Biosynthesis of para-cresolyl cobamide in Sporomusa ovata

Abstract

Para-cresolyl-cobamide is a unique corrinoid from Sporomusa ovata because a cresol-riboside is present in place of a benzimidazole-nucleotide. The biosynthesis of the complete corrinoid was studied by growing cells with the possible [¹⁴C]-labeled precursors tyrosine, para-cresol, threonine, glutamate and glycine, respectively. The specific radioactivities of these precursors in their cellular pools were evaluated by determining the specific radioactivities of various amino acids isolated from cellular proteins. Those measurements were compared to the specific radioactivities of the complete corrinoid and its degradation product cobinamide. Hence, the incorporation of a particular precursor either in the cobinamide or in the cresol-riboside was verified. Tyrosine and p-cresol were incorporated into the cresol-riboside moiety of the complete corrinoid, and tyrosine rather than p-cresol was incorporated also into the cell protein. This finding suggested that the cellular tyrosine was degraded irreversibly to p-cresol. The p-cresol thereafter required an “α-O-transglycosidase”-like activation reaction to yield the cresol-riboside with an O-glycosidic bond. Both, glutamate and threonine were incorporated into the protein fraction and into the cobinamide fraction, but not into the cresolriboside moiety. Glycine, however, was excluded as a direct precursor of the p-cresolyl-cobamide, suggesting the C-5 pathway of δ-aminolevulinic acid synthesis in Sporomusa.