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Ribulose Diphosphate Carboxylase from Freshly Ruptured Spinach Chloroplasts Having an in Vivo Km[CO₂] ¹

^a Department of Chemistry, The University of Arizona, Tucson, Arizona 85721

The properties of a form of ribulose diphosphate carboxylase having a high affinity for CO₂ have been studied. Its apparent Km(HCO₃^–) of 0.5 to 0.8 mM (pH 7.8) and calculated Km(CO₂) 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 Km(HCO₃^–) 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 Km(HCO₃^–) 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.