Ser214 Is Crucial for Substrate Binding to Serine Proteases*

Highly conserved amino acids that form crucial structural elements of the catalytic apparatus can be used to account for the evolutionary history of serine proteases and the cascades into which they are organized. One such evolutionary marker in chymotrypsin-like proteases is Ser214, located adjacent to the active site and forming part of the primary specificity pocket. Here we report the mutation of Ser214 in thrombin to Ala, Thr, Cys, Asp, Glu, and Lys. None of the mutants seriously compromises active site catalytic function as measured by the kinetic parameter k cat. However, the least conservative mutations result in large increases in K m because of lower rates of substrate diffusion into the active site. Therefore, the role of Ser214 is to promote the productive formation of the enzyme-substrate complex. The S214C mutant is catalytically inactive, which suggests that during evolution the TCN→AGY codon transitions for Ser214 occurred through Thr intermediates.