UNSPECIFIED. (1991) KINETIC-PROPERTIES OF RIBULOSE BISPHOSPHATE CARBOXYLASE OXYGENASE FROM THIOBACILLUS-THYASIRIS, THE PUTATIVE SYMBIONT OF THYASIRA-FLEXUOSA (MONTAGU), A BIVALVE MUSSEL. JOURNAL OF GENERAL MICROBIOLOGY, 137 (Part 7). pp. 1491-1496. ISSN 0022-1287

Abstract

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-mu-M, K(m)(CO2) 125.5-mu-M, K(m)(02) 900-mu-M 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 inhibited by increasing CO2 concentration. Divalent metal ions 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-degrees-C, although the enzyme rapidly lost activity at this temperature. An Arrhenius plot was biphasic, with a break at 40-degrees-C. The activation energies were 55.5 x 10(3) J mol-1 and 32.9 x 10(3) J mol-1 over the temperature ranges 10-40-degrees-C and 40-55-degrees-C, respectively. Q10 was 2.12 for any 10-degrees-C increment between 10-40-degrees-C, and 1.47 between 40-55-degrees-C. RuBP carboxylase activity was stable at 35-degrees-C, the optimum growth temperature of T. thyasiris and at 7.5-degrees-C, the temperature of the habitat of Thyasira flexuosa, but the activity was 40% and 3.5%, respectively, of the potential activity at 55-degrees-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.

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