The Vitamin B 12 Coenzyme

INTRODUCTION In 19'58 Barker and his associates l 3 recognized a new coenzyme which controlled the conversion of glutamate into ,8-methylaspartate by Clostridium tetanomorph'im. The coenzyme was shown4 to be related to ",-vitamin B12, i.e. contair ing an adenine nucleotide grouping in place of the 5,6-dimethylbe:nziminalole nucleotide of vitamin B12, although similar coenzymes containing b{ nziminazole or 5,6-dimethylbenziminazole were produced by growing: C tetanomorphum in the presence of the appropriate base5, Other variations of the nucleotide base have been achieved using Propionibacterium arabinosllm in the presence of other purines and benziminazoles6 • The presence of ,:he coenzymes in a wide variety of micro-organisms such as several species of Actinomycetes including Streptomyces olivaceus and S. griseus has been demonstrated by the glutamate isomerase assay7 or by isolation. It appears thit vitamin B12 and its analogues are always biosynthesized in the form of their coenzymes. Preliminary physical and chemical studies suggested that in the 5,6-dimethylbenziminazolyl cobamide coenzyme the cyanide gr )UP of vitamin B12, cyanocobalamin, was replaced by an adenine nucleoside:, 5, 8 and the determination9 of the complete structure (I; R = 5'-de·)xyadenosyl) of the coenzyme by X-ray analysis revealed the existence (f an essentially covalent bond between the cobalt atom and the 5/,·carbon 1tom of the additional S'-deoxyadenosine group. The molecule