Replacement of tryptophan 392 located in the active site cavity of pyruvate
decarboxylase (PDC; EC 4.1.1.1) from Zymomonas mobilis by methionine or gl
utamine yields enzymes with smaller catalytic constants of 8.5 s(-1) and 3.
6 s(-1) at 4 degrees C, compared to that of the wild-type enzyme (17 s(-1))
. The rate constants of the H/D exchange at the C2 of the coenzyme thiamine
diphosphate have been determined to be 130 s(-1) for the wild-type enzyme,
56 s(-1) for the methionine and 30 s(-1) for the glutamine mutant, respect
ively, A group with a pK(a) of about 5 has been identified to be essential
for C2 deprotonation of the enzyme-bound thiamine diphosphate from the pH d
ependence of the H/D exchange. (C) 1998 Federation of European Biochemical
Societies.