A Metal-Free Hydrogenase

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 

H2 activation. The experimental free energy in the enzymatic process is very low (1.3 kcal/mol) as expected for a reversible system. The computed reaction enthalpies 

An Fe-Fe bond (2.6 A) is present in both C. pasteurianum and D. desulfuricans and is typical of dithio-bridged organometallic Fe-Fe systems. The dinuclear Fe 

The reverse reaction, formation of H2 from 2H+ and 2e, involves protonation of the 2Fe center to form a metal hydride. The most basic site for initial protonation in the enzyme active sites may be the electrons in the M-M bonds, which can readily be reversibly protonated to form hydride-bridged species.43 Electrophilic [M-HJ+ systems often prefer to form p-H complexes rather than M-H on deprotonation of t]2-H2 by external bases  

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, 

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Berkessel A, Thauer RK (1995) On the mechanism of catalysis by a metal-free hydrogenase from methanogenic archaea enzymatic transformation of H2 without a metal and its analogy to the chemistry of alkanes in superacidic solution. Angew Chem Int Ed 34 2247... 

As discussed previously, superelectrophilic activation in biological systems has been found even with a metal-free hydrogenase enzyme found in methanogenic archea, an enzymatic system that converts CO2 to methane.57 It was found that /V5./V10-menthyl tetrahydromethanopterin (42) undergoes an enzyme-catalyzed reaction with H2 by hydride transfer to the pro-R position and release of a proton to form the reduced product (43 eq 36). 

Proposed mechanism for the reversible reaction of N, N -methenyltetrahydromethanopterin (methenyl-H4MPT ) with H2 to N, N -methylenetetrahydromethanopterin (methylene-H4MPT) and a proton catalysed by the metal-free hydrogenase from methanogenic archaea... 

Buurman, G., Shima, S., Thauei R. K. (2000) The metal-free hydrogenase from methanogenic archaea evidence for a bound cofactor. FEBS Lett. 485, 200M... 

Ciloslowski, J., Boche, G. (1997) Geometry-tunahle Lewis acidity of amidinium cations and its relevance to redox reactions of the Thauer metal-free hydrogenase - a theoretical study. Angeiv. Chem. Int. Ed. Engl. 36, 107-9. 

Teles, J. H., Borde, S., Berkessel, A. (1998) Hydrogenation without a metal catalyst - an ab initio study on the mechanism of the metal-free hydrogenase from Methanobacterium ther-moautotrophicum. J. Am. Chem. Soc. 120, 1345-6. 

As described in Chapter 2, a unique gitonic superelectrophile is considered to be involved in an enzyme system that converts CO2 to methane. Berkessel and Thauer have studied this metal-free hydrogenase enzyme from methanogenic archaea and a mechanism is proposed involving activation through a vicinal-superelectrophilic system (eq 34).50... 

Lyon EJ, Shima S, Buurman G, Chowdhuri S, Batschauer A, Steinbach K, Thauer RK (2004) UV-A/blue-light inactivation of the metal-free hydrogenase (Hmd) from methanogenic archaea. The enzyme contains functional iron after all. Eur. J. Biochem. 271 195-204... 

Ge2-alkyne analogs to give a mixture of Gc2 and primary germane products (Scheme 11.4) has been described by Power and coworkers [23]. It is also noteworthy that a unique metal-free hydrogenase has been identified from methanogenic archaea. This enzyme catalyzes reactions with H2 [24, 25], and it is suggested that a folate-Hke cofactor is important in the the reversible activation or liberation of H2 [26, 27]. 

Le Goff A, Artero V, Jousselme B, et al. From hydrogenases to noble metal-free catalytic nanomaterials for H2 production and uptake. Science. 2009 326 1384-7. 

Berkessel, A., Thauer, R. K. (1995). On the Mechanism of Catalysis by aMetal-Free Hydrogenase from Methanogenic Archaea Enzymatic Transformation of without a Metal and Its Analogy to the Chemistry of Alkanes in Superacidic Solution. Angewandte Chemie International Edition, 34, 2247-2250. 

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