Wg. Niehaus et al., SLOW-BINDING INHIBITION OF 6-PHOSPHOGLUCONATE DEHYDROGENASE BY ZINC ION, Archives of biochemistry and biophysics, 333(2), 1996, pp. 333-337
6-Phosphogluconate dehydrogenase (EC 1.1.1.44) has been purified from
Cryptococcus neoformans, an encapsulated yeast that is an opportunisti
c pathogen of AIDS patients, The dimeric enzyme had a subunit molecula
r weight of 50,000, a specific activity of 50 units mg(-1), and K-m va
lues of 13 mu M for 6-phosphogluconate and 0.89 mu M for NADP, This en
zyme, like many fungal dehydrogenases, was inhibited by Zn2+, with the
inhibition pattern being competitive versus the nonnucleotide substra
te, In the presence of micromolar Zn2+, the reaction was biphasic, wit
h the reduction of NADP proceeding initially at the control rate, but,
over the time course of 20-300 s, this initial nonlinear phase reache
d a final, linear steady state with a slower velocity, This pattern is
indicative of a slow binding inhibition process, for which we have ca
lculated the following kinetic constants: k(6), the limiting rate cons
tant for the transition from initial to final steady state was 0.0024
s(-1), corresponding to a half-time of 300 s; K-i, the overall equili
brium constant for the dissociation of EZn2+ to E + Zn2+ was 0.021 mu
M. (C) 1996 Academic Press, Inc.