Acetogenic bacteria nickel

The most important physiological role of CODH in the metabolism of acetogenic bacteria was unknown until 1985, when it was shown that the enzyme is bifunctional and has acetyl-CoA synthase activity (121). It was previously thought that acetyl-CoA was synthesized at the cobalt center of a vitamin-Bi2-containing protein. In the same paper, it was proposed that nickel is the active site of CO oxidation and acetyl-CoA synthesis. 

Nickel in Carbon Monoxide Dehydrogenase from Acetogenic Bacteria 645... 

This is a noncyclic pathway that also results in the fixation of two molecules of C02 to form acetyl-CoA. It was elucidated by Wood, Ljungdahl, Thauer and others as a pathway which is used by acetogenic bacteria to synthesize acetate from C02 in their energy metabolism [21]. The acetyl-CoA pathway resembles the Monsanto process of acetate synthesis from CO and methanol, with one molecule of C02 being reduced to the level of methyltetrahydropterin, while another C02 molecule is reduced to the level of carbon monoxide in the reaction catalyzed by the nickel-dependent carbon monoxide dehydrogenase (Figure 3.3). 

Carbon monoxide dehydrogenase has been isolated from several acetogenic bacteria and purified in some cases, as shown in Table 20. This also contains details of carbon monoxide dehydrogenases from methanogens. The enzymes from C. thermoaceticum and A woodii have been compared. They both have molecular weights about 450 000 with six subunits, (0)8)3, but can exist as dimers, 0)3. The larger subunits appear to be identical in both cases, but the smaller ones differ. Nickel is present in concentrations that suggest two Ni per molecule of dimeric enzyme. There also appear to be two or three [4Fe-4S] clusters per dimeric enzyme, while the A. woodii enzyme may have a 3Fe centre. 

Nickel occurs as a cofactor in four enzymes known to date (Walsh and Orme-Johnson, 1987). Ureases from plant and animal sources use two Ni atoms as Lewis acids, a role more typical of Zn. In the other three enzymes, all bacterial. Ni is redox active. In methyl coenzyme M reductase of methanogenic bacteria, Ni is found in a tetrapyrrole (factor F430) and cycles between Ni(ll) and Ni(l). Many bacteria contain Ni-dependent hydroge-nase(s) and methanogenic and acetogenic bacteria have a specific Ni-containing CO... 

Nickel is found in at least four enzymes urease, certain hydrogenases, methyl-CoM reductase (in its cofactor F430) of methanogenic bacteria, and carbon monoxide dehydrogenase of acetogenic and methanogenic bacteria 434... 

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