NiFe proton transfers

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... 

The initial proton release associated with H2 cleavage is promoted by a base. For hydrogen evolution, net proton uptake from the medium is necessary. Conversely, for Hj oxidation protons are transferred from the active site to solution. The transfer of protons within a protein is considered to involve small (<1 ) movements of the amino acids that participate in the pathway (Williams, 1995). The proton transfer would involve a rotation of each individual donor and acceptor. Analyses of the crystal structures have suggested proton transfer pathways for the NiFe and Fe-only hydrogenases. 

The active site of the NiFe enzyme is 30 from the surface. Volbeda et al. proposed a proton transfer chain beginning at the NiFe cluster to His72 to His 536 through two water molecules finally to Glu46 at the surface. All of these residues are highly conserved among the NiFe hydrogenases. Two other histidine residues were also considered to have a possible role in proton transfer. 

Transfer of a proton from f/2-H2 to the ju-thiolates in H-ases is also possible, and calculations support such heterolysis (although it is endothermic by 15 kcal/mol).41 Transfer of a proton to CN is nearly isoenergetic but a high barrier is computed (38 kcal/mol, compared to 17 kcal/mol for transfer to sulfide). The next steps involve movement of protons away from the active site and synchronous or asynchronous electron transfer to the cubane cluster and away from the site via other Fe-S clusters. The electrons in the H-H bond could essentially flow through the Fe-Fe bond and, depending on whether one- or two-electron transfer process takes place, one-electron Fe---Fe bonds (2.9-3.1 A)42 may be present in the intermediates (one-electron transfer steps are shown in Scheme 2). The flexibility of the M-M separation (2.6-3.2A, corresponding to 0, 1, or 2e M-M bonds) could facilitate electron/proton transfer here and in the [NiFe] H-ases. 

Catalytic Cycle of the [NiFe] Hydrogenases. Figure 3A shows a simplified diagram of the sites of electron and proton transfer in [NiFe]-hydrogenase. Figure 4 shows the reaction with H2. The consumption of H2 by the enzyme is the reverse of this process. The steps are indicated by circled numbers. 

Proton and Electron Transfers in [NiFe] Hydrogenase Per E. M. Siegbahn... 

The actual catalytic cycle of [NiFe] hydrogenase encompasses only three states Ni-SIa, Ni-C and Ni-R, which are interconverted by one-electron/one-proton equilibria (Figure 3.4.7A) [123, 124], In the catalytic process, the approaching H2 is attached to the Ni, and the bond is polarized followed by base-assisted heterolytic cleavage of the H2 molecule leading to a bridging hydride species. One of the candidates for acting as a base is a terminal cysteine at the Ni. Alternatively, a water molecule bound to the iron has been proposed [120]. Concomitant electron transfer to the proximal FeS cluster then leads to the Ni-C state, which has been shown to... 

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