An enzyme (E) combines with its substrate (S) to form an intermediate enzyme-substrate complex (ES) , which then decomposes into reaction products(P) and the free enzyme, as seen in the equation below.
E S ————— > ES ———- > E + P
An enzyme causes the substrate upon which it is acting to be much more reactive than when it is free. One postulate accounting for this is that the enzyme holds the substrate in a position which strains and weakens the substrate’s molecular bonds. This weakening of the bonds within the substrate makes them easier to cleave and results in a general lowering of the energy of activation of the reaction. This postulate is extremely simplistic – the actual forces at work are much more numerous and complex.
When the substrate binds to the enzyme, it combines with only a relatively small part of the enzyme molecule the active site. Information about the active site, such as its location and the nature and sequence of amino acids in it, provides an indication of the mechanism of binding and catalysis. The binding of the substrate to the enzyme’s active, site depends on many forces: hydrogen bonding, the interaction of hydrophobic (water-repelling) groups, and the electrostatic interaction between charged groups on the amino acids. Many active sites also contain metal ions which aid in binding the substrate or expediting the catalytic reaction by withdrawing or stabilizing electrons. For example, the enzyme carboxypeptidase, which hydrolyzes polypeptide bonds of proteins in food, contains a zinc atom in its active site. The electrophilic (electron-attracting) zinc atom coordinates electrons from the carbonyl of the peptide bond, weakening the bond for attack by a specific amino acid of the enzyme at the active site. Such a mechanism, however, is beyond the scope of elementary biology and one would require a good course in biochemistry to understand fully.
Some enzymes, the regulatory or allosteric enzymes, have two binding sites: an active site and a regulatory site. Regulatory enzymes are a key controlling factor in metabolic pathways. If the end product of a pathway is in excess, it inhibits the action of the regulatory enzyme by binding to its regulatory site. The end product shuts off the catalytic activity of the active site by altering the arrangement of the enzyme’s polypeptide chains, thus deforming and inactivating the enzyme (see diagram below).
Fig. Schematic diagram showing binding at the active site (a)
and regulatory site (b) of an enzyme. Note the change in
enzyme conformation accompanying binding of product to the