Kinetics aspects of Gamma-hydroxybutyrate dehydrogenase

Two groups of metabolically related enzymes, the Group III family of Fe²⁺-dependent alcohol dehydrogenases (ADHs) and the separate subfamily of nucleoside diphosphates linked to x (nudix) hydrolases that activate Group III ADHs are under-characterized. Here we report the steady-state initial-velocity forward direction (alcohol → aldehyde) reaction of a Group III ADH, namely gamma-hydroxybutyrate dehydrogenase (GHBDH, UniProt: Q59104), cloned from Cupriavidus necator as a fusion protein. We also report the effects of nudix hydrolases on the GHBDH reaction. At optimal pH 9.0, the GHBDH reaction is activated ~2-fold by two different saturating purified nudix hydrolases, namely Bacillus methanolicus activator (ACT, UniProt: I3EA59) and Escherichia coli NudF (UniProt Q93K97) proteins. At physiological pH values of ~7.0, ACT activates by >3.5-fold. Initial-rate characterization at pH 9.0 of the forward direction un-activated and ACT-activated reactions show for both cases competitive inhibition by the product succinic semialdehyde versus GHB, and noncompetitive inhibitions by the three other substrate-product combinations. This pattern is consistent with NAD⁺ binding first in Mono-Iso Theorell-Chance kinetics. Mutants of some possibly important residues in GHBDH also were characterized. H265, conserved among all Group III ADHs and previously proposed to be a critical general base, is only ~4-fold helpful for GHBDH activity relevant to H265A. The four previously proposed conserved Fe²⁺ chelators (D193, H197, H261 and H280) each are essential for GHBDH activity. A 2-step explanation for cross-species stimulation by sub-stoichiometric ACT in the forward direction and confirmed lack of ACT stimulation in the reverse direction reaction is proposed.