Some Properties and Physiological Roles of Phosphoenolpyruvate Carboxylase in Rhodopseudomonas sp. No. 7 Grown on Ethanol under Anaerobic-light Conditions

Abstract

Phosphoenolpyruvate carboxylase (EC 4.1.1.31, PEPC) and phosphoenolpyruvate kinase (EC 4.1.1.49<ATP>, PEPK) were detected in cells of Rhodopseudomonas sp. No. 7 grown photoanaerobically on ethanol and acetate. The activity of PEPC was about 3 times higher in the ethanol-grown cells than the acetate-grown cells. PEPC was purified as an electrophoretically homogeneous and stable protein (Mr, about 400, 000; subunit Mr, about 102, 000). The enzyme absolutely required Mn²⁺ or Mg²⁺ for the appearance of its activity. Acetyl CoA (40 μM) reduced the K_m against phosphoenolpyruvate and HCO^-₃ to about 1/20 (0.23mM) and 1/2 (1.7mM), respectively. At that time, the V_max increased to about 30 times (90 μmol/min/mg protein). In the presence of acetyl CoA the circular dichroism spectrum of the enzyme showed the decrease of an α-helix structure. The enzyme was not activated by fructose-1, 6-bisphosphate. The enzyme was inhibited by aspartate (K_i, 0.208mM). Besides, the enzyme was inhibited by nucleotides (ATP and GTP). The enzyme activity was activated to about 15 times by 3.25M ethanol.