Ribulose diphosphate carboxylase from freshly ruptured spinach chloroplasts having an in vivo Km

The properties of a form of ribulose diphosphate carboxylase having a high affinity for CO2 have been studied. Its apparent $\text{K}m(\text{HCO}_{3}{}^{-})$ of 0.5 to 0.8 mM (pH 7.8) and calculated $\text{K}m(\text{CO}_{2})$ of 11 to 18 μM are comparable to the values exhibited by intact chloroplasts during photosynthesis. This form of the enzyme was released from chloroplasts in hypotonic media and was unstable, rapidly converting to a form having a high $\text{K}m(\text{HCO}_{3}{}^{-})$ of 20 to 25 mM similar to that for the purified enzyme. Incubation of the enzyme with MgCl2 and HCO3 - yielded a third form with an intermediate $\text{K}m(\text{HCO}_{3}{}^{-})$ of 2.5 to 3.0 mM. The low Km form had sufficient activity both at air levels of CO2 and at saturating CO2 to account for the rates of photosynthesis by intact chloroplasts. The low Km form could be stabilized in the presence of ribose 5-phosphate, adenosine triphosphate, and MgCl2, at low temperatures for up to 2 hours.