Ribulose Diphosphate Carboxylase from Freshly Ruptured Spinach Chloroplasts Having an in Vivo Km[CO2]

The properties of a form of ribulose diphosphate carboxylase having a high affinity for CO(2) have been studied. Its apparent Km(HCO(3) (-)) of 0.5 to 0.8 mm (pH 7.8) and calculated Km(CO(2)) of 11 to 18 mum 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 Km(HCO(3) (-)) of 20 to 25 mm similar to that for the purified enzyme. Incubation of the enzyme with MgCl(2) and HCO(3) (-) yielded a third form with an intermediate Km(HCO(3) (-)) of 2.5 to 3.0 mm.The low Km form had sufficient activity both at air levels of CO(2) and at saturating CO(2) 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 MgCl(2), at low temperatures for up to 2 hours.