In the diagrams that follow A = CH2CO2H and P = CH2CH2CO2H
Enzymes: A, S-Adenosylmethionine-uroporphyrinogen III methyl transferase (SUMT) or CysG (in E. coli); B, S-Adenosylmethionine-precorrin-2 methyl transferase (SP2MT); C, CysG (in E. coli).
After precorrin-3A the pathway differs in the aerobic organism Pseudomonas denitrificans and the anaerobic (or microaerophilic) organism Propionibacterium shermanii. The aerobic pathway is as follows.
Notable features are the oxidative step from precorrin-3A to precorrin-3B using O2 catalysed by CobG, an enzyme containing an iron-sulphur cluster. This sets up the ring-contraction by CobJ via a pinacol-type rearrangement. The same enzyme also catalyses the methylation at C-17. CobM then introduces the methyl group at C-11 which later on migrates to C-12 where it is found in vitamin B12. CobF then hydrolyses off the acetyl group from C-1 as acetic acid, replacing it by a methyl group from SAM. The product, precorrin-6A, was the first intermediate discovered in this part of the pathway, in 1990. Cob K then reduces the 18/19 double bond and CobL performs two methylations, at C-5 and C-15, and decarboxylation of the acetate side-chain on C-12.
The anaerobic pathway differs from the aerobic one above in that the cobalt ion is inserted much earlier at the dihydroisobacteriochlorin stage, i.e. either precorrin-2 or -3A. The oxidation step also differs as it cannot use O2 and generates a 6- rather than a 5-membered lactone as shown below. The pathway shown is consistent with
It is thought that from precorrin-6A onwards the intermediates on the anaerobic pathway are the cobalt complexes of the intermediates on the aerobic pathway. It has been demonstrated that a hydride transfer from NAD(P)H to C-19 occurs on both pathways and that the cobalt-free intermediates can be utilised by a cell-free preparation from the anaerobic organism.
In the aerobic Pseudomonas denitrificans the corrin macrocycle is formed by a [1,4]sigmatropic rearrangement of the methyl group from C-11 to C-12, catalysed by CobH, giving hydrogenobyrinic acid (see below). Amidation of two of the acetate groups is followed by insertion of cobalt(II) by a complex of three proteins (CobN, S and T). Reduction to Co(I) and adenosylation using ATP then gives adenosylcobyrinic acid diamide. Four further amidations then give adenosylcobyric acid.
Attachment of propanolamine by another complex of three proteins gives adenosylcobinamide, which is first phosphorylated and then converted into the GDP derivative. Displacement of GMP by an -OH group of alpha-ribazole then gives coenzyme B12 (adenosyl cobalamin).