This study examined how oxygen and glutamate regulate 2-oxoglutarate dehydrogenase synthesis in the facultatively anaerobic bacterium Citrobacter freundii. Researchers grew cultures under anaerobic conditions while varying the purity of nitrogen gas used for sparging and the presence of glutamate supplementation. Glutamate alone induced a 50-fold increase in 2-oxoglutarate dehydrogenase activity during anaerobic growth. Trace amounts of oxygen contaminating commercial nitrogen gas (less than 20 ppm) were sufficient to further increase enzyme synthesis and significantly alter the expression of other respiratory enzymes and cytochromes. When oxygen was completely removed using chemical purifiers, alcohol dehydrogenase and succinate dehydrogenase activities increased, while fumarate hydratase decreased. The study demonstrates that multiple factors—oxygen availability, glutamate presence, and redox potential—coordinately regulate tricarboxylic acid cycle enzyme synthesis. The authors conclude that previously observed variations in 2-oxoglutarate dehydrogenase activity in anaerobic cultures were attributable to both unremoved oxygen traces in commercial gases and glutamate supplementation, rather than simple anaerobic conditions.

Key findings

• Glutamate induces 50-fold increase in 2-oxoglutarate dehydrogenase synthesis in anaerobically growing Citrobacter freundii
• Trace oxygen contamination (<20 ppm) in commercial nitrogen gas significantly affects respiratory enzyme synthesis and cytochrome composition
• Removal of trace oxygen shifts enzyme expression patterns toward enhanced fermentative metabolism (increased alcohol and succinate dehydrogenase, decreased fumarate hydratase)
• Soluble cytochromes a₁ and c₅₅₂.₅ appear specifically under highly anaerobic conditions when trace oxygen is removed
• Redox potential better correlates with enzyme synthesis patterns than dissolved oxygen concentration measurements