The molecular events that accompany the conversion of substrate into products constitute the mechanism of enzyme action. Enzyme action on its substrate results either in the formation or degradation of chemical bonds in the substrate molecules.
According to Michaelis – Menton theory, the enzyme E combines with the substrate S to form an intermediate enzyme substrate complex ES. This complex then breaks down into product P and enzyme E is regenerated. The enzyme can again combine with the fresh molecule of the substrate in similar manner. The formation of enzyme substrate complex as an intermediate during the reaction has been proved by spectroscopic studies. So, a simple enzymatic reaction might be written as
Where E, S and P represent enzyme, substrate and product respectively. ES and EP are complexes of the enzyme with substrate and product respectively. At the end of the reaction along with the required products the enzyme is regenerated in its original form and can involve in another round of catalysis. ES complex is a highly energised, transiently existing complex which can be easily degraded to form the product.
In the formation of enzyme substrate complexes, the substrate molecules attach at certain specific sites on the enzyme molecules. These specific points on enzyme molecules where the substrate molecules attach are known as active site or catalytic site.
Active sites on the enzymes are usually provided by certain functional group of amino acids present in the enzyme protein. For example, free hydroxyl group of serine, phenolic group of tyrosine, sulfhydryl group of cysteine and imidazolyl group of histidine are some of the important catalytic groups present in enzyme active sites.
In 1894, Fischer proposed that the substrate fits into the active site of the enzyme as a key fits into the lock (Fig 9.4). Because of this model, the theory is known as lock and key theory of enzyme action.
According to lock and key theory, there are exact functional groups and structural features in the enzyme into which substrate molecule must fit. The region of the enzyme that complexes with the substrate is called active site or catalytic site. The theory cannot be applied for all the enzymatic reactions because in some reactions the substrate molecules and the active site are not structurally similar to fit in with each other. Moreover, in certain cases the catalytic activity is observed even though a fit is impossible.
Later, lock and key theory was modified by Koshland in 1963 in the form of ‘induced fit mechanism’. The essential feature of this theory is the flexibility of the enzyme active site. In Fisher model, the active site is presumed to be a rigid preshaped structure to fit the substrate, while in the induced fit model the substrate induces the conformational change in the enzyme (Fig 9.5), so that the substrate and active site come close to each other in such a way that the substrate fits the active site in a more convenient manner.
The active site on the enzyme molecule exerts a binding force on the substrate molecule by hydrophilic and hydrophobic catalytic groups. Enzyme substrate complexes are formed by multiple bonding i.e., covalent, electrostatic and hydrogen bonding with the substrate. The functional group at the active site are arranged in a definite spatial manner so that the ES complex formation is favourable.
Many enzymes require non proteinous group called as coenzymes for their maximal activity. The enzymes requiring coenzymes for their activity also possess sites for the attachment of co-enzymes. The complexes formed in such cases are known as enzyme-substrate-coenzyme complexes.
Certain enzymes require a metal ion, in addition to coenzyme for their full activity. These metallic ions are called positive modifiers of enzyme activity. Examples of such enzymes include alcohol dehydrogenase, peroxidase, catalase and xanthine oxidase etc., which contain sites for binding metal ions. The removal of metal from these enzymes often results in partial or total loss of enzymatic activity. These enzymes are otherwise called as metallo enzymes. The common metallic ions required for enzymatic activity are K+, Cu+, Mg++, Ca++ etc.