Slow-binding inhibition of 6-phosphogluconate dehydrogenase by zinc ion.
6-Phosphogluconate dehydrogenase (EC 1.1.1.44) has been purified from Cryptococcus neoformans, an encapsulated yeast that is an opportunistic pathogen of AIDS patients. The dimeric enzyme had a subunit molecular weight of 50,000, a specific activity of 50 units mg-1, and Km values of 13 microM for 6-phosphogluconate and 0.89 microM for NADP. This enzyme, like many fungal dehydrogenases, was inhibited by Zn2+, with the inhibition pattern being competitive versus the nonnucleotide substrate. In the presence of micromolar Zn2+, the reaction was biphasic, with the reduction of NADP proceeding initially at the control rate, but, over the time course of 20-300 s, this initial nonlinear phase reached a final, linear steady state with a slower velocity. This pattern is indicative of a slow binding inhibition process, for which we have calculated the following kinetic constants: k6, the limiting rate constant for the transition from initial to final steady state was 0.0024 s-1, corresponding to a half-time of 300 s; Ki*, the overall equilibrium constant for the dissociation of E*Zn2+ to E + Zn2+ was 0.021 microM.
Niehaus
W G
WG
Department of Biochemistry, Virginia Polytechnic Institute and State University, Blacksburg 24061-0308, USA. wniehaus@vt.edu
Richardson
S B
SB
Wolz
R L
RL
D017088:AIDS-Related Opportunistic Infections; D003035:Cobalt / Q000494:pharmacology; D003453:Cryptococcosis; D003455:Cryptococcus neoformans / Q000201:enzymology* / Q000472:pathogenicity; D006801:Humans; D007700:Kinetics; D046911:Macromolecular Substances; D008956:Models, Chemical; D008970:Molecular Weight; D009243:NAD / Q000378:metabolism; D009532:Nickel / Q000494:pharmacology; D010734:Phosphogluconate Dehydrogenase / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D015032:Zinc / Q000378:metabolism* / Q000494:pharmacology
D046911:Macromolecular Substances; D009243:NAD; D003035:Cobalt; D009532:Nickel; D010734:Phosphogluconate Dehydrogenase; D015032:Zinc