The biosynthesis of valine from isobutyrate by Peptostreptococcus elsdenii and Bacteroides ruminicola

Abstract

1. Growing cultures of Peptostreptococcus elsdenii and Bacteroides ruminicola incorporate 14C from [1-14C]isobutyrate into the valine of cell protein. With P. elsdenii some of the 14C is also incorporated into leucine. 2. Crude cell-free extracts of both organisms in the presence of glutamine, carbon dioxide and suitable sources of energy and electrons incorporate 14C from [1-14C]isobutyrate into valine but not into leucine. 3. With extracts of P. elsdenii treated with DEAE-cellulose the reaction is dependent on ATP, CoA, thiamin pyrophosphate, molecular hydrogen and a low-potential electron carrier (ferredoxin, flavodoxin or benzyl viologen). 4. The same extracts incorporate 14C from NaH14CO3 into valine in the presence of isobutyrate plus ATP, CoA, glutamine and ferredoxin; isobutyryl-CoA or isobutyryl phosphate plus CoA will replace the isobutyrate plus CoA and ATP. With acetyl phosphate in place of isobutyryl phosphate, 14C is incorporated into alanine. With isovalerate or 2-methylbutyrate in place of isobutyrate, 14C is incorporated into leucine and isoleucine respectively. 5. When carrier 2-oxoisovalerate is added to the carboxylating system 14C from [1-14C]isobutyrate passes into the oxo acid fraction. 6. It is concluded that these two organisms form valine from isobutyrate by the sequence isobutyrate→isobutyryl-CoA→2-oxoisovalerate→valine and that the reductive carboxylation of isobutyrate is catalysed by a system similar to the pyruvate synthetase of clostridia and photosynthetic bacteria.