Biomethylation reactions

The biomethylation reaction between platinum and methylcobalamin involves both platinum(II) and platinum(IV) oxidation states. An outer-sphere complex is formed between the charged platinum(II) salts and the corrin macrocycle, which catalytically labilizes the Co—C o bond to electrophilic attack. A two-electron redox switch mechanism has been proposed between platinum(II) and platinum(IV). However, a mechanism consistent with the kinetic data is direct electrophilic attack by PtClg on the Co—C a bond in MeBu. Studies on [Pt(NH3)2(OH2)2] indicate that the bases on cobalt interact in the coordination sphere of platinum(II). Since both platinum(ll) and platinum(rV) are together required to effect methyl transfer from methylcobalamin to platinuni, Pt and C NMR spectroscopy have been used to show that the methyl group is transferred to the platinum of the platinum(n) reactant. The kinetics of demethylation by mixtures of platinum(II) and platinum(IV) complexes show a lack of dependence on the axial ligand. The authors conclude therefore that it is unlikely that the reaction involves direct attack by the bound platinum on the Co—C bond, and instead favor electron transfer from an orbital on the corrin ring to the boimd platinum group in the slow step, followed by rapid methyl transfer. ... 

Methylcobalamin is also involved in other biomethylation reactions. Examples include the methylation of inorganic As salts to volatile methyl arsines, as well as the biomethylation of Hg salts to yield the highly toxic methyl-mercury cation [CH3-Hg] (see also Section 1.31.3.5). In this reaction, methylcobalamin serves as a Grignard-type reagent and the methyl group is transferred as a carbanion. 

Arsenite is also an intermediate in the fungal biomethylation of arsenic (Bentley and Chasteen 2002) and oxidation to the less toxic arsenate can be accomplished by heterotrophic bacteria including Alcaligenes faecalis. Exceptionally, arsenite can serve as electron donor for chemolithotrophic growth of an organism designated NT-26 (Santini et al. 2000), and both selenate and arsenate can be involved in dissimilation reactions as alternative electron acceptors. 

Antimony does not appear to be an essential ultratrace element and does not appear to undergo metabolic reactions analogous to those of arsenic for example, no biomethylation of antimony has been detected.178 Antimony trioxide is now suspected to be carcinogenic to humans. 

After a long fallow period, other workers have recently begun to extend Challenger s arsenic biomethylation work (1-3). Cox and Alexander have studied this reaction by the mold Candida humicola (59-9/). The overall reaction may be summed up by the reaction shown in Eq. (6). Both cell extracts and whole cells of Methanobacterium strain M.o.H. converted arsenate to dimethylarsine (92). Investigations on C. humicola and other molds determined that the rate of (CH s production follows the growth rate pattern of the mold, and decreases when the mold reaches the resting phase (93). The methylated arsenic intermediates were identified by use of... 

The fate of intermediates in transmethylation, especially ones that have not been isolated, may be considered at this point. One possible scheme is proposed in Eq. (10) where n represents the maximum valence of the metal. At enzymatic sites where biomethylation occurs, appropriate coordination of the metal with ligands presumably stabilizes the intermediate by suppressing reductive elimination. In exocellular reactions, a sufficient solution concentration of biogenic ligands such as thiols (197) or comparably effective donors [e.g., CN (198)] may provide the necessary stabilization to enable successive methylations. It is not clear, however, how such complexation, which also lowers the electrophilic character of the intermediate methylmetal (10, 69), could readily provide requisite stability without severe dimunition of the overall rate of methylation. This question remains actively debated (29) and will offer a research challenge for some time to come. 

Methyl(chloride) pincer iridium complex, protonation, 7, 313 jV-Mcthylcobalammc, in metal biomethylation, 12, 609 Methyl coenzyme M reductase, characteristics and reactions, 1, 890... 

The enzymology of arsenic biomethylation is complicated because of its many oxidation states, its propensity to react with sulfur compounds, and low concentrations of arsenic compounds in biological specimens. The chemical intermediates and reactions in the metabolism of arsenate are similar in microorganisms and animals. However, in microorganisms, the reactions tend to proceed to methylarsines, whereas in mammalian species the major urinary metabolite is generally dimethylarsinate and only a very small amount of it is reduced further. The arsenate reductase and methylarsonate reductase were thought to play an important role in arsenic biomethylation however, with the exception of arsenate reductase most of the enzymatic experiments involved mammalian systems. 

Fig. 2.10 Chemical processes which can alter speciation of various elements differ among different soil layers various sediment layers can control passage of heteroelements into deeper soil or sediment strata by chemical reactions, with phenols, phosphate or As(V) retained in corresponding layers whereas other kinds of transformation either directly invoke biological activity (e.g. biomethylation), not to be mimicked by simple element-...

A third process, biomethylation, has been the subject of interest in recent years in view of its implications with respect to the entry of toxic metals into the food cheun. It is also a potentially important reaction for mobilizing and transporting certain elements through the environment. 

Biomethylation. Methylation and methyl transfer are important reactions in the organic metabolism of organisms (Mudd, 1973) and it has been known for some time that dimethyl selenide, dimethyl telluride and di- and trimethyl arsine are biosynthetic products of the metabolism of inorganic compounds selenium, tellurium and arsenic, respectively, by microorganisms (Challenger, 1951). 

The implications are that these organoarsonic acids are natural products and hence have a biogeochemical origin in the oil shale taphonomy (fossilization) process. It is also interesting to note that no examples of biophenylation have been reported, whereas biomethylation of arsenic compounds is a well known reaction. (31) How the phenylarsonic acid forms will have to be answered with the examination of precursors to the oil shale such as freshwater marine algal mats as well as other biogeochemical samples ... 

A preliminary report on microbial reaction with Cd indicated a strain of Pseudomonas produced trace amounts of a volatile Cd species from inorganic Cd(II) in the presence of vitamin B12, but provided no evidence for biomethylation of Cd °. The prospect of Cd methylation has been considered by others". 

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