Hydrogenases relevance

Organism Type of hydrogenase Relevant characteristics Physiological function Effectors and/or conditions affecting hydrogenase gene regulation RefererKe ... 

We will use here the main results obtained for two complex and distinct situations the structural and spectroscopic information gathered for D. gigas [NiFe] hydrogenase and AOR, in order to discuss relevant aspects related to magnetic interaction between the redox centers, intramolecular electron transfer, and, finally, interaction with other redox partners in direct relation with intermolecular electron transfer and processing of substrates to products. 

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... 

In this respect the possible presence of electron spin density at the iron in the Ni-Fe cluster is of importance. This question has been answered by ENDOR experiments performed on Fe enriched [NiFe] hydrogenase (Huyett et al. 1997). In this study no significant Fe hfc was detected in the NiA, NiB and NiC states (A57p s 1 MHz). This shows that in all relevant states of the enzyme the iron is in the low-spin Fe state. 

We should also remember that not all of the states that we see when freezing the enzyme (Section 7.4) are necessarily part of the mechanism. The most stable enzyme molecule is a dead one, so we must be aware that some of the spectroscopic signals represent damaged molecules. In the [NiFe] hydrogenases, the NiA and NiB states probably are not involved in the catalytic cycle, because they react slowly, if at all, with H2. In the mechanism shown in Fig. 8.3, it is assumed that the relevant active states are NiSR, NiA and NiR. 

In conclusion, the presented dinuclear iron structure is the first example of a bio-mimetic iron compound, which can be regarded as a first generation model for the class of [Fe]-only hydrogenases. The complex incorporates both relevant carbon monoxide ligands, as well as three bridging thiolato ligands, which could be possibly present in the active site of these enzymes. 

Ciloslowski, J., Boche, G. (1997) Geometry-tunahle Lewis acidity of amidinium cations and its relevance to redox reactions of the Thauer metal-free hydrogenase - a theoretical study. Angeiv. Chem. Int. Ed. Engl. 36, 107-9. 

As ascribed, the EPR spectrum with g = 2.10 can be low-spin Fec(III). When the isolated enzyme is reductively titrated this signal disappears at a potential Emj -0.3 V [65]. This would seem to indicate that the putative Fec(III) form is not relevant, at least not to hydrogen-production activity. The cubane is a one-electron acceptor as it can shuttle between the 2+ and 1 + oxidation states. Therefore, if the active center were to take up a total of two electrons, then the oxidation state of the Fec would, as least formally, shuttle between II and I. Recently, a redox transition in Fe hydrogenase with an Em below the H2/H+ potential has been observed in direct electrochemistry [89]. This superreduced state has not been studied by spectroscopy. It might well correspond to the formal Fec(I) state. For NiFe hydrogenases Fec(I) has recently been proposed as a key intermediate in the catalytic cycle [90] (cf. Chapter 9). 

For the Ru system the thiol hydride could not be detected, while for the Rh system and also [IrH2(HS(CH2)3SH)(PCy3)2]+(which similarly catalyzes D2/H + exchange (79)), the H2 complex could not be seen but is a transient. A related system, Ni(NHP Pr3)(S3) clearly shows that heterolysis of D2 can also occur at nickel sites, which may be relevant to H2 activation in [FeNi] hydrogenases (78). 

A potentially promising route to learn more about the complex redox chemistry of hydrogenase is to look at the direct electron transfer between the enzyme and an electrode. A study of this phenomenon may also be relevant to a possible coupling of solar cells with hydrogenasecatalyzed H2 production. Results from this line of research have thus far been very limited. A response was obtained in differential-pulse polarography on the dropping mercury electrode modified with polylysine (cf. van Dijk et al., 1985). Attempts to use... 

Barton BE, Whaley CM, Rauchfuss TB, Gray DF. Nickel-iron dithiolato hydrides relevant to the [NiFe]-hydrogenase active site. J Am Chem Soc. 2009 131(20) 6942-3. 

In this context, it is worth noting that the nickel coordination geometries of [Ni(NHPPr3)(S3)] and the [Ni(Cys)4] site in [NiFe] hydrogenase are distorted in a similar and characteristic way. Such distortions influence the relative energies of the nickel acceptor and donor orbitals that have to interact with the a and a orbitals of H2 in order to enable primary coordination of H2 (147). Thus, in the end, there has to exist not only a vacant site but the metal center has to have orbitals suited for coordinating the relevant substrates. 

Hydrogen reduction Model studies of hydrogenase provide insights relevant to the pathogenic organism Helicobacter pylori and its ability to survive in the gastric mucosa. 

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