Poly-β-hydroxybutyrate biosynthesis and the regulation of glucose metabolism in Azotobacter beijerinckii

Abstract

Azotobacter beijerinckii possesses the enzymes of both the Entner-Doudoroff and the oxidative pentose phosphate cycle pathways of glucose catabolism and both pathways are subject to feedback inhibition by products of glucose oxidation. The allosteric glucose 6-phosphate dehydrogenase utilizes both NADP(+) and NAD(+) as electron acceptors and is inhibited by ATP, ADP, NADH and NADPH. 6-Phosphogluconate dehydrogenase (NADP-specific) is unaffected by adenosine nucleotides but is strongly inhibited by NADH and NADPH. The formation of pyruvate and glyceraldehyde 3-phosphate from 6-phosphogluconate by the action of the Entner-Doudoroff enzymes is inhibited by ATP, citrate, isocitrate and cis-aconitate. Glyceraldehyde 3-phosphate dehydrogenase is unaffected by adenosine and nicotinamide nucleotides but the enzyme is non-specific with respect to NADP and NAD. Citrate synthase is strongly inhibited by NADH and the inhibition is reversed by the addition of AMP. Isocitrate dehydrogenase, a highly active NADP-specific enzyme, is inhibited by NADPH, NADH, ATP and by high concentrations of NADP(+). These findings are discussed in relation to the massive synthesis of poly-beta-hydroxybutyrate that occurs under certain nutritional conditions. We propose that synthesis of this reserve material, to the extent of 70% of the dry weight of the organism, serves as an electron and carbon ;sink' when conditions prevail that would otherwise inhibit nitrogen fixation and growth.

Effect of Oxygen Concentration and Growth Rate on Glucose Metabolism, Poly- -hydroxybutyrate Biosynthesis and Respiration of Azotobacter beijerinckii

“…Carbohydrate metabolism in A . beijerinckii is controlled at the hexose phosphate level by glucose-6-phosphate dehydrogenase, an allosteric enzyme modulated by ATP, ADP, NADH and NADPH (Senior & Dawes, 1971). It is noteworthy, therefore, that the level of this enzyme is unaffected by nutrient limitation at a constant dilution rate (Fig.…”

“…beijerinckii (Senior & Dawes, 1971 ;Stephenson et al, 1978), while the effects of transition from oxygen to nitrogen limitation and vice versa on these two enzymes of PHB synthesis were investigated by Jackson & Dawes (1976). As high oxygen concentrations invoke the mechanism of respiratory protection of the nitrogenase system of nitrogen-fixing Azotobacters (Parker, 1954;Phillips & Johnson, 1961;Dalton & Postgate, 1968;Drozd & Postgate, 1970), characterized by very high respiratory rates at the expense of the available carbon source, it was important to discover whether a switch to oxygen limitation affected either the pattern of glucose metabolism or the levels of the enzymes concerned or both.…”

An Analysis of Intermediary Metabolism and its Control in a Fat-synthesizing Yeast (Candida 107) Growing on Glucose or Alkanes

“…In Candida 107, inhibition is non-competitive and, although we do not know the NADPH/NADP+ ratio in the cell or the absolute concentrations of these reactants, the system here is rather different. In this connexion Senior & Dawes (1971), examining the metabolism of Azotobacter beijerinckii which accumulates poly-P-hydroxybutyrate as a lipid storage compound, also found the glucose 6-phosphate dehydrogenase to be non-competitively inhibited by NAD(P)H with no decrease in the K , value. However, in Azotobacter the enzyme was subject to allosteric inhibition by ATP.…”

The Respiratory‐Chain NADPH Dehydrogenase ofAzotobacter vinelandii

“…Under oxygen-limiting conditions regulation of the NADPH/NADP+ ratio in both these organisms is probably effected via the predominantly NADPH-dependent acetoacetyl-CoA reductase associated with poly 3-hydroxybutyrate synthesis [19]. I n A. vinelandii therefore, NADPH dehydrogenase and acetoacetyl-CoA reductase may serve similar regulatory functions albeit under strikingly different conditions of oxygen availability.…”