Subtilisin (Mr 27600) is a bacterial protease that can catalyze hydrolysis of certain amino acid esters and amides. Subtilisin does have a problem, in that it becomes inactivated by oxidation of a methionine Met-222. Subtilisin mutant with Cys replacing Met-222 shows 138% of activity of the wild type (due to increase in kcat for mutant). Subtilisin mutants with either Ala, Ser, Gly, or Thr in place of Met-222 exhibit enzymatic activity of 53, 35, 30, and 28% of subtilisin, respectively, while any other amino acid replacement leads to a significant reduction in enzymatic activity of mutant (0.3 - 15% of subtilisin activity). Explain in detail the reason(s) for such a loss of activity in:
a). Oxidized subtilisin
b). Ala, Ser, Gly, and Thr mutants
c). All other mutants.
d). Why Cys-222 mutant may have higher kcat than a wild-type enzyme?
e). Why this (Cys-222) mutation has not been selected as the best in evolution?