The Metal-Free Hydrogenase

In superacids, electrophilic carbenium ions can react with H2 to form a two-electron three atom pentavalent carbonium ion, in essence 

FIGURE 7. Cleavage of Hj by a carbenium ion in superacid solution. The reaction involves a pentavalent carbonium ion, formed by transferring a hydride equivalent to the carbenium ion (Olah et al., 1995). 

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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... 

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... 

Geierstanger, B. H., Prasch, T., Griesinger, C., Hartmann, G. C., Buurman, G. and Thauer, R. K. (1998) Catalytic mechanism of the metal-free hydrogenase from Methanogenic Archaea Reversed stereospecificity of the catalytic and non-catalytic reaction. Angew. Chem. Int., 37, 3300-3. 

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. 

The metal-free hydrogenase enzyme system was also studied by theoretical calculations.52 This study involved the reactions of two model systems (115 and 116) with molecular hydrogen (Table 3). Cation 115... 

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... 

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. 

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). 

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]. 

It appears that transition metals are central to all biological processes involving H2, CO, N2, and CH4, which are extremely important feedstocks in industry as well. The one exception, metal-free hydrogenases, was in fact, after several years of research, shown to contain Fe and to be inhibited by CO [2]. 

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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