Hydrogenase Subject

Aerobic bacteria which often use H2 as an alternative energy source express hydrogenase genes, with a few exceptions, when the substrate is provided (Table 3.1). How do these organisms recognize the presence of H2, the smallest molecule on Earth The underlying molecular mechanisms are subject of current research and will be discussed in Sections 3.2 and 3.3. 

The cytoplasmic NAD-reducing hydrogenase (SH) of the bacterium R. eutropha is a heterotetrameric enzyme, which contains several cofactors (Friedrich et al. 1996 Thiemermann et al. 1996). The Ni-containing subunit is called HoxH. This subunit plus the small subunit HoxY form the strictly conserved hydrogenase module with the Ni-Fe centre and a proximal [4Fe-4S] cluster. HoxF and HoxU represents the Fe-S/flavoprotein moiety which is closely related to a similar moiety in NADHrubiquinone oxidoreductase. The SH has been subject to molecular biological techniques in order to study its modular structure, mechanism and biosynthesis. 

Cyanobacteria also contain a reversible/bidirectional hydrogenase, which is soluble and has the capacity to both take up and produce H2 (Bothe et al. 1991 Flores and Herrero 1994 Appel and Schulz 1998 Hansel and Lindblad 1998). The latter enzyme was called bidirectional hydrogenase until the respective structural genes were sequenced and characterized by Schmitz et al. (1995). It has been the subject of some interest as another possible means to produce H2 photosynthetically. 

He presented the design of the hydrogenases and pointed out that one of the major problems is that these systems are subject to dynamic equilibria. All of the substrates, electrons included, are transported in and out of the active site in a very specific way based on numerous studies. 

The elucidation of the role of Ni in hydrogenases and CODHs is complicated by the presence of extensive redox chemistry. The issue as to whether Ni is redox active in these enzymes has been subjected to considerable controversy. Prior to the discovery of a Fe ion in the active site of D. gigas hydrogenase, some authors assigned all the redox activity to Ni. This view has been challenged, however, by Ni model chemistry and XAS experiments which indicated both the... 

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 elucidation of the pathway leading to 2,4-DAHAT requires further research. As described by Angermaier and Simon (3), the nitro radical anion was the first product of the reduction of / -nitrobenzoate. This nitro radical anion is subject to various dismutation and other chemical reactions (Fig. 4), until the final reduction product /7-hydroxyl aminobenzoate is formed. Since DANT is reduced by the ferredoxin/hydrogenase system (39), the possibility cannot be excluded that a radical anion is involved and that the formation of 2,4-DAHAT from this radical anion is due to dismutation and not to biologically mediated reactions. 

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