In hydrogenases

The Ni ion stays in the common Ni(II) state throughout the catalytic cycle. Redox processes that are purely Ni-based would imply three different Ni redox states Ni-SI [Ni(II)] Ni-C [Ni(I)l Ni-R [Ni(0)]. Such changes, comprising potentials confined to those observed in hydrogenase (-100 to -400 mV), would be totally unprecedented (66, 94, 95). Also, successive one-electron changes at the Ni... 

A template synthesis employing Ni(OAc)2, 2,5-dihydroxy-2,5-dimethyl-1,4-dithiane, and 3,3 -iminobis(propylamine) gave the water-soluble five-coordinate complex [Ni(495)], the crystal structure of which shows trigonal bipyramidal coordination of Ni11 with the central amine and terminal thiolates in plane and the two imino nitrogens in axial positions. Solvatochromism of the complex is interpreted in terms of S" H bonding, which may be of relevance to the catalytic cycle in hydrogenases.1341... 

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).

Increase in hydrogenase and (3-gaIactosidase activities during growth with H2 of B10(pAC142) cells containing the hupSv.lacZ fusion... 

Nature uses the transition-metal elements iron and nickel, rather than noble metals, and in their ionic form rather than the metals. As will be seen in this book, for the simplest chemical reaction, the metal-ion centres in hydrogenases are some of the most complex catalysts known. Their structures, which have just been elucidated, have proved to be an elegant and totally unexpected solution to the problem. The construction of these catalysts is in itself a molecular assembly line of extraordinary sophistication. 

Figure 3.7 Increase in hydrogenase and P-galactosidase activities during growth with H2 of BIO (pACI42) cells containing the hupS /acZ fusion. ONP, o-nitrophenol MB, methylene blue prot, protein solid symbols, H2 added open symbols, no H2 added.

Let us, for example, determine the of a [4Fe-4S] cluster in hydrogenase. The cluster has an EPR signal in its reduced form so we must rewrite the Nernst equation as... 

An alternative approach to find out which ligands of the active site in hydrogenases are of importance for an optimal activity, is to study enzyme with point mutations. Several groups are actively working in this field and a detailed example is provided in this section. 

The number of atoms of Xe or H2 in the putative gas channels represents a higher solubility for H2 in hydrogenase, than in water. The channels appear to concentrate the gas. It is interesting that one of the ways that has been discovered for storing H2... 

The mechanism of action, and organization of the catalytic sites, in hydrogenases are different from a solid catalyst such as platinum. For a start, the reaction of H2 with hydrogenase involves heterolytic cleavage into a hydron and a hydride. This contrasts with the reaction of H2 at the surface of a metal such as platinum, which is usually considered to involve the homolytic cleavage into two hydrogen atoms. Moreover in the enzyme, the catalyst is a cluster of metal ions (with oxidation states +2 or -h3) rather than the metal (oxidation state 0). 

Albracht, S. P. J., Kroger, A., Van der Zwaan, J. W., Unden, G., Bocher, R., Mell, H. and Fontijn, R. D. (1986) Direct evidence for sulfur as a ligand to nickel in hydrogenase An EPR study of the enzyme from Wolinella succinogenes enriched in O2. Biochim. Biophys. Acta, 874, 116-27. 

Bagley, K. A., Van Garderen, C. J., Chen, M., Duin, E. C., Albracht, S. P. and Woodruff, W. H. (1994) Infrared studies on the interaction of carbon monoxide with divalent nickel in hydrogenase from Chromatium vinosum. Biochemistry, 33, 9229-36. 

Black, L. K., Fu, C. and Maier, R. J. (1994) Sequences and characterization of hupU and hupV genes of Bradyrhizobium japonicum encoding a possible nickel-sensing complex involved in hydrogenase expression./. BacterioL, 176, 7102-6. 

Cammack, R., Patil, D., Aguirre, R. and Hatchikian, E. C. (1982) Redox properties of the ESR-detectable nickel in hydrogenase from Desulfovibrio gigas. FEBS Lett., 142, 289-92. 

Erbes, D. L., Burris, R. H. and Orme-Johnson, W. H. (1975) On the iron-sulfur cluster in hydrogenase from Clostridium pasteurianum W5. Proc. Natl. Acad. Sci. USA, 72, 4795-9. 

Higuchi, Y, Yagi, T. and Yasuoka, N. (1997) Unusual ligand structure in Ni-Ee active center and an additional Mg site in hydrogenase revealed by high resolution X-ray structure analysis. Structure (London), 5, 1671-80. 

Medina, M., Hatchikian, E. C. and Cammack, R. (1996) Studies of light-induced nickel EPR signals in hydrogenase Comparison of enzymes with and without selenium. Biochim. Biophys. Acta, 1275, 227-36. 

Rossmann, R., Maier, T., Lottspeich, F. and Bock, A. (1995) Characterization of a protease from Escherichia coli involved in hydrogenase maturation, Eur. J. Biochem., 227, 545-50. 

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