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 (M_r, about 400,000; subunit M_r, about 102,000). The enzyme absolutely required Mn²⁺ or Mg²⁺ for the appearance of its activity. Acetyl CoA (40 μμ) reduced the K_m against phosphoenolpyruvate and to about 1/20 (0.23 mm) and 1/2 (1.7 mm), 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, 0.208 mm). Besides, the enzyme was inhibited by nucleotides (ATP and GTP). The enzyme activity was activated to about 15 times by 3.25 m ethanol.

Key words:

• acetyl CoA
• ethanol assimilation
• photosynthetic bacteria
• phosphoenolpyruvate carboxylase
• Rhodopseudomonas sp.