Kinetic properties of ribulose bisphosphate carboxylase/oxygenase from Thiobacillus thyasiris, the putative symbiont of Thyasira flexuosa (Montagu), a bivalve mussel

Summary: Some kinetic properties of ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase from Thiobacillus thyasiris, a marine, facultatively heterotrophic, sulphur-oxidizing bacterium and putative symbiont of Thyasira flexuosa (Montagu), a bivalve mussel, have been determined. The kinetic parameters for the CO2/Mg2+-activated enzyme were: K m(RuBP) 24·3 μM, K m(CO2) 125·5 μM, K m(O2) 900 μM and K m (Mg2+) 1·53 mM. The low CO2 affinity suggests that T. thyasiris may possess a CO2-concentrating mechanism. RuBP oxygenase activity was inhbited by increasing CO2 concentration. Divalent metal ions were essential for RuBP carboxylase activity; activity of the Mg2+-free enzyme could be restored by the addition of Mg2+, Mn2+ or Ca2+. The pH optimum was 7·8. The temperature optimum for RuBP carboxylase activity was 55 °C, although the enzyme rapidly lost activity at this temperature. An Arrhenius plot was biphasic, with a break at 40 °C. The activation energies were 55·5 × 103 J mol-1 and 32·9 × 103 J mol-1> over the temperature ranges 10-40 °C, and 1·47 between 40-55 °C, RuBP carboxylase activity was stable at 35 °C, the optimum growth temperature of T. thyasiris and at 7·5 °C, the temperature of the habitat of Thyasira flexuosa, but the activity was 40% and 3·5%, respectively, of the potential activity at 55 °C. RuBP carboxylase activity was stimulated by NaCl concentrations of up to 0·3 m, with a maximum (33%), occurring between 0·1 and 0·2 m-NaCl. At higher concentrations of NaCl (>0·3 m) RuBP carboxylase activity was inhibited.