title: Interaction of constituent subunits in ribulose 1,5-bisphosphate carboxylase from Aphanothece halophytica.
abstract: Ribulose 1,5-bisphosphate carboxylase-oxygenase (RuBisCO) from the halophilic cyanobacterium, Aphanothece halophytica, dissociates into catalytic core (large subunit A oligomer) and small subunit B under low ionic strength during sucrose density gradient centrifugation. Supplementation of KCl, NaCl, or K2SO4 ( [I] = 0.3 M) partly prevents the dissociation, the preventive effect of divalent cation salts such as MgCl2 and CaCl2 being more effective than monovalent cation salts. RuBisCO with its higher-plant-type molecular form can be isolated from the cyanobacterial extracts using gradient medium containing 0.3 M KCl, 20 mM MgCl2, and 10 mM CaCl2. The isolated enzyme contains large subunit A and small subunit B in a molar ratio of approximately 1:1, estimated from the densitometric scanning of Coomassie blue-stained gels. During the second sucrose density gradient centrifugation to remove minor contaminants, a small amount of subunit B is depleted from the holoenzyme. Determination of the molecular weight by equilibrium centrifugation and electron microscopic observation have confirmed that the cyanobacterial RuBisCO has an A8B8-type structure. The enzyme activity per se is found to be sensitive to concentrations of salts, and small subunit B is obligatory for the enzyme catalysis. It has been shown that the more the enzyme activity is inhibited by salts, the tighter the association of subunit B becomes. It is likely that the active enzyme retains the loose conformational structure to such an extent that the dissociable release of subunit B from the holoenzyme in vivo is not allowed.
mesh_terms: D002122:Calcium Chloride / Q000494:pharmacology; D000458:Cyanobacteria / Q000201:enzymology*; D046911:Macromolecular Substances; D008274:Magnesium / Q000494:pharmacology; D015636:Magnesium Chloride; D008854:Microscopy, Electron; D008970:Molecular Weight; D011189:Potassium Chloride / Q000494:pharmacology; D012273:Ribulose-Bisphosphate Carboxylase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013431:Sulfates / Q000494:pharmacology
chemical_list: D046911:Macromolecular Substances; D013431:Sulfates; D015636:Magnesium Chloride; C031512:potassium sulfate; D011189:Potassium Chloride; D012273:Ribulose-Bisphosphate Carboxylase; D008274:Magnesium; D002122:Calcium Chloride