Effect of Betaine on Enzyme Activity and Subunit Interaction of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase from Aphanothece halophytica

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Abstract
The presence of betaine, a quaternary ammonium compound, at a concentration (0.5 molar) reported to accumulate inside Aphanothece halophytica in response to increasing external salinity, slightly promoted ribulose-1,5-bisphosphate (RuBP) carboxylase activity. KCl at 0.25 molar inhibited RuBP carboxylase about 55%. Betaine relieved the inhibition by 0.25 m KCl and the original uninhibited activity was restored at 1 m betaine. Other osmoregulatory solutes such as sucrose and glycerol also reduced KCl inhibition, though to a lesser extent than betaine. Proline had no effect. The protective effect of betaine against KCl inhibition of RuBP carboxylase activity was also observed in other cyanobacteria, i.e. Synechococcus ACMM 323, Plectonema boryanum, and Anabaena variabilis, and in the photosynthetic bacterium Rhodospirillum rubrum but not in Chromatium vinosum. Apart from betaine, other quaternary ammonium compounds, i.e. sarcosine and trimethylamine-N-oxide (TMAO), but not glycine, also protected the enzyme against KCl inhibition and the effectiveness of such compounds appeared to correlate with the extent of N-methylation. Heat and cold inactivation of the enzyme could be protected by either betaine or KCl. However, best protection occurred when both betaine and KCl were present together. The Km (CO2) was not altered by either betaine or KCl, nor when they were present together. However, the Km (RuBP) was increased about 5-fold by KCl, but was unaffected by betaine. The presence of betaine together with KCl lowered the KCl-raised Km (RuBP) by about half. The extent of the dissociation of the enzyme molecule under the condition of low ionic strength was reduced by either betaine or KCl alone and more so when they were present together. Glycine, sarcosine, and TMAO were more effective than betaine or KCl in lowering the extent of the dissociation of the enzyme molecule.