LIMITATION OF THE RATE OF RIBULOSEBISPHOSPHATE CARBOXYLASE ACTIVATIONBY CARBAMYLATION AND RIBULOSEBISPHOSPHATE CARBOXYLASE ACTIVASE ACTIVITY - DEVELOPMENT AND TESTS OF A MECHANISTIC MODEL

A mechanistically-based model of light-mediated activation of ribulose -1,5-bisphosphate carboxylase/oxygenase (Rubisco) is developed. The mo del describes the kinetics of Rubisco activation following a relativel y rapid increase in photon flux density (PPFD) from an initially low l evel. Underlying the model is the assumption that there are two slow p rocesses that could potentially limit the rate of light-mediated Rubis co activation. These processes are the addition of the activator CO2 t o the large subunit of Rubisco, which is accompanied by a conformation al change in the enzyme (carbamylation), and activase-mediated removal of ribulose 1,5-bisphosphate from the inactive form of the enzyme. Th e contribution of these slow processes to the overall activation kinet ics of Rubisco was resolved by measuring Rubisco activation in whole s pinach leaves using non-steady-state CO2 exchange. It was found that w hen the change in PPFD was relatively small and a correspondingly smal l proportion of the Rubisco pool was activated, the kinetics of activa tion were highly sensitive to the intercellular CO2 concentration (c(i )). The apparent rate constant for activation under these conditions w as found to be similar to that for the carbamylation of purified spina ch Rubisco. When the change in PPFD and the proportion of Rubisco acti vated was relatively large, however, the kinetics of Rubisco activatio n were almost completely CO2 insensitive and were consistent with thos e of an enzyme-catalysed reaction. It is suggested that (1) CO2-insens itive activation reflects the operation of Rubisco activase and (2) th e increasing CO2 sensitivity seen as the change in PPFD decreases refl ects a transition to limitation by carbamylation.