Methyl corrinoids

The third reason for favoring a non-radical pathway is based on studies of a mutant version of the CFeSP. This mutant was generated by changing a cysteine residue to an alanine, which converts the 4Fe-4S cluster of the CFeSP into a 3Fe-4S cluster (14). This mutation causes the redox potential of the 3Fe-4S cluster to increase by about 500 mV. The mutant is incapable of coupling the reduction of the cobalt center to the oxidation of CO by CODH. Correspondingly, it is unable to participate in acetate synthesis from CH3-H4 folate, CO, and CoA unless chemical reductants are present. If mechanism 3 (discussed earlier) is correct, then the methyl transfer from the methylated corrinoid protein to CODH should be crippled. However, this reaction occurred at equal rates with the wild-type protein and the CFeSP variant. We feel that this result rules out the possibility of a radical methyl transfer mechanics and offers strong support for mechanism 1. 

Another important group of methyl transfer reactions are those from methyl corrinoids to mercury, tin, arsenic, selenium, and tellurium. For example, Eq. 16-44 describes the methylation of Hg2+. These reactions are of special interest because of the generation of toxic methyl and dimethyl mercury and dimethylarsine. 

Notice that whereas in Eq. 16-43 the methyl group is transferred as CH3+ by nucleophilic displacement on a carbon atom, the transfer to Hg2+ in Eq. 16-44 is that of a carbanion, CH3, with no valence change occurring in the cobalt. However, it is also possible that transfer occurs as a methyl radical.420 Methyl corrinoids are able to undergo this type of reaction nonenzymatically, and the ability to transfer a methyl anion is a property of methyl corrinoids not shared by other transmethy-... 

The anaerobic bacterium Clostridium thermoaceticum obtains its energy for growth by reduction of C02 with hydrogen (Eq. 16-46). One of the C02 molecules is reduced to formate which is converted via 5-methyl-THF to the methyl corrinoid 5-methoxybenzimidazolyl-... 

The reaction [Eq. (7)] requires a disulfide-reducing system such as dithiothreitol or disulfide reductase and a reducing agent such as NADPH or reduced ferredoxin. It is proposed [Eq. (5)] that carbon monoxide oxidoreductase binds CO as a one-carbon intermediate [C,], which can be either oxidized to C02 or condensed with the methyl group of a methylated corrinoid protein and CoA in the final step of acetyl-CoA synthesis. 

Menon, S., and Ragsdale, S. W., 1999, The role of an iron-suUur cluster in an enzymatic methylation reaction methylation of CO dehydrogenase/acetyl-CoA synthase by the methylated corrinoid iron-sulfur protein, J. Biol. Chem. 274(17) 11513nll518. 

ACS catalyzes the synthesis of acetyl-CoA from CO, CoA, and a methyl group donated by a methylated corrinoid iron sulfur protein (Reaction 4, Table 1). In the CODH-only enzymes like CODHI and II from C hydrogenoformans and R. rubrum, there is evidence for a channel between the surface and the C-cluster (above). On the other hand, in CODH/ACS,... 

We were interested in the specificity of methyl corrinoid derivatives and tested methyl factor III and methyl factor B as substrates for methane formation. Specificity for activation of the methyl group did not reside in the lower axial ligand since the methyl group was converted readily to methane in the absence of the dimethylbenzimidazole moiety of the lower axial ligand (11). 

The reaction between mercuric ion and methyl corrinoids is an example of carbanion methyl -transfer (Figure 2). Because mercuric ion is a good electrophile, it also coordinates to the nitrogen of the 5,6-dimethylbenzimidazole base to give a mixture of "base off" and "base on" methylcobalamin. The "base on" species reacts 1000 times faster than the "base off" species to give methylmercury as the product. Other metals which are known to react with methyl--cobalamin by a similar mechanism to mercuric ion are lead (Pb ), thallium (Tl ) and palladium... 

The second important type of organometallic reactivity of B 12-derivatives concerns the highly nucleophilic/nucleofugal Co(I)-corrins [75,91,132[. These provide the basis of the heterolytic mode of formation/cleavage of the Co - C bond, important in methyl-corrinoids in enzyme-catalyzed methyl-transfer reactions [125-127]. This mode is represented by the reaction of Co(I)-corrins with alkylating agents in the formation of the Co - CH3 bond and the nucleophile-induced demethylation of methyl Co(III)-corrins for the... 

The crystallographic results on the structure of MetH and the finding of the base-off/His-on binding of the cofactor in a Bi2-dependent methyl transferase were consistent with earlier ESR spectroscopic evidence for histidine binding to the cobalt center of p-cresolyl-cobamide (52) in the aceto-gen Sporomusa ovata [145,169]. Various other B -dependent methyltrans-ferases are indicated to have either a base-off/His-on bound methyl-Co(III)-corrinoid, or even a methyl-corrinoid cofactor in base-off form (where His-coordination is absent) [156]. 

Fig. 8. Determination of the thermod5mamic effect of the nucleotide coordination on the homolytic bond dissociation of methylcobalamin (3) by equilibration of methyl corrinoids and Co - corrinoids equilibrium constant if (20°C) = 0.56, AH = 2.5 kcal-mol" S AS = 7.1 cal/(K-mol) (18,31).

The bacterial metalloenzyme acetyl coenzyme A synthase/CO dehydrogenase (ACS/CODH) catalyzes two very important biological processes, namely the reduction of atmospheric CO2 to CO and the synthesis of acetyl coerrzyme A from CO, a methyl from a methylated corrinoid iron-sulfur protein, and the thiol coenzyme A [166-168]. This bifunctional errzyme is the key to the Wood-Ljiungahl pathway of anaerobic CO2 fixation (Scheme 1.18) and a major component of the global carbon cycle. Reactions catalyzed by CODH and ACS are shown in Eqs. (1.4) and (1.5) below. 

The cortinoid protein [Co —E] must be reduced to accept a methyl group from the CH3—Hjfolate (Reaction [vi]). Methylation of the reduced coninoid protein is then catalyzed by transmethylase to produce the Co-methyl group of the methylated corrinoid protein [CH3—Co—E] (Reaction [vii]) (Abubackar et al., 2011 Ljungdhal, 1986). 

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