NiFe hydrogenase catalytic mechanism

For [NiFe] hydrogenase, many mechanisms of the catalytic reaction have been suggested. 36,37 One of these schemes is shown in Figure 9-3.37,38 The oxidized system (Ni-A and Ni-B) is reduced under an atmosphere of H2 and switches to a reduced system. This scheme has four paramagnetic states Ni-A, Ni-B, Ni-C and Ni-L and three EPR-silent states Ni-SU, Ni-SI and Ni-R. Volbeda et al. have found that Ni-SI has the two different states, denoted by Ni-SI and Ni-SIn.10,11 The Ni-A... 

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. 

Volbeda A, Fontecilla-Camps JC (2003) The active site and catalytic mechanism of NiFe hydrogenases. Dalton Trans. 4030-4038... 

The mechanism of the NiFe hydrogenase has been treated by caleula-tional methods, with some interesting conclusions (Pavlov et al., 1998). Scheme 1 of this reference proposes a catalytic cycle based on these results. It was proposed that Fe binds H2 and that a low spin Fe is essential for het-erolytic cleavage of the H6H bond. The next step is proposed to be hydride transfer to Fe and proton transfer to a ligated cysteine thiolate, whieh leads to decoordination of the cysteine and concurrent bridging of the N of CN... 

Montet, Y., Garcin, E., Volbeda, A., Hatchikian, E. C., Frey, M., and Fontecilla-Camps, J. C., 1998, Structural basis for the catalytic mechanism of NiFe hydrogenase. Pure and Applied... 

By comparing the spectroscopic features of the enzyme various redox states and using similar models, Amara et derived a catalytic mechanism of [NiFe] hydrogenase with the active forms of the site. Their mechanism is presented in Scheme 9. 

Figure 3 shows a proposed catalytic mechanism for [NiFe]-hydrogenases. For hydrogen oxidation, the catalytic cycle starts at the Ni-SU state, a Ni(II) state with no bridging ligand. Hydrogen binds the active site and is heteroly tic ally cleaved to produce the Ni-R state, a Ni(II) species with a bridging hydride and likely a... 

The biologically uncommon Ni center associated with FeS clusters is a powerful and unique catalytic unity. In this chapter we have reviewed the structural and mechanistic aspects of three NiFeS centers the active site of hydrogenase and Clusters A and C of CODH/ACS. In the former, the association of a Ni center with the most unusual FeCOCN2 unit is a fascinating one. Model chemists, spectroscopists, and crystallographers have joined efforts to try and elucidate the reaction mechanism. Although a consensus is being slowly reached, the exact roles of the different active site components have not yet been fully established. Ni appears to be the catalytic center proper, whereas the unusual Fe center may be specially suited to bind a by-... 

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