Metabolism of Glutamate in Purple Nonsulfur Bacteria

Participation of Vitamin B₁₂

抄録

Purple nonsulfur bacteria, Rhodospirillum rubrum and Rhodopseudomonas spheroides were found to possess coenzyme B₁₂-dependent glutamate mutase activity. Cell-free extracts of these bacteria grown on Co²⁺-containing media catalyzed the conversion of glutamate to β-methylaspartate and further to mesaconate. The activity of the cell-free extracts of these organisms cultivated on Co²⁺-deficient media was markedly lower than that of the normal cells. Addition of coenzyme B₁₂ to the former reaction mixture enhanced the mesaconate formation via β-methylaspartate. These results indicate the involvement of coenzyme B¹²-dependent glutamate mutase of these bacteria in the dissimilation of glutamate to acetyl-CoA and pyruvate through the following pathway.
glutamate→β-methylaspartate→mesaconate→citramalate→→acetyl-CoA, pyruvate
On the other hand, a greater part of glutamate was converted to α-hydroxyglutarate and succinate with the cell-free extracts of these photosynthetic bacteria. This fact, taking account of the presence of propionyl-CoA carboxylase in these bacteria, implies the participation of coenzyme B₁₂-dependent (R)-methylmalonyl-CoA mutase in the formation of succinate via the following route.
glutamate→α-ketoglutarate→α-hydroxyglutarate→propionate→propionyl-CoA→
(S)-methylmalonyl-CoA→(R)-methylmalonyl-CoA→succinyl-CoA