Corrinoids reduced forms

Fig. 11. Active sites and reactions of the bifunctional CODH/ACS. For synthesis of acetyl-CoA, two electrons are transferred from external electron donors to Cluster B of the CODH subunit. Electrons are relayed to Cluster C which reduces CO2 to CO. The CO is proposed to be channeled to Cluster A of the ACS subunit to form a metal-CO adduct that combines with the methyl group of the CFeSP and CoA to form acetyl-CoA. For utilization of acetyl-CoA, these reactions are reversed. The abbreviations are CODH, CO dehydrogenase ACS, acetyl-CoA synthase CFeSP, the corrinoid iron-sulfur protein CoA, Coenzyme A.

Figure 15.6 The Wood-Ljungdahl pathway. One molecule of C02 (blue) is converted to formate and then reduced to a methyl group, which is then transferred to the corrinoid-iron-sulphur protein CFeSP. CFeSP transfers the methyl group to the A-cluster of the bifunctional CODH/ACS. The other molecule of C02 (red) is reduced to CO by the C-cluster of the CODH subunit. The CO is then transferred to the A-cluster through a long channel, some 70 A long, where with the methyl group and CoA it forms acetylCoA. (From Drennan et al., 2004. With kind permission of Springer Science and Business Media.)...

The cis- and trans-dichloroethenes reacting by this addition mechanism are transformed more slowly than the tetra- and trichloroethenes that could form them and more slowly than the vinyl chloride that their reactions would form. Thus, these particular dichlorocompounds accumulate when microorganisms reduce tetra- and trichloroethene in anoxic environments contaminated by these solvents (see Glod et al., 1997b and references therein). Reduction reactions of 1,1-dichloroethene with the super-reduced corrinoids are more likely to form the very toxic product, vinyl chloride. 

So far, for the highly oxygen-sensitive Co -forms of cobalt-corrinoids (such as Co -cobalamin, (6)), information from X-ray analysis on their three-dimensional structure is not available. However, in these strongly nucleophihc, highly reduced cobalt complexes, the diamagnetic Co -center (a d metal ion, isoelectronic with a Ni"-ion) presumably is ligated by the corrin ligand only, in a tetracoordinated, nearly square-planar fashion. As a consequence, the Co -forms of complete corrins would have to be base-off, that is, the nucleotide function would not be attached to the central cobalt ion. 

MetH catalyzes the methylation of the bound and reduced cob(I)alamin cofactor by (N -protonated) N -methyltetrahydrofolate to give enzyme-bound methylcobalamin (3) in a base-off/His-on form (see later) [125,153-155]. The methyl-Co(III)corrinoid is demethylated by homocysteine, whose sulfur is activated and deprotonated due to the coordination to a zinc ion (held by three cysteine residues) of the homocysteine binding domain [164] (see Fig. 15). The two methyl-transfer reactions occur in a sequential mechanism [124,125,153,154]. Intermittently, the bound Cob(I)alamin (40 ) is oxidized to enzymatically inactive cob(II)alamin (23) and requires reactivation by reductive methylation with SAM and a flavodoxin as a reducing agent [125,153-155,165]. 

Most of the corrinoids in propionic acid bacteria are represented by AdoCbl MeCbl is usually present in low amounts. The principal form is pseudo-vitamin B12, containing adenine nucleotide as the lower ligand. MeCbl is formed by the reaction of S-adenosylmethionine with the cobalamin that contains reduced Co(I). 

In these overall pathways (Fig. 4) there appear to be at least two novel transmethylation reactions. The methyl group of methanol may be transferred to either Bias, forming methyl-B12 (Blaylock, 1968) or be incorporated into methane (Blaylock and Stadtman, 1966). The former reaction is dependent upon ATP and Mg" (Blaylock and Stadtman, 1966), which has led to the speculation that possibly methanol is activated prior to methyl transfer by formation of methyl-ADP (Barker, 1967). The formation of methane from CH3-B12 requires several protein fractions (including a Co-corrinoid protein), a reducing system, and ATP, which plays an unexplained role. In addition, a heat-stable cofactor may be involved (Blaylock, 1968). Very recently it has been briefly reported (McBride and Wolfe, 1970) that the methyl group of CH3-B12 is transferred to a soluble cofactor prior to its reduction to methane. The unidentified methylation cofactor could not be replaced by any of the known methyl transfer factors (McBride and Wolfe, 1970). Further study of these unusual types of reactions will be required before the details of the methyl transfer steps are clarified. 

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