6 Catalytic proteins
Among those proteins of known function, the majority are enzymes. Enzymes act as catalysts, i.e. they increase the rates of reactions, making and breaking bonds, without themselves undergoing any permanent change. They are highly specific for particular reactions and are excellent examples of how a protein's function is entirely dependent on its structure.
First of all, a protein must bind its substrate (or substrates) in a specific fashion; it must then convert the substrate(s) into the product (or products) and finally release these. In the case of enzymes, the binding site for a substrate is also referred to as the active site, as an indication that the substrate undergoes chemical modification.
An enzyme-catalysed reaction can be represented as follows:
where E is the enzyme, S is the substrate, P is the product, ES is the enzyme with substrate bound (enzyme–substrate complex) and EP is the enzyme with product bound (enzyme–product complex). ES‡ (called the transition state) is the highest-energy intermediate in the enzyme-catalysed reaction and is very unstable, due to the strained conformation of both the substrate and the enzyme in this complex.
The transition state in an enzyme-catalysed reaction has a lower energy than that for the same reaction in the absence of enzyme. Thus the enzyme reduces the energy barrier presented by the conversion of substrate to product and hence speeds up the reaction. We will consider how the structure of an enzyme, and in particular the configuration of the amino acid residues at the active site, facilitate catalysis.