Many Bacteria and Archaea employ the heterodisulfide reductase (Hdr)-like sulfur oxidation pathway. The relevant genes are inevitably associated with genes encoding lipoate-binding proteins (LbpA). Here, deletion of the gene identified LbpA as an essential component of the Hdr-like sulfur-oxidizing system in the Alphaproteobacterium Hyphomicrobium denitrificans. Thus, a biological function was established for the universally conserved cofactor lipoate that is markedly different from its canonical roles in central metabolism. LbpAs likely function as sulfur-binding entities presenting substrate to different catalytic sites of the Hdr-like complex, similar to the substrate-channeling function of lipoate in carbon-metabolizing multienzyme complexes, for example pyruvate dehydrogenase. LbpAs serve a specific function in sulfur oxidation, cannot functionally replace the related GcvH protein in Bacillus subtilis and are not modified by the canonical E. coli and B. subtilis lipoyl attachment machineries. Instead, LplA-like lipoate-protein ligases encoded in or in immediate vicinity of hdr-lpbA gene clusters act specifically on these proteins.

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Most of our knowledge on microbial physiology and biochemistry is based on studies performed under laboratory conditions. For growing hydrogen-oxidizing anaerobic, autotrophic prokaryotes, an H2:CO2 (80:20, v/v) gas mixture is typically used. However, hydrogen concentrations in natural environments are usually low, but may vary in a wide range. Here we show that the thermophilic anaerobic bacterium Ammonifex degensii balances its autotrophic carbon fixation between two pathways depending on the H2 partial pressure. At 80% H2, favoring ferredoxin reduction, it uses the ferredoxin-dependent Wood-Ljungdahl pathway. In contrast, during growth at 10% H2, it switches to the more ATP-demanding, ferredoxin-independent Calvin-Benson cycle. The study reveals that the H2 redox potential is an important factor influencing the usage of different autotrophic pathways. This type of metabolic adjustment may be widespread in the microbial world.