NiFe hydride

Figure 6.10 The catalytic site of [NiFe] and [NiFeSe] hydrogenases in oxidised inactive (top) and reduced active (bottom) states. Note the three non-protein diatomic ligands to the iron.The site bridging the Ni and Fe is occupied by an oxygen or sulfur species in the most oxidised states and probably by a hydride or molecular hydrogen in the most reduced states.

The minimal functional module in [NiFe] hydrogenases always contains the NiFe(CN)2(CO) site plus the proximal [4Fe-4S] cluster. The active site in [Fe] hydrogenases consists of the Fe-Fe site linked to a [4Fe-4S] cluster. Oxidation of the hydride is either an action of the dinuclear site alone, or a concerted action of this site plus the proximal cluster. 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

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). 

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,... 

Fig. 6. Fully optimized transition state (structure 6-7) for hydride transfer to Cys492 in [NiFe] hydrogenase. The oxidation states are Ni(I-III) and Fe(II). Distances in A and spins larger than 0.1 are marked.

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... 

FeFe] Hydrogenase Less information on the electronic structure is available from EPR on [FeFe] hydrogenase as compared to [NiFe] hydrogenase since only the Hox state (and Hox-CO) is paramagnetic. In particular, the hydride-carrying species (Hred) cannot be characterized by EPR techniques. However, FTIR and Moss-bauer spectroscopy have revealed important information on the intermediate states of the H cluster [128, 129]. 

The work on biomimetic models for [NiFe] and [FeFe] hydrogenase has been described in several review articles [15b 158]. In this work, many of the structural features important for proper function found in the native systems have been successfully incorporated for example, the bimetallic Ni-Fe or Fe-Fe core with rather short metal-metal distances and an open coordination site at one metal, the sulfur-rich environment (terminal and bridging thiolate ligands), CO/CN ligation of the iron(s), and the incorporation of a base for acceptance of the proton and, more recently, of hydride bridges. Still-existing problems of many model systems are the O2 sensitivity, the high overpotentials, and lack of activity (low turnover rates). 

Pandelia ME, Infossi P, Stein M, Giudici-Orticoni MT, Lubitz W. Spectroscopic characterization of the key catalytic intermediate Ni-C in the 02-tolerant [NiFe] hydrogenase I from Aquifex aeolicus evidence of a weakly bound hydride. Chem Comm. 2012 48(6) 823-5. 

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. 

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... 

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