Nickel-iron proteins, hydrogenase

A second approach created a fusion protein from a PSI subunit (PsaE) and a nickel-iron [NiFe] hydrogenase [12], This new protein was then assembled into a PSI mutant lacking the PsaE subunit. The fused enzymatic system was attached to a gold surface in the same way as the PSn electrode described above using a His-tag on PSI, Ni(n) and NTA functionalities on the surface (Fig. 4a, right side). A soluble electron shuttle was used to transfer electrons from the electrode to PSI. From these two approaches the fusion protein is to date the most effective artificial enzymatic system for photo-driven hydrogen production and the activity is comparable to the electrocatalytic activity of the hydrogenase alone immobilized directly on an electrode. 

Nickel-iron hydrogenases [NiFe] (Figure 8.2) are present in several bacteria. Their structure is known [22, 23] to be a heterodimeric protein formed by four subunits, three of which are small [Fe] and one contains the bimetallic active center consisting of a dimeric cluster formed by a six coordinated Fe linked to a pentacoordinated Ni (III) through two cysteine-S and a third ligand whose nature changes with the oxidation state of the metals in the reduced state it is a hydride, H, whereas in the oxidized state it may be either an oxo, 0, or a sulfide,... 

Structural models, which are synthesized to imitate features of the proposed structure of the active site. These may be used to demonstrate the chemical conditions, which allow such structures to exist, to investigate their chemical properties and to give a better understanding of the spectroscopic characteristics of the native proteins. Examples of these include the mixed carbonyl/cyano complexes of iron, used to verify the infrared spectra to the hydrogenases (Fig 7.4) (Lai et al. 1998) and the nickel-thiolate complexes which have low redox potentials like the hydrogenases (Franolic et al. 1992). 

This enzyme [EC 1.18.99.1], also known as hydrogenly-ase, catalyzes the reaction of H2 with two oxidized ferre-doxin to produce two H+ and two reduced ferredoxin. This enzyme is an iron-sulfur protein and requires nickel ions. It can use molecular hydrogen to reduce a variety of substances. See also Hydrogen Dehydrogenase Cytochrome C3 Hydrogenase... 

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