RIBULOSE 1,5-BISPHOSPHATE CARBOXYLASE FROM THE THERMOPHILIC, ACIDOPHILIC ALGA, CYANIDIUM-CALDARIUM (GEITLER) - PURIFICATION, CHARACTERIZATION AND THERMOSTABILITY OF THE ENZYME

As an initial stage in the study of proteins from thermophilic algae, the enzyme ribulose 1,5-bisphosphate carboxylase 2-phospho-d-glycerate carboxylyase (dimerizing, EC 4.1.1.39) was purified 11-fold from the thermophilic alga Cyanidium caldarium, with a 24% recovery. This purified enzyme appeared homogeneous on polyacrylamide gels and could be dissociated into two subunit types of molecular weights 55 000 and 14 900. The optimal assay temperature was 42.5°C, whilst enzyme purified from Chlorella spp. showed maximum activity at 35°C. The thermostability of Cyanidium ribulose 1,5-bisphosphate carboxylase was considerably greater than that of the Chlorella enzyme, and the presence of Mg2+ and HCO3- further enhanced this heat stability. A break in the Arrhenius plot occured at 20°C for Chlorella ribulose 1,5-bisphosphate carboxylase and at 36°C for the enzyme from Cyanidium. It is suggested that the thermostability of Cyanidium ribulose 1,5-bisphosphate carboxylase is a result of an inherent stability of the enzyme molecule which permits efficient CO2 fixation at high temperatures but results in low activity i n the mesophilic temperature range. © 1979.