Hydrogenase catalytic cycle

FIGURE 7.5 Schematic view of the [Ni-Fe] hydrogenase catalytic cycle, as suggested in Ref. [2]. The EPR detectable states are shown in italics the EPR silent states are shown with bold fonts. The exact mechanism and the transient species involved are not yet known. More details in the text. 

Fig. 12. Hydrogenase catalytic cycle. The hydride is postulated to bridge the two metal centers. Proton balance is based on the pH dependence of the redox potentials [87]. A + B = 2n...

The Ni ion stays in the common Ni(II) state throughout the catalytic cycle. Redox processes that are purely Ni-based would imply three different Ni redox states Ni-SI [Ni(II)] Ni-C [Ni(I)l Ni-R [Ni(0)]. Such changes, comprising potentials confined to those observed in hydrogenase (-100 to -400 mV), would be totally unprecedented (66, 94, 95). Also, successive one-electron changes at the Ni... 

Moreover, an electron transfer chain could be reconstituted in vitro that is able to oxidize aldehydes to carboxylic acids with concomitant reduction of protons and net production of dihydrogen (213, 243). The first enzyme in this chain is an aldehyde oxidoreductase (AOR), a homodimer (100 kDa) containing one Mo cofactor (MOD) and two [2Fe—2S] centers per subunit (199). The enzyme catalytic cycle can be regenerated by transferring electrons to flavodoxin, an FMN-con-taining protein of 16 kDa (and afterwards to a multiheme cytochrome and then to hydrogenase) ... 

A template synthesis employing Ni(OAc)2, 2,5-dihydroxy-2,5-dimethyl-1,4-dithiane, and 3,3 -iminobis(propylamine) gave the water-soluble five-coordinate complex [Ni(495)], the crystal structure of which shows trigonal bipyramidal coordination of Ni11 with the central amine and terminal thiolates in plane and the two imino nitrogens in axial positions. Solvatochromism of the complex is interpreted in terms of S" H bonding, which may be of relevance to the catalytic cycle in hydrogenases.1341... 

In the years since the discovery of nickel and iron in the catalytic centres, numerous different descriptions of the catalytic cycle of hydrogenase have been proposed. These were based on different oxidation states of the metal centres, and different sequences of transfer of electrons and hydrous. Although the reaction cycle has not been definitively resolved, the spectroscopic evidence places constraints on possible models that should be considered. 

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. 

As ascribed, the EPR spectrum with g = 2.10 can be low-spin Fec(III). When the isolated enzyme is reductively titrated this signal disappears at a potential Emj -0.3 V [65]. This would seem to indicate that the putative Fec(III) form is not relevant, at least not to hydrogen-production activity. The cubane is a one-electron acceptor as it can shuttle between the 2+ and 1 + oxidation states. Therefore, if the active center were to take up a total of two electrons, then the oxidation state of the Fec would, as least formally, shuttle between II and I. Recently, a redox transition in Fe hydrogenase with an Em below the H2/H+ potential has been observed in direct electrochemistry [89]. This superreduced state has not been studied by spectroscopy. It might well correspond to the formal Fec(I) state. For NiFe hydrogenases Fec(I) has recently been proposed as a key intermediate in the catalytic cycle [90] (cf. Chapter 9). 

Enzymological and redox potentiometric studies by EPR have indicated that the catalytic sites of the oxygen-stable [Ni-Fe] hydrogenases exhibit at least six enzy-mologically distinct states [75-78], These are schematically represented in Figure 3, which depicts a speculative model for the hydrogenase redox cycle, as discussed later. 

Fig. 8. Suggested catalytic cycle for [NiFe] hydrogenase with an uncharged His77.

Fig. 10. Energetics for the suggested catalytic cycle for [NiFe] hydrogenase. The numbers for the structures are those from Fig. 9. The full line corresponds to a charged His77, while the dashed line is for an uncharged His77.

Fan H-J, Hall MB (2001) Recent theoretical predictions of the active site for the observed forms in the catalytic cycle of Ni-Fe hydrogenase. J. Biol. Inorg. Chem. 6 467 173... 

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... 

In another model (Figure 3.4.7B) proposed by Fontecilla-Camps et al. [114], the Ni-R state is formed directly from Ni-SIa with H2. In the catalytic cycle, the hydride remains in the bridge between Ni and Fe and acts as a base for the next incoming H2. In this mechanism, the hydrogenase cycles between Ni-R, Ni-C, and a transient Ni-X state the latter has a second hydride bound to the Ni. This model assumes that the two protons are released in two subsequent oxidation steps. H2 production will occur through the same reverse pathway. 

Pandelia ME, Ogata H, Lubitz W. Intermediates in the catalytic cycle of [NiFe] hydrogenase functional spectroscopy of the active site. ChemPhysChem. 2010 11(6) 1127-40. 

Pardo A, de Lacey AL, Fernandez VM, Fan H. J, Fan Y, Hall MB. Density functional study of the catalytic cycle of nickel-iron [NiFe] hydrogenases and the involvement of high-spin nickel(II). J Biol Inorg Chem. 2006 ll(3) 286-306. 

Adamska A, Silakov A, Lambertz C, Rudiger O, Happe T, Reijerse EJ, Lubitz W. Identification and characterization of the super-reduced state of the H-cluster in [FeFe] hydrogenase a new building block for the catalytic cycle Angew Chem Int Ed 2012 in press. 

FIGURE 3.3 Catalytic cycle for FeFe-hydrogenases, with two most likely stopping points in the cycle highlighted in red. SOURCE Presentation of Thomas Rauchfuss, University of Illinois, Urbana-Champagne. 

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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