Roles of the Large and Small Subunits of Ribulose-1, 5-Bisphosphate Carboxylase in the Activation by CO2 and Mg2+1

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Abstract
Spinach leaf ribulose-1,5-bisphosphate (RuP2) carboxylase, which normally has a quaternary structure composed of eight large subunit (A) and eight small ones (B), was activated by CO2 and Mg2+ when subunit B was depleted. RuP2 carboxylase isolated from the purple non-sulfur photosynthetic bacterium, Rhodospirillum rubrum, which was reported to be a dimer of subunit A, was also activated by these activators. In the catalytic process of the enzyme reaction after activation by CO2 and Mg2+, spinach RuP2 carboxylase lacking subunit B or the enzyme treated with anti-subunit B serum (anti-[B]), exhibited a higher Km(CO2) value than the carboxylase which had been reconstituted or treated with non-immunized control serum. R. rubrum RuP2 carboxylase also showed a much higher Km(CO2) value than the native spinach enzyme. In the activation process, anti-[B]-treated spinach RuP2 carboxylase exhibited a lower degree of activation by CO2 or Mg2+ than the control serum-treated enzyme, although equilibrium constants for CO2 and Mg2+ did not change. No interaction between subunit B and CO2 molecules was observed by a fluorometric study with 2-p-toluidinonaphthalene-6-sulfonate. R. rubrum RuP2 carboxylase, like spinach carboxylase, was activated by NADPH or 6-P-gluconate. R. rubrum RuP2 carboxylase/oxygenase possessed properties basically similar to those of the spinach enzyme. Our results thus lead to the tentative conclusion that subunit A is primarily responsible for the enzyme activation and catalysis, and that subunit B is involved in sustaining the conformation of the enzyme molecule which is necessary for effective activation.