Mechanism for the interaction of thiols with methylcobalamin

Activation of the methylreductase system from Methanobacterium bryantii by corrins

Corrins activated the methylreductase system from Methanobacterium bryantii three- to fivefold in extracts resolved from low-molecular-weight factors. Corrins did not substitute for ATP and component B, which were also required for maximal activity. The concentration of diaquacobinamides required for one-half maximal activity was 1 microM. The concentrations of cyanocobalamin, methylcobalamin, Co alpha-(5-hydroxybenzimidazoyl)-Co beta-cyanocobamide, and 5'-deoxyadenosylcobinamide required for one-half maximal activity were between 4 and 7 microM. Deoxyadenosylcobalamin was nearly inactive. Activation was independent of thiols, coenzyme M, and ATP. Activation was also observed after partial purification of the methylreductase system by agarose column chromatography. Corrins were required in catalytic concentrations, methylcobalamin was not required, and methanogenesis was enzymatic. Corrin activation of the methylreductase is a novel effect on methanogenesis. However, the physiological significance of the corrin activation is uncertain.

Role of vitamin B12 in methyl transfer for methane biosynthesis by Methanosarcina barkeri

When Methanosarcina barkeri is grown on methanol as the sole carbon source, a B12-containing protein is synthesized by this organism. This B12 protein contains bound aquocobalamin, and when this cofactor is reduced and methylated with [14C]methyl iodide, the resultant [14C]methyl B12 protein is extremely active in the biosynthesis of 14C-labeled methane. These findings indicate that a B12-dependent system is operative in the biological formation of methane in addition to other systems that are B12-independent.

The biochemistry of toxic elements

In 1968 it was discovered that mercury could be biomethylated to give methylmercury as the major product (Wood, Kennedy & Rosen, 1968; Jensen & Jernelöv, 1969). This discovery has proved to be important in two respects: (a) it provided us with the recognition that biological systems are capable of synthesizing very toxic organometallic compounds from relatively innocuous inorganic precursors, and (b) it opened the door to the study of organometallic compounds produced by biological mechanisms in the aqueous environment.