NiFe electron transfers

The spatial arrangement of the Fe-S clusters in D. gigas NiFe-hydrogenase (see Fig. 1) suggests an active role for the [Fe3S4] ° cluster in mediating electron transfer from the NiFe active site to the... 

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. 

Leger C, Jones AK, Albracht SPJ, Armstrong FAA. 2002. Effect of a dispersion of interfacial electron transfer rates on steady state catalytic electron transport in [NiFe]-hydrogenase and other enzymes. J Phys Chem B 106 13058-13063. 

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

The enzyme adsorbed on the electrode showed large IT2 oxidation currents, with an activity greater than the catalytic activity with electron acceptors and donors (Pershad et al. 1999). This demonstrates that the electron transfer to [NiFe] active site, and reaction with IT2 are extremely efficient, and the factor that limits the enzyme-catalysed rate is the diffusion of the electron donors and acceptors to the enzyme. 

Figure 8.3 Outline reaction cycle of NiFe hydrogenase.The minimal hydrogenase is depicted, consisting of the [NiFe] centre in the large subunit, and the proximal [4Fe-4S] cluster (C) in the small subunit.The reaction is written in the direction of the oxidation of H2. Electrons are transferred out through the other iron-sulfur clusters to an acceptor protein (not shown).The equivalent states of the NiFe centre B, SR, R and C are indicated. Reduced centres are shaded. Electron transfers are accompanied by transfers of hydrons (not shown).

A similar reaction can be written for the [Fe] hydrogenases with a Fe-[4Fe-4S] complex replacing the nickel. Note that the nickel atom in the NiFe cluster, and the Fe-[4Fe-4S] sites are nearest to the electron carrier [4Fe-4S] clusters, indicating that electron transfer occurs through these atoms. The other atom in each of the centres is an iron atom with -CN and -CO ligands, and it seems likely that this is a binding site for hydride (Fig. 8.1). 

Pereira et al. (1998) provided biochemical evidence that electron transfer from either iron-only or NiFe-hydrogenases to HmcA is possible, although at a slow rate. The increase in electron-transfer rate by addition of cytochrome C3 indicates that a more probable electron-transport path is from hydrogen through hydrogenase and cytochrome C3 to HmcA. 

Electron-transferring subunit, nickel-containing hydrogenases, 38 409-410 Electron transport blue copper proteins, 36 378 NiFe hydrogenase, 47 16-17 Electron volt, 16 73... 

Fig. 5. Approximate transition state (structure 4) for the first electron transfer in [NiFe] hydrogenase. The oxidation states are Ni(II III) and Fe(II). Distances in A and spins larger than 0.1 are marked.

In [NiFe] hydrogenase, catalyzation does not necessarily occur in the neutral state of the active site. Therefore we have to consider electron transfer during catalyzation. Hence, we calculated the energy of the anion and dianion complexes in the catalytic reaction, where the structures of each state are assumed to be optimized structures and transition states of the neutral state. This is the starting point in the analysis of the string model.105-107... 

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