Hydrogenase Systems

There has been considerable effort in the last few years toward achieving an understanding of hydrogenase enzyme systems which have the ability to activate H2 for exchange with water, para-ortho conversion, and reduction reactions when coupled to an electron carrier E such as NAD+, cytochrome c3, or ferredoxins (7, p. 396 470-473). Reaction (81) can be catalyzed in either direction  

Knowledge of the active site allows for speculation on the mechanism of H2-D20 exchange which these Fe4 systems catalyze 473,483). Ruthe-nium(III) systems catalyze such an exchange via a ruthenium(III) hydride intermediate (7, p. 73 Section II,A), as exemplified in reactions (82) and (83), and iron hydrides must be involved in the hydrogenase systems. Ruthenium(III) also catalyzes the H2 reduction of ruthenium(IV) via reaction (82), followed by reaction (84) (3), and using these ruthenium systems as models, a very tentative scheme has been proposed 473) for 

Reactions (85) and (86) could occur within the enzyme and result in H2 uptake or evolution, catalyzed (or autocatalyzed, as in the H2 reduction of Ru,v) by ta3. Reaction (88) is the coupling reaction which with reaction (87) yields net reactions such as (81). The model is an attractive one and can be tested using the synthetic analogs the oxidation level of ta remains to be established (473). 

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There is clearly a growing interest in photocatalysis involving metal hydrides (see Section VIII) and hydrogenase systems (see Section IX), and the search for systems to utilize solar energy to dissociate water into its elements will undoubtedly intensify further efforts. 

The importance of H2 metabolism to some organisms is highlighted by their possession of more than one hydrogenase system. Four hydrogenase systems are known in Escherichia coli and Methanococcus voltae, three in Desufovibrio vulgaris and... 

Hydrogenase accessory genes can also be used to study the evolutionary relationships between various hydrogenase systems. This is particularly the case with the bypA, B, C, D, E and F genes most of which were originally identified as being required for the maturation of the several NiEe hydrogenases in E. coli (Bohm et al. 1990 Lutz et al. 1991). 

Brito, B., Monza, J., Imperial, J., Ruiz-Argueso, T. and Palacios, J. M. (2000) Nickel availability and hupSL activation by heterologous regulators limit symbiotic expression of the Rhizobium leguminosarum bv. viciae hydrogenase system in Hup( —) rhizobia. Appl. Environ. Microbiol., 66, 937-42. 

Rao, K.K., Gogotov, I.N. and Hall, D.O. 1978. Hydrogen evolution by chloroplast-hydrogenase systems improvements and additional observations. Biochimie, 60,291-296. 

Benemann, J.R., Berenson, J.A., Kaplan, N.O. and Kamen, M.D. 1973. Hydrogen evolution by a chloroplalst-ferredoxin-hydrogenase system. Proc. Natl. Acad. Sci. USA 70, 2317-2320... 

Note that pH 11 was used for the purely chemical titration in Fig. 8 (A) in order to reach a sufficiently negative potential while in the electrochemical titration in fig. 8 (B) pH 9.5 was sufficient to extend the potential to -700 mV. Previously, Lozier and Butler had performed a redox titration ofthe PS-I reaction and obtained results very similar to those shown in Fig. 8 (A). These workers, using the Clostridial H/HV hydrogenase system as the reductant and l,l -trimethylene-2-2 -dipyridylium dibromide as the mediator, achieved specific potentials by gradually varying the pH between 8 and 10 to effect reduction ofthe photosystem-I electron acceptors. 

Eric M. Shepard was born in 1977. He received a BS degree in chemistry from Rocky Mountain College. He studied copper- and TPQjContaining amine oxidases under Dr. David M. Dooley at Montana State University, where he was supported by an NSF IGERT fellowship on complex biological systems. He received his Ph.D. degree in biochemistry from MSU and is currently a postdoctoral research associate under Dr. Joan B. Broderick. His research interests are metal cluster assembly in the [FeFe] hydrogenase system and [Fe-S] cluster spectroscopy and reactivity. 

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