Hydrogenase composition

Hydrogenase isoenzymes are also common among the metabolically more versatile bacteria (see Chapter 2). For instance, H2 metabolism and isoenzyme composition in enteric bacteria, including Escherichia coli and Salmonella typhimurium, appear to be differentially regulated under the two modes of anaerobic life, fermentation and anaerobic respiration (Table 3.1). Furthermore, biosynthesis of the individual isoenzymes appears to be controlled at a global level by the quality of the carbon source. 

Among the diversity of Ni hydrogenases, there is a common pattern of protein composition, to which most conform, which consists of two protein subunits of relative molecular mass approximately 60,000 and 30,000 Da (14). There is some evidence (38) that the nickel is situated in the 60,000-Da subunit. More complex hydrogenases, such as the soluble hydrogenase of Nocardia opaca (Table I), contain other subunits which are concerned with the reduction of specific electron acceptors. 

In those hydrogenases in which the nickel is EPR detectable (Table I), the remarkable similarity in the lineshapes of the spectra is a strong indication that the nickel environment is highly conserved. Therefore, although there are substantial differences in the catalytic activities and specificity of hydrogenases from different organisms, it seems likely that there are, at most, only a few different types of nickel centers. It therefore seems reasonable to correlate spectroscopic information on nickel in hydrogenases from different species in order to obtain a composite picture. 

For hydrogen oxidation bi-component metal doped systems deposited on Raney nickel for AFC Mo and W carbides for AFC prepared by method of precipitation from a gas phase Radicals of following composition -OH, -OSO3H, -COOH, -OPO(OH)3 for PEMFC Some organic catalysts like biologically active [NiFe]-hydrogenase, pyropolymers, etc. ... 

As for the uptake hydrogenase, the molecular studies helped to clarify the picture of the subunit composition/molecular mass of the bidirectional hydrogenase. In agreement with the molecular data, previous works referred to large subunits of about 50-56 kDa in Anabaena variabilis, Anacystis nidulans. Microcystis aeruginosa, and Spirulina plantensis (Asada et al., 1987 Llama et al., 1979 Kentemich et al., 1989), and small subunits of about 17 kDa in Anabaena variabilis and Anacystis nidulans (Kentemich et al., 1989). In Anabaena variabilis the genes hoxH and hoxY encode predicted polypetides of 54.8 kDa and 22.5 kDa, respectively. 

In both these latter cases, reducing equivalents are produced from H2 via the hydrogenase enzyme, resulting in greater carbon conversion to ethanol. Thus, the theoretical yield of carbon in the producer gas towards ethanol production depends upon the composition of the producer gas. 

The presence of nickel in hydrogenases has only been recognized relatively recently. Purified preparations of the active enzymes were the subject of quite intensive studies for years before the Ni content was discovered by nutritional studies (see Reference 189 for a history). Some workers even tried (in vain) to purify out impurity EPR signals that were later found to be from the Ni. In contrast to the Fe hydrogenases discussed in the previous section, the Ni enzymes possess a variety of compositions, molecular weights, activation behavior, and redox potentials. As Table 7.5 shows, some of the Ni hydro-... 

The hydrogenases are classified according to their composition. Three distinct classes of metal-containing hydrogenases have been identified [3] Fe-only hydrogenases NiFe-hydrogenases and NiFeSe-hydrogenases. 

Figure 2 Composition of the electron transport chain catalyzing polysulHde sulfur respiration with H2 or formate in W. succinogenes. FdhA/B/C, formate dehydrogenase PsrA/B/C, polysulHde reductase HydA/B/C, hydrogenase Ni, nickel-iron center Fe/S, iron-sulfur centers Mo, molybdenum ion bound to MGD the dark squares designate heme b groups Sud, polysulfide sulfur transferase.

The compositions of the described electron transport chains show participations of similar NiPe hydrogenases and similar polysulfide sulfur reductases in the case of W. succinogenes and A. ambivalens, whereas the sulfur reductase of P. abyss may be different. In this microorganism the electron transfer between the hydrogenases and the polysulfide sulfur reductases mediated by 8-methyl-menaquinone, sulfolobusquinone or cytochromes b and c, respectively, seems to be adapted to the synthesis capabilites of the species. 

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