Pyruvate carboxylase from a thermophilic Bacillus. Studies on the specificity of activation by acyl derivatives of coenzyme A and on the properties of catalysis in the absence of activator
Authors: Libor Susan M. , Sundaram Trichur K. , Scrutton Michael C. 
Published: August 2015

1. Oxaloacetate synthesis catalysed by pyruvate carboxylase from a thermophilic Bacillus in the absence of acetyl-CoA required addition of high concentrations of pyruvate, MgATP2− and HCO3−, and at 45°C occurred at a maximum rate approx. 20% of that in the presence of a saturating concentration of acetyl-CoA. The apparent Km for HCO3− at pH7.8 was 400mm without acetyl-CoA, and 16mm with a saturating activator concentration. The relationship between reciprocal initial rate and reciprocal MgATP2− concentration was non-linear (convex-down) in the absence of acetyl-CoA, but the extent of deviation decreased as the activator concentration was increased. The relationship between reciprocal initial rate and reciprocal pyruvate concentration was non-linear (convex-down) in the presence or absence of acetyl-CoA. 2. The optimum pH for catalysis of oxaloacetate synthesis was similar in the presence or absence of acetyl-CoA. The variation with pH of apparent Km for HCO3− implicated residue(s) with pKa 8.6 in catalysis of the activator-independent oxaloacetate synthesis. 3. Linear Arrhenius and van't Hoff plots were observed for the temperature-dependence of oxaloacetate synthesis in the absence of acetyl-CoA over the range 25–55°C. Ea (activation energy) was 56.3kJ/mol and ΔH‡ (HCO3−) (enthalpy of activation) was −38.6kJ/mol. In the presence of acetyl-CoA, biphasic Arrhenius and van't Hoff plots are observed with a change of slope at 30°C in each case. Ea was 43.7 and 106.3kJ/mol above and below 30°C respectively. 4. Incubation of Bacillus pyruvate carboxylase with trinitrobenzenesulphonate caused specific inactivation of acetyl-CoA-dependent catalytic activity associated with the incorporation of 1.3±0.2 trinitrophenyl residues per subunit. Activator-independent catalysis and regulatory inhibition by l-aspartate were unaffected. The rate of inactivation of acetyl-CoA-dependent catalysis by trinitrobenzenesulphonate was specifically decreased by addition of acetyl-CoA and other acetyl-CoA and other acyl-CoA species, but complete protection was not obtained. 5. All alkylacyl derivatives of CoA tested activated Bacillus pyruvate carboxylase; acetyl-CoA was the most effective. The apparent Ka exhibited a biphasic relationship with acyl-chain length for the straight-chain homologues. Certain long-chain acyl-CoA species showed additional activation at a high concentration. Weak activation occurred on addition of CoA or adenosine 3′,5′-bisphosphate, but carboxyacyl-CoA species and derivatives containing a modified phosphoadenosyl group were inhibitory. Thioesters of CoA with non-carboxylic acids, e.g. methanesulphonyl-CoA, serve as activators of the thermophilic Bacillus and Saccharomyces cerevisiae pyruvate carboxylases, but as inhibitors of pyruvate carboxylases obtained from chicken and rat liver. 6. α-Oxoglutarate mimics the effect of l-aspartate as a regulatory inhibitor of the pyruvate carboxylases from both the thermophilic Bacillus and Saccharomyces cerevisiae. l-Glutamate was ineffective in both cases.