Hydrogenase enzyme

Thiolate-bridged dinickel complexes and, in particular, heterobimetallic Ni/Fe complexes have attracted much interest as model systems for the hydrogenase enzymes. A review covering this... 

Other transformations supplied by these enzymes are para-ortho-hydrogen conversion, and the exchange reaction between H2 and protons of water.409-412 The hydrogenase enzymes found in various microorganisms are very different in their protein structure and in the types of electron carrier they use. 

The metal-based hydrogenase enzymes known can be subdivided into three main groups ... 

Nonenzymatic regeneration of NAD(P)H requires the regioselective transfer of two electrons and a proton to NAD(P)+. Various rhodium(III) complexes are effective electrocatalysts capable of mimicking hydrogenase enzymes.48-54... 

There has been considerable effort in the last few years toward achieving an understanding of hydrogenase enzyme systems which have the ability to activate H2 for exchange with water, para-ortho conversion, and reduction reactions when coupled to an electron carrier E such as NAD+, cytochrome c3, or ferredoxins (7, p. 396 470-473). Reaction (81) can be catalyzed in either direction ... 

In the European program COST (European CO-operation in the Field of Scientific and Technical Research) 8.41 Biological and Biochemical Diversity of Hydrogen Metabolism the main objective is to pool interrelated European expertise in order to understand the structural and molecular basis of the functions, as well as the factors that influence the activity and stability of hydrogenase enzymes. 

Until now, only a few versatile, selective and effective transition-metal complexes have been applied in nicotinamide cofactor reduction. The TOFs are well within the same order of magnitude for all systems studied, and are within the same range as reported for the hydrogenase enzyme thus, the catalytic efficiency is comparable. The most versatile complex Cp Rh(bpy) (9) stands out due to its acceptance of NAD+ and NADP+, acceptance of various redox equivalents (formate, hydrogen and electrons), and its high selectivity towards enzymatically active 1,4-NAD(P)H. 

Fu C, Javedan S, Moshiri F, Maier RJ. 1994. Bacterial genes involved in incorporation of nickel into a hydrogenase enzyme. Proc Natl Acad Sci USA. 91 5099-103. 

The importance of the hydrogenase enzyme was also demonstrated in experiments in which mixed cultures of SRB recently isolated from the production water of two pipeline systems in Alberta were circulated through two Robbins Devices (McCoy et al. 1981) at a flow rate of 4Lmin Both loops showed detectable SRB attached to corrosion coupons. One population of the SRB had a high level of hydrogenase activity, which correlated well with the subsequent high corrosion of the metal coupons the other population of SRB had low levels of hydrogenase and low levels of iron loss detected (Bryant et al. 1991). 

Hydrogenase was named by Stephenson and Stickland in 1931, discovered in their experiments using anaerobic colon bacterium Escherichia Coli that evolved hydrogen [144-146]. Hydrogenase enzymes are metalloproteins that contain sulfur and nickel and/or iron [147]. To date over 80 hydrogenase enzymes have been identified. These enzymes reversibly catalyze hydrogen production/uptake reactions ... 

The presence of this gene, however, does not itself imply the existence of hy-drogenosomes, because the [FeFe] hydrogenase enzyme could function in the cytoplasm, as in Giardia and Entamoeba (Townson et al. 1996 Rodriguez et al. 1998). 

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). 

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... 

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... 

Figure 3 Speculative model for the hydrogenase enzyme cycle such as that from D. gigas. The highest oxidation states of the enzyme are at the top, and each step down corresponds to a one-electron reduction. Some hydrons that are transferred to sites in the protein are not shown. Redox states of the iron-sulfur clusters are omitted.

In view of its application to fuel cell development, research into hydrogen activation remains a forefront area for chemists, physicists, and biologists (7). A rekindling of opportunity and excitement in this field of chemistry has come from the delineation of simple catalytic sites of hydrogenase enzymes as displayed by protein crystal structures published within the last decade (2 10). These active sites hold out promise of using complexes comprised of base metals such as iron or... 

The hydrogenase enzymes (1) catalyse the heterolytic cleavage of dihydrogen into protons and electrons,... 

This is a reasonable energy, fairly close to half of the binding energy of H2 (103.5 kcal/mol) as expected for a hydrogenase enzyme. Further implications of these energies will be discussed at the end of this section where an energy diagram is constructed. 

The above electron and proton transfers complete one cycle of the hydrogenase enzyme. The entire mechanism as suggested above is... 

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