J. Biol. Chem., Vol. 261, Issue 20, 9087-9089, Jul, 1986
Benzoylamidoacetonitrile is bound as a thioimidate in the active site of papain
JR Brisson, PR Carey and AC Storer
13C NMR spectroscopy has been used to demonstrate that 13CN-labeled
benzoylamidoacetonitrile forms a covalent adduct with the thiol group of
cysteine 25 in the active site of papain. Spectral comparison with model
compounds indicates that the adduct is a thioimidate. On the basis of a
proposed mechanism for the formation of the thioimidate, it is concluded
that the -CH2C(= NH)S--imino nitrogen does not sit in the active site in
the same manner as the thiol ester carbonyl oxygen of the thiol acyl enzyme
(or the oxyanion of the tetrahedral intermediate). Thus, in this sense the
stabilization of the thioimidate does not reflect a similarity in structure
between the bound thioimidate and the transition state.