Substrate assisted catalysis: Implications for biotechnology and drug design

In enzyme catalysis, the convention is that the enzyme supplies all the functional groups that are needed to convert a substrate into a product. This convention, however, is now recognized to have some exceptions. In a growing number of cases it is evident that the substrate also provides one or more functional groups that actively participate in the catalytic process. These cases are grouped together under the title 'substrate-assisted catalysis' (SAC). Examples of SAC have been described both for native and for engineered enzymes that were rendered inactive by mutations. Such mutations eliminate amino acid sidechains that participate in the catalytic process and thereby cause partial or total loss of enzymatic activity. For several of these mutant enzymes, a modified substrate bearing functional groups similar to those that had been eliminated by the mutation was found to rescue enzyme activity. A notable example is the mutual specificity found between mutant serine proteases and their modified substrates. This creates a highly specific site for proteolytic cleavage, a desirable property in the processing of recombinant fusion products. An attractive target for SAC is the G-protein family. It was applied to two of its members - Gsα and p21 -Ras. In both cases it was possible to restore the GTPase activity of the mutants back to the level of the wild-type proteins. Beyond restoring activity by SAC, further modifications of the substrate were introduced to support or refute particular roles of the functional groups in the GTPase reaction. This approach was also applied as a molecular tool to discriminate between specific enzymatic mechanisms and as a guideline to incorporate particular functional groups into the substrate. Taken together, these studies pave the way to novel therapeutic and biotechnological approaches aimed at restoring the activity of mutant inactive enzymes. (C) 2000 Wiley-Liss, Inc.