Expression of Acetaldehyde Dehydrogenase Gene Increases Hydrogen Production and Acetate Consumption by <i>Rhodobacter sphaeroides</i>

Abstract

Rhodobacter sphaeroides RV (RV) produces high yields of hydrogen from organic acids in the presence of light. The hydrogen production from acetate is lower than that from lactate, probably because of its low ability to metabolize acetate. In this study, gene of acetaldehyde dehydrogenase (ACDH, EC 1.2.1.10) that catalyzes the reversible conversion of acetaldehyde and CoA to acetyl-CoA with the concurrent reduction of NAD to NADH, is overexpressed in the RV strain. The produced acetyl-CoA can be oxidized to carbon dioxide in the tricarboxylic acid (TCA) cycle, wherein electrons are generated and used for hydrogen production. The byproduct NADH can be used as reducing agent for acetate to produce acetaldehyde by acetate dehydrogenase. The recombinant RV strain (RVAC) expressing the ACDH gene showed ACDH activity with a specific activity of 3.2 mU/ mg, and the RV and the recombinant RV strain that harbored the intact (empty) plasmid pLP-1.2 (RVI) showed no detectable ACDH activity. The hydrogen yields of the RVAC strain from 21-mM acetate were 1.5-fold higher than that of the wild type RV strain and also that of the RVI strain. In contrast, hydrogen yield from 21-mM lactate was 30% lower than that in the control strains.