A paradigm for aldehyde oxidation: histidinol dehydrogenase

Structural and kinetic insights into the stereospecific oxidation of R-2,3-dihydroxypropanesulfonate by DHPS-3-dehydrogenase from Cupriavidus pinatubonensis

2,3-Dihydroxypropanesulfonate (DHPS) and sulfolactate (SL) are environmentally important organosulfur compounds that play key roles as metabolic currencies in the sulfur cycle. Despite their prevalence, the pathways governing DHPS and SL production remain poorly understood. Here, we study DHPS-3-dehydrogenase from Cupriavidus pinatubonensis (CpHpsN), a bacterium capable of utilizing DHPS as a sole carbon source. Kinetic analysis of CpHpsN reveals a strict preference for R-DHPS, catalyzing its 4-electron oxidation to R-SL, with high specificity for NAD+ over NADP+. The 3D structure of CpHpsN in complex with Zn2+, NADH and R-SL, elucidated through X-ray crystallography, reveals a fold akin to bacterial and plant histidinol dehydrogenases with similar coordination geometry around the octahedral Zn2+ centre and involving the sulfonate group as a ligand. A key residue, His126, distinguishes DHPS dehydrogenases from histidinol dehydrogenases, by structural recognition of the sulfonate substrate of the former. Site-directed mutagenesis pinpoints Glu318, His319, and Asp352 as active-site residues important for the catalytic activity of CpHpsN. Taxonomic and pathway distribution analysis reveals the prevalence of HpsN homologues within different pathways of DHPS catabolism and across bacterial classes including Alpha-, Beta-, Gamma-, and Deltaproteobacteria and Desulfobacteria, emphasizing its importance in the biogeochemical sulfur cycle.

pH and deuterium kinetic isotope effects studies on the oxidation of choline to betaine-aldehyde catalyzed by choline oxidase

The FAD-dependent choline oxidase catalyzes the four-electron oxidation of choline to glycine-betaine, with betaine-aldehyde as intermediate. The enzyme is capable of accepting either choline or betaine-aldehyde as a substrate, allowing the investigation of the reaction mechanism for both the conversion of choline to betaine-aldehyde and of betaine-aldehyde to glycine-betaine. In the present study, pH and deuterium kinetic isotope effects with [1,2-2H(4)]-choline were used to study the mechanism of oxidation of choline to betaine-aldehyde. The V/K and V(max) pH-profiles increased to limiting values with increasing pH, suggesting the presence of a catalytic base essential for catalysis at the enzyme active site. From the V/K pH-profile with [1,2-2H(4)]-choline, a pK(a) of 8.0 was determined for the catalytic base. This pK(a) was shifted to 7.5 in the V/K pH-profile with choline, indicating a significant commitment to catalysis with this substrate. In agreement with this conclusion, the D(V/K) values decreased from a limiting value of 12.4 below pH 6.5 to a limiting value of 4.1 above pH 9.5. The large D(V/K) values at low pH are consistent with carbon-hydrogen bond cleavage of choline being nearly irreversible and fully rate-limiting at low pH. Based on comparison of amino acid sequences and previous structural and mechanistic studies on other members of the GMC oxidoreductase superfamily, the identity of the catalytic base of choline oxidase is proposed.