Free and membrane-bound multienzyme complexes with Calvin cycle activities in cotton leaves

Free and membrane-bound forms of Calvin-cycle multienzyme complexes with a mol wt of 520 +/- 20 kD and 640 +/- 25 M respectively, were isolated from the cotton (Gossypium hirsutum L.) leaves. Both complexes exhibited the following enzymatic activities: ribose phosphate isomerase, phosphoribulokinase, ribulose bisphosphate carboxylase (Rubisco), phosphoglycerate kinase, and glyceraldehyde phosphate dehydrogenase. The activities of the membrane-bound multienzyme complex were significantly higher than the activities of the free complex. This difference was especially pronounced in the case of carboxylase activity. An increase in the enzymatic activity of membrane-bound multienzyme complex in comparison with the free complex is presumably due to the different number of their constituent parts. Another possible cause is the membrane-level regulation of the functional activity of the enzymes composing the complex.