Abstract
The hydroxyl of the serine in the active center of subtilisin has been converted to a sulfhydryl group. The resulting enzyme, thiol-sub-tillsin, is at most 1/100 as active as native subtilisin toward normal ester and peptlde substrates. Since model experiments indicated that the thiol group should in general be more reactive than a hydroxyl group, the loss of activity of thiol-subtUisln indicates the enormous sensitivity of catalytic residues to subtle changes in structure.