The Natural Evolution Process Initially Produces Poor Enzymes

Changes in substrate selectivity or reaction chemistry often require amino acid substitutions at two or more specific locations along the amino acid chain. During evolution, point mutations leading to amino acid substitutions occur at random amino acid positions, so the probability of accumulating specific amino acid changes at two predefined locations with two random mutations is very low indeed. Consequently, many mutations accumulate in the gene before changes that can affect the specificity of the enzyme occur. This helps explain why related enzymes with different specificities often differ in sequence identity by >50%. If we consider any particular amino acid location, the chances of a substitution increasing stability and/or activity of the enzyme are less likely than decreasing its stability and/or activity (Taverna and Goldstein, 2002a). Thus, by the time a gene accumulates sufficient numbers of mutations to achieve a new functionality, its catalytic properties (K_m and K_cat), in addition to its stability, are impaired. This decline in functionality is inevitable because selection for the new functionality can only occur after the new catalysis arises. Only at this time can selection pressure for the product of the new reaction lead to subsequent selection of mutants with improved catalytic properties (Taverna and Goldstein, 2002b).