Figure 5.

Temperature dependence of the phosphatase activity of the isolated membrane enzyme and the enzyme bound to thylakoid membranes. Phosphatase assays were performed with ³²P-labeled phosphopeptides as a substrate. The ³²P-labeled phosphopeptides were obtained from radioactively labeled thylakoid membranes by trypsin treatment. The phosphatase activity of isolated phosphatase (A) and of intact thylakoids (B) was measured at 22°C, 27°C, 35°C, or 42°C. The initial phosphopeptide concentrations were 10 μm, based on the ³²P content. C, Comparison of the increase in dephosphorylation rate constants at different temperatures for the isolated phosphatase and the membrane-bound enzyme.