Parallels and Decisive Differences in Vitamin B 12 Biosyntheses

Single reactions that are, in principle, similar to those in the cobalt‐free biosynthesis of hydrogenbyrinic acid ( 1 ) in the aerobe P. denitrificans , but with cobalt‐coordinated substrates, occur in the biosynthesis of cobyrinic acid ( 2 ) in the anaerobe P. shermanii . This information was obtained from incorporation experiments with 3 H/ 14 C doubly labeled precursors from the biosynthetic pathway of P. denitrificans . Previously it had been established that in P. shermanii and the biosynthesis of 2 , the cobalt ion is incorporated much earlier than in P. denitrificans .