H-ases

The X-ray structure of the enzyme from D. gigas provided the first crucial details about the coordination environment around the metals even though it was for the inactive air-oxidized unready form.15,16 The overall protein unit is a heterodimer with subunits of masses 60,000 and 30,000, with the former containing one Ni and one Fe, and the latter the Fe-S clusters (one 3Fe-4S and two 4Fe-4S). Until the 

All evidence points to the Fe(COXCN)2 group as shown in 2, where the S ligands are endogenous cysteinyl sulfurs (or sclcnocysteinyl in rare cases) and X is 

Although these energetics appear proper, the calculations do not take into account the interactions of the active site with the protein environment that closely surrounds the CO/CN ligands with little freedom for movement.30 Although this may not be the case for other H-ases (some [Fe] H-ases have only one point of attachment to the protein), terminal-bridging ligand reorientations might require 

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Both CO and cyanide ligands have been identified in the dinuclear active sites of iron-only H-ases that are remarkably organometallic-like and could bind and heterolytically split H2, most likely at an octahedral site trans to the bridging CO (83). 

Northup et al. postulated that H Ase may play a role in cancer invasiveness [126], Stern et al. described elevated levels of HAse in urine of children with Wilm s tumor and suggested that HAse may be used as a tumor marker [127]. 

The study of H-ases is a very active, vast research field (> 3000 publications) that has been the subject of many reviews.2 11 Only the aspects directly involving the biological activation and production of H2 will be discussed here. Most notably, the active sites of H-ases feature the first biological systems with CO and cyanide ligands. These are coordinated to dinuclear M-M bonded centers, such as shown in 1... 

H-ases are redox enzymes that catalyze reversible interconversion of H2 and protons to either utilize H2 as an energy source or dispose excess electrons as H2 ... 

Three basic types of H-ase active sites have been identified. The most prevalent contain Ni in combination with Fe, but a select few contain only Fe, and are classified as iron-only ([Fe]) H-ases. Selenium replaces sulfur in the Ni-bound cysteine in some NiIe] H-ases, and one H-ase active site contains no metals at all The activation of H2 is as multifaceted in biology as it is in industrial catalysis. The general structural components involved in the H-ase mechanism are shown in Figure... 

Figure 10.2. Various states of the [Ni/Fe] H-ase active site and plausible catalytic cyclic. The experimental values of vq, for the CO ligand are shown as subscripts.

Figure 10.3. A possible mechanism for H-ase function as suggested by the calculations of Niu et al.29...

Remarkably, one H-ase found in methanogenic archaea, Methanobacterium thermoautotrophicum, does not contain transition metals at all 8 It catalyzes the reduction of a pterin compound by H2 and also produces a proton, as a step in methane formation from C02 and H2. One proposed mechanism is analogous to that of Olah33 for the reversible formation of carbocations and H2 from alkanes in superacid media, e.g., isobutane conversion [Eq. (10.7)]. However, the enzyme is... 

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. 

The Fe-Fe bonds in [CpFe(CO)(PR3)(/i-CO)] 2 are as basic as weak amines (p-Kj, 6) and concomitant shift of p-CO to terminal positions occurs on protonation [Eq. (10.12)]44 Protonation of the Fe-Fe bond in [Fe(CO)2(PR3)(/i-SR )]2 occurs in preference to protonation of the sulfur ligands [Eq. (10.13)].45 These axe, however, Fe1 centers, and analogous protonation of Feu-Fen is not well known. The basicities of M-M bonds such as in [CpRu(CO)2]2 are substantially higher than that of the metal sites in related 18e mononuclear complexes and are highly sensitive to the nature of the ancillary ligands.43 As discussed above, theoretical studies of [NiFe] H-ase mechanisms indicate that p-H intermediates are energetically favorable. As shown in the reverse sequence of Scheme 2. the hydride could then shift to a terminal position and be protonated to a readily dissociable H2 ligand, and the cycle would continue. 

Such bridging/terminal shifts involving CO as well as H may be more likely in the [Fe] H-ase sites, which are attached to the protein only via the 4Fe-4S cluster,... 

The above concepts serve to bridge the fields of organometallic and biochemistry, and modeling of the functional aspects of H-ases is still in its infancy,9 e.g., 8.25... 

In the absence of a substrate, N-ase acts like a H-ase and evolves H2 by proton reduction, which cannot be completely suppressed even at high N2 pressures. At least... 

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