Genetic Variants of Human Erythrocyte Glucose‐6‐Phosphate Dehydrogenase

Abstract

Analysis of thermal inactivation profiles of human erythrocyte glucose‐6‐phosphate dehydrogenase, type B, reveals three distinct forms of the enzyme. The form with the lowest stability is observed at very low NADP+ concentration, and probably consists of the monomeric molecular species. As the NADP+ concentration is increased, the enzyme undergoes two successive transitions to forms of higher stability, which probably consist of two different conformational states of the dimer, in equilibrium with tetramer. Enzyme type A behaves similarly, but may have less tendency to monomer formation. NADPH, like NADP+, is competent to stabilize the enzyme under suitable conditions and to induce dimer formation, but probably not to induce formation of the dimer with the highest thermostability. NADPH nay be integral part of the enzyme structure. Stopped‐flow experiments suggest that the monomer of glucose‐6‐phosphate dehydrogenase is catalytically inactive, but that it recovers activity within 100 ms when exposed to a high concentration of NADPH+. By contrast, with the enzyme in dimeric form, the reaction starts immediately upon addition of glucose‐6‐phosphate.

What is the role of the second “structural” NADP+‐binding site in human glucose 6‐phosphate dehydrogenase?

“…Previous studies demonstrated that G6PD stripped of structural NADP + seemed to be inactive and monomeric (Bonsignore et al 1971b; Wrigley et al 1972; Cancedda et al 1973). However, the fact that the enzyme can be totally stripped of NADP + in a gentle fashion by catalytic turnover would appear to imply that it must remain active even after the coenzyme has vacated the structural site: Initially, NADP + leaving one G6PD subunit might be reduced at the catalytic site of a second, still‐intact subunit, but this would not easily account for rapid total stripping of the entire population of enzyme molecules.…”

Three-Dimensional Modeling of Glucose-6-phosphate Dehydrogenase-Deficient Variants from German Ancestry

“…Both structures are without any mutation, but 2BHL has 25 N-terminal residues missing. Since the smallest human glucose-6-phosphate dehydrogenase functional unit is a dimer [24], [25], we started with a dimeric structure (Figure 1A). The structure was obtained by positioning two 2BH9 monomers (along with their NADP + units) as in the dimer of 1QKI.…”

Glucose‐6‐Phosphate Dehydrogenases

“…Yoshida and Hoagland were unable to exchange the tightly bound NADP' with free NADP' and in their experiments the G6PD existed largely as a tetramer in the presence of NADPH (397). Cancedda et al (59) also reported that NADPH could stabilize the enzyme. In part, these discrepancies might result from differences in conditions, particularly the initial state of aggregation of the enzyme.…”