Regulation of the Tricarboxylic Acid Cycle and Poly- -hydroxybutyrate Metabolism in Azotobacter beijerinckii Grown under Nitrogen or Oxygen Limitation

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Abstract
A. beijerinckii was grown in NH3-free glucose/mineral salts media in chemostat culture under O2 or N2 limitation. Selected enzymes of the tricarboxylic acid cycle and poly-.beta.-hydroxybutyrate metabolism were monitored in relation to O2 supply for both steady and transition states. Two dissolved O2 concentrations were used for the N2-limited steady state to investigate the possible effects of respiratory protection of nitrogenase on these enzymes. The levels of NADH oxidase [EC 1.6.99.3], isocitrate dehydrogenase [EC 1.1.1.42] and 2-oxoglutarate dehydrogenase [EC 1.2.4.2] increased markedly on relaxation of O2 limitation while pyruvate dehydrogenase [EC 1.2.4.1] and citrate synthase [EC 4.1.3.7] were relatively unaffected. .beta.-Ketothiolase [EC 2.3.1.9] and acetoacetyl-CoA reductase [EC 1.1.1.36] levels decreased as O2 limitation was relaxed. Respiratory activity, as measured by the QO2 [O2 uptake rate] value, increased with O2 supply rate. Imposition of O2 limitation on a N2-limited culture caused an immediate increase in the NADH/NAD ratio but this rapidly readjusted to its previous steady-state value. These changes are discussed in relation to respiratory protection of nitrogenase and poly-.beta.-hydroxybutyrate metabolism in A. beijerinckii.
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