Glucose‐6‐phosphate dehydrogenase

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Abstract
The conformation of NADP+ in glucose‐6‐phosphate‐dehydrogenase—NADP+ binary complexes has been investigated using proton‐proton transferred nuclear Overhauser enhancement measurements to determine inter‐proton distance ratios between bound NADP+ protons. The enzymes from Saccharomyces cerevisiae (brewer's yeast and baker's yeast) and Hansenula jadinii (Candida utilis, Torula utilis) form binary complexes with NADP+ in which the glycosidic bond of the adenine moiety is in the anti conformation whereas that of the nicotinamide moiety exists as a syn (69–70%)/anti (30–40%) mixture. The enzymes have similar subunit sizes (Mr∼58000) and it is shown that they bind NADP+ in essentially similar conformations. Inactivation of the baker's yeast enzyme with acetylsalicylic acid caused little if any alteration in the conformation of bound NADP+, and the presence of NADP+ during inactivation afforded very little protection to the enzyme. Inactivation rates were, however, lower in the presence of glucose 6‐phosphate. It is concluded that the ɛ‐amino group of the lysine residue that is acetylated during the inactivation reaction with acetylsalicylic acid is not necessary for binary complex formation between the enzyme and NADP+, but that it is situated in a part of the molecule affected by formation of the enzyme—glucose‐6‐phosphate complex. The implication of the findings for the catalytic process, and related evolutionary aspects, are discussed briefly.