydrogenase, at ordinary temperature, because the enzyme lowers activation energy.
