Aerobic and anaerobic alcohol dehydrogenases inEscherichia coli

Fermentative Pyruvate and Acetyl-Coenzyme A Metabolism

Pyruvate and acetyl-CoA form the backbone of central metabolism. The nonoxidative cleavage of pyruvate to acetyl-CoA and formate by the glycyl radical enzyme pyruvate formate lyase is one of the signature reactions of mixed-acid fermentation in enterobacteria. Under these conditions, formic acid accounts for up to one-third of the carbon derived from glucose. The further metabolism of acetyl-CoA to acetate via acetyl-phosphate catalyzed by phosphotransacetylase and acetate kinase is an exemplar of substrate-level phosphorylation. Acetyl-CoA can also be used as an acceptor of the reducing equivalents generated during glycolysis, whereby ethanol is formed by the polymeric acetaldehyde/alcohol dehydrogenase (AdhE) enzyme. The metabolism of acetyl-CoA via either the acetate or the ethanol branches is governed by the cellular demand for ATP and the necessity to reoxidize NADH. Consequently, in the absence of an electron acceptor mutants lacking either branch of acetyl-CoA metabolism fail to cleave pyruvate, despite the presence of PFL, and instead reduce it to D-lactate by the D-lactate dehydrogenase. The conversion of PFL to the active, radical-bearing species is controlled by a radical-SAM enzyme, PFL-activase. All of these reactions are regulated in response to the prevalent cellular NADH:NAD+ ratio. In contrast to Escherichia coli and Salmonella species, some genera of enterobacteria, e.g., Klebsiella and Enterobacter, produce the more neutral product 2,3-butanediol and considerable amounts of CO2 as fermentation products. In these bacteria, two molecules of pyruvate are converted to α-acetolactate (AL) by α-acetolactate synthase (ALS). AL is then decarboxylated and subsequently reduced to the product 2,3-butandiol.

Aerobic activity of Escherichia coli alcohol dehydrogenase is determined by a single amino acid

Expression of the alcohol dehydrogenase gene, adhE, in Escherichia coli is anaerobically regulated at both the transcriptional and the translational levels. To study the AdhE protein, the adhE(+) structural gene was cloned into expression vectors under the control of the lacZ and trp(c) promoters. Wild-type AdhE protein produced under aerobic conditions from these constructs was inactive. Constitutive mutants (adhC) that produced high levels of AdhE under both aerobic and anaerobic conditions were previously isolated. When only the adhE structural gene from one of the adhC mutants was cloned into expression vectors, highly functional AdhE protein was isolated under both aerobic and anaerobic conditions. Sequence analysis revealed that the adhE gene from the adhC mutant contained two mutations resulting in two amino acid substitutions, Ala267Thr and Glu568Lys. Thus, adhC strains contain a promoter mutation and two mutations in the structural gene. The mutant structural gene from adhC strains was designated adhE*. Fragment exchange experiments revealed that the substitution responsible for aerobic expression in the adhE* clones is Glu568Lys. Genetic selection and site-directed mutagenesis experiments showed that virtually any amino acid substitution for Glu568 produced AdhE that was active under both aerobic and anaerobic conditions. These findings suggest that adhE expression is also regulated posttranslationally and that strict regulation of alcohol dehydrogenase activity in E. coli is physiologically significant.

Regulation of the adhE gene, which encodes ethanol dehydrogenase in Escherichia coli

The respiratory control of adhE, which encodes ethanol (alcohol) dehydrogenase in Escherichia coli, was examined at the transcriptional level by using various phi (adhE'-lacZ) adhE+ merodiploid strains. Expression of the adhE'-lacZ operon fusion was increased about eightfold by anaerobic growth. Under anaerobic growth conditions, provision of nitrate lowered the level of expression. Nitrate repression was more severe than aerobic repression. From analyses of various mutants with mutations related to nitrate reduction, nitrate repression appears to result from two effects. (i) When nitrate was present, NarL, the positive regulator of the nar operon, exerted a direct repression on adhE expression, which was demonstrable even aerobically. (ii) The chemical reduction of nitrate exerted an indirect effect by altering the cellular redox potential.

Cloning and characterization of the gene for a methanol-utilising alcohol dehydrogenase from Bacillus stearothermophilus

The cloning and characterization of the alcohol dehydrogenase (ADH) gene (adh) from Bacillus stearothermophilus strain DSM2334, an obligate aerobe, are described. The clone directed the synthesis of ADH as judged on Western blots, activity gels and tetrazolium plates. It specified an enzyme that oxidised methanol as well as ethanol. The enzyme was found to be encoded by a single gene in B. stearothermophilus which did not cross-hybridize to adh clones from Escherichia coli, yeast or maize. The cloned gene was expressed in E. coli but activity was not detected in Bacillus subtilis, despite stable maintenance of the recombinant plasmid in this host. The gene is catabolite-repressed in DSM2334.