Roles of HynAB and Ech, the Only Two Hydrogenases Found in the Model Sulfate Reducer Desulfovibrio gigas

Abstract

Sulfate-reducing bacteria are characterized by a high number of hydrogenases, which have been proposed to contribute to the overall energy metabolism of the cell, but exactly in what role is not clear. Desulfovibrio spp. can produce or consume H 2 when growing on organic or inorganic substrates in the presence or absence of sulfate. Because of the presence of only two hydrogenases encoded in its genome, the periplasmic HynAB and cytoplasmic Ech hydrogenases, Desulfovibrio gigas is an excellent model organism for investigation of the specific function of each of these enzymes during growth. In this study, we analyzed the physiological response to the deletion of the genes that encode the two hydrogenases in D. gigas, through the generation of ⌬echBC and ⌬hynAB single mutant strains. These strains were analyzed for the ability to grow on different substrates, such as lactate, pyruvate, and hydrogen, under respiratory and fermentative conditions. Furthermore, the expression of both hydrogenase genes in the three strains studied was assessed through quantitative reverse transcription-PCR. The results demonstrate that neither hydrogenase is essential for growth on lactate-sulfate, indicating that hydrogen cycling is not indispensable. In addition, the periplasmic HynAB enzyme has a bifunctional activity and is required for growth on H 2 or by fermentation of pyruvate. Therefore, this enzyme seems to play a dominant role in D. gigas hydrogen metabolism. Hydrogenases are key enzymes in the hydrogen metabolism of Desulfovibrio spp. that catalyze the reversible oxidation of molecular hydrogen into protons and electrons (1). However, their role during sulfate respiration has not been clearly established. Odom and Peck proposed a hydrogen cycling model to explain energy conservation during growth on lactate and sulfate by Desulfovibrio spp., which belong to the deltaproteobacteria subgroup of the sulfate-reducing bacteria (SRB) (2). The model predicts that protons and electrons produced in the oxidation of lactate are used for the production of molecular hydrogen by a cytoplasmic hydrogenase. This hydrogen then diffuses across the membrane to the periplasm, where it is reoxidized by a periplasmic hydrogenase. Electrons are transferred back to the cytoplasm for sulfate reduction, thus creating a proton gradient across the membrane that leads to ATP formation. In this model, the presence of at least two hydrogenases on opposite sides of the membrane is a requirement for growth. In contrast, other studies suggested that the physiological role of these enzymes was to regulate the redox potential of the cell, controlling the flow of protons and electrons and generating a proton motive force (3). More recent models, proposed for Desulfovibrio vulgaris, suggested dual pathways for electron transfer from lactate to sulfate, one involving the cycling of H 2 and the other a route involving a membrane-associated electron transfer chain (4, 5). Several membrane complexes have been identified in SRB that could be involved in this proce...

Genome sequence of the model sulfate reducer Desulfovibrio gigas: a comparative analysis within the Desulfovibrio genus

“…Genes encoding the hydrogenases (Table S24), as well as energy conserving transmembrane complexes (Table S25) present a higher CAI value, around 0.701, than the average, suggesting that these genes have a higher expression potential which was experimentally shown in the case of both the hydrogenases Ech and Hyn, (Morais‐Silva et al. ).The genome is very compact as observed by its gene density of 1128 bp per gene and the average length of each gene is 993 bp. It contains 17 transposases (Table S3), whereas in other SRB genomes this number is in average 34 (Bennett ).…”

“…(Morais‐Silva et al. ). This could be a reflex of the unusual low number of these enzymes in D. gigas .…”

“…; Morais‐Silva et al. ). However, despite the accumulated information about this bacterium, a clear whole‐genome context of the genes and metabolic complexes is not yet available for D. gigas .…”

Genome-Guided Analysis and Whole Transcriptome Profiling of the Mesophilic Syntrophic Acetate Oxidising Bacterium Syntrophaceticus schinkii

“…barkeri [84], the sulphate reducer Desulfovibrio gigas [85], and the thermophilic SAOB T . phaeum [40] (S6 Table, S5 Fig), and which appears to be of importance for energy conservation in S .…”

[FeFe]- and [NiFe]-hydrogenase diversity, mechanism, and maturation

“…One is a membrane bound Ech-type group 4 enzyme while the other is a periplasmic group 1 enzyme (HynAB). By mutational analysis it was shown that the periplasmic hydrogenase is essential for growth on H 2 with sulfate and functions to donate electrons from H 2 to the quinone pool via a periplasmic cytochrome c [72]. Aquifex aolicus is one the most thermophilic bacteria known and it gains energy by oxidizing H 2 using low concentrations of oxygen or oxidized sulfur compounds as electron acceptors [73].…”