Hydrogenase-like proteins

The hydrogenase-like proteins are known to catalyze the H/D exchange reaction, typical of hydrogenase (58). It remains to be determined to what extent the hydrogen binding site of these proteins re-... 

PFO puruvateiferredoxin oxidoreductase, HCP hybrid cluster protein, PNO pyruvate NADH oxidoreductase, Nbp35 P-loop NTPase, Nar/Narf hydrogenase-like protein, Rli ATP-binding cassette protein... 

Concerning the general topic of ferredoxin and redox proteins, it is important to note the existence of several of these in various methanogens. Ferredoxin has been reported in M barkeri [iQ2], M. thermophila [249,303], and Methanococcus thermolithotrophicus [304,305]. In one case, ferredoxin is required in electron transfer from carbon monoxide dehydrogenase to a membrane-bound hydrogenase in M. thermophila [306]. Another redox protein, referred to as a glutaredoxin-like protein , has been isolated from M. thermoautotrophicum [307], but has no known function. As mentioned above, MVH gene clusters contain a sequence for a polyferredoxin of unknown function. 

Fig. 5.7. In green sulfur bacteria and in some archaebacteria, a reverse citric acid cycle is used for the assimilation of C02. It must be assumed that this was the original function of the citric acid cycle that only secondarily took over the role as a dissimulatory and oxidative process for the degradation of organic matter. A major enzyme here is 2-oxoglutarate ferredoxin for C02 fixation. Note that it, like several other enzymes in the cycle, uses Fe/S proteins. One is the initial so-called complex I which has eight different Fe/S centres of different kinds but no haem (see also other early electron-transfer chains, e.g. in hydrogenases).

Two [Fe] hydrogenase structures have so far been determined from C. pasteurianum (Cp) (Peters et al. 1998) and D. desulfuricans (Dd) (Nicolet et al. 1999). They have in common a large domain, which contains the catalytic site and three [4Fe-4S] iron sulfur clusters. The catalytic site and the closest (proximal) cluster are deeply buried inside the protein between two domains (or lobes), with access to a third, ferredoxin-like, domain that contains the two remaining (medial and distal) clusters. By contrast with [NiFe] hydrogenases the proximal [4Fe-4S] cluster is directly bridged to the bin-uclear active site by a cysteic thiolate (Fig. 6.12). 

The splitting of H2 by hydrogenases is heterolytic (into H and H" ), rather than homolytic (into two H. radicals). The hydride is considered to deliver two electrons at a time to the enzyme. A [4Fe-4S] cluster in proteins can, however, accept only one electron. Other redox enzymes, e.g. flavoproteins, dealing with two-electron donors (like NADH) solve this problem by first accommodating both electrons onto the flavin, whereafter these electrons are transferred to an Fe-S cluster one at a time. 

Structural models, which are synthesized to imitate features of the proposed structure of the active site. These may be used to demonstrate the chemical conditions, which allow such structures to exist, to investigate their chemical properties and to give a better understanding of the spectroscopic characteristics of the native proteins. Examples of these include the mixed carbonyl/cyano complexes of iron, used to verify the infrared spectra to the hydrogenases (Fig 7.4) (Lai et al. 1998) and the nickel-thiolate complexes which have low redox potentials like the hydrogenases (Franolic et al. 1992). 

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