CbiE (precorrin-6Y methyltransferase, also known as cobalt-precorrin-7 C(5)-methyltransferase, also known as cobalt-precorrin-6Y C(5)-methyltransferase) catalyzes the methylation of C-5 in cobalt-precorrin-7 to form cobalt-precorrin-8. It participates in the pathway toward the biosynthesis of cobalamin (vitamin B12). There are two distinct cobalamin biosynthetic pathways in bacteria. The aerobic pathway requires oxygen, and cobalt is inserted late in the pathway; the anaerobic pathway does not require oxygen, and cobalt insertion is the first committed step towards cobalamin synthesis. CbiE functions in the anaerobic pathway, it is a subunit of precorrin-6Y C5,15-methyltransferase, a bifunctional enzyme: cobalt-precorrin-7 C(5)-methyltransferase (CbiE)/cobalt-precorrin-6B C(15)-methyltransferase (decarboxylating) (CbiT), that catalyzes two methylations (at C-5 and C-15) in precorrin-6Y, as well as the decarboxylation of the acetate side chain located in ring C, in order to generate precorrin-8X. CbiE and CbiT can be found fused (CbiET, also called CobL), or on separate protein chains (CbiE and CbiT). In the aerobic pathway, a single enzyme called CobL catalyzes the methylations at C-5 and C-15, and the decarboxylation of the C-12 acetate side chain of precorrin-6B.
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