Acetyl‐cobalamin from Photoinduced Carbonylation of Methyl‐cobalamin

Abstract

SummaryIrradiation with visible light of a deoxygenated aqueous solution of methyl-cobalamin (1) under an atmosphere of CO and at ambient temperature converts 1 into acetyl-cobalamin (2). This carbonylation has potential relevance with respect to bacterial acetate biosynthesis from CO,.Thauer recently suggested') bacterial synthesis of CH,COOH (or of a coenzyme A bound CH,CO-group) from two CO, to proceed via carbonylution of methyl-Co (111)-corrinoids analogous to methyl-cobalamin (1). This contrasted with earlier proposals, where C-C bond formation in the course of bacterial acetate synthesis from CO, was considered to occur by carboxylation of vitamin B,, derivatives (reviewed in [I] [l] [5b] concerning the involvement of vitamin B,, derivatives as intervening CH,-group carriers in bacterial acetate synthesis from 2 CO,'), discovered earlier by way of the (intact [9]) incorporation of the cobalt-bound CH,-group of methyl-cobalamin (1) during acetate biosynthesis in Clostridium thermoaceticum [ 101.On the other hand, for the presumed participation of methyl-cobyrinates in the assembly of the CH,CO-group from 'one-carbon' precursors, a chemical model has not yet been established (experimentally). In particular, a carboxylation of the axial CH,-group of the organo-corrinate was considered originally [ 11 [I I] as the C-C bond forming step, but in extensive (non-enzymatic) studies with methyl-Co(II1)-corrinoids and with CO,, formation of CH,COOH was achieved merely as a minor, barely detec-I)

Unusual selectivities of radical reactions. Experimental studies on alkyl versus phenoxyl reaction systems

“…For the tert-butyl/2,4,6-tri-tert-butylphenoxyl system p1 = 5. 6 -lop8 cm and p2 = 9.2 . cm were estimated like in previous work [10,12], and for the diffusion coefficients those of the model compounds isobutane and 2,6-di-tert-butyl-4-methylphenol in rt-heptane were used [10,12].…”

Acetogenesis and the Wood–Ljungdahl pathway of CO2 fixation

“…The proposal of a corrinoid-catalyzed methyl radical mechanism also seemed feasible since photolysis of methylcobalamin (in solution) in the presence of a high CO pressure (31 atm.) was shown to generate acetyl-cobalamin via a radical mechanism [82]. Thus, it seemed reasonable that the assembly of acetate occurred on a corrinoid protein though it was unknown whether the synthesis occurred from an acetylcobalt, acetoxycobalt, or a carboxymethylcobalt intermediate.…”

Geological and Chemical Factors that Impacted the Biological Utilization of Cobalt in the Archean Eon

“…Global redox state affected not only the availability of transition metal cofactors but also the Figure 6. Photo-induced carbonylation of methyl-cobalamine (adapted from Kräutler, 1984).…”

“…Loss of axial ligands allows the production by photolysis of alkyl radicals, which are then available for subsequent reactions (Halpern, ; Rétey, ). The photo‐induced carbonylation of methylcorrin presumably operates by way of the intermediate free methyl radical and Cbl(II), where carbon monoxide functions as a nonoxidative trap for the free methyl radical (Figure adapted from Kräutler, , ). This carbonylation pathway has potential relevance with respect to acetate biosynthesis in anaerobic bacteria (Kräutler, ; Thauer, ) and could be a potential precursor to early metabolism.…”