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Limited proteolysis of ribulose‐bisphosphate carboxylase with trypsin or endopeptidase Lys C causes the loss of the carboxylase and oxygenase activities, without disrupting the binding of bisphosphates or the reactions normally associated with activation of the enzyme. Gel electrophoresis of the non‐denatured enzyme indicates that the L8S9quaternary structure of large and small subunits is unaffected by this treatment. However, electrophoresis in denaturing conditions shows that the L subunit is smaller as a result of the removal of two short polypeptides, each approx. 1000 Da. The loss of activity is associated with the removal of the second peptide. The amino acid composition of the isolated peptides indicates that both originate from the N‐terminus of the L subunit. Over protracted periods of exposure to the proteases a third smaller peptide is also released from the N‐terminus of the L subunit. The S subunit is not affected by these treatments. The carboxylase isolated from a number of photosynthetic species exhibits a differential response to this limited proteolysis.

The reactions catalysed by ribulosebisphosphate carboxylase/oxygenase have been studied by high resolution proton NMR spectroscopy. The protons of the substrate ribulose-P 2and the products of the carboxylase and oxygenase reactions are readily assigned to the various carbon centres. Half of the 3-phosphoglycerate molecules generated during carboxylase turnover of ribulose-P 2in saturating CO 2concentrations acquire deuterium from the solution at the C 2position, whereas the other 3-phosphoglycerate molecules and also the 3-phosphoglycerate produced as a result of the oxygenase activity contain no deuterium of this type. However, the addition of activated enzyme (i.e., in the presence of an effector) to a solution of ribulose-P 2in the near absence of either CO 2or O 2catalyses the exchange of the C 3proton with the deuterium of the solution.

Purified RuBP carboxylase requires activation by reaction with CO 2and a divalent metal ion. Mg 2+is the most effective metal ion, and is probably involved in activation in vivo. Ca 2+is reported not to be an activator. Several oxyanions, including phosphate esters, are effectors of activation of RuBP carboxylase by Co 2and Mg 2+. It is now shown that Ca 2+is an effective activator of RuBP carboxylase and that PO 43−, FBP, NADPH and 6-phosphogluconate are effectors of this activation. The ratio of oxygenase to carboxylase activity of enzyme activated with Ca 2+is similar to that for the enzyme activated with Mg 2+.