DYNAMICS OF SELF-REGULATION OF PHOTOSYNTHETIC CARBON METABOLISM

Studying photosynthesis in intact plants under a diurnal cycle reveale d important emergent properties of the intact system that would not be seen from the study of individual components alone. The plant achieve s acclimation during the changing diurnal light cycle through a combin ation of light-mediated and self-regulating mechanisms. Carbon flux th rough the assimilatory segment achieves a general correspondence with the course of diurnal irradiance by altering the activation state of t he three light-activated enzymes, phosphoribulokinase (EC 2.7.1.19), r ibulose 1,5-bisphosphate carboxylase (EC 4.1.1.39) and glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.13), in response to changing light . Because individual enzymes are activated somewhat independently and do not respond precisely to increasing irradiance level, momentary imb alances in flux occur. One emergent mechanism of the intact plant, whi ch integrates these complex interactions and thus refines regulation, is self-regulation. This type of mechanism provides a means of restori ng and maintaining balance within the Calvin-cycle to adjust fluxes pr ecisely among the enzymes within the segment. Simple fluid transfer mo dels allow one to see how environmental control and self-regulation in teract to regulate the Calvin-cycle in a precise, responsive and flexi ble manner. Enzyme activities are influenced by a variety of regulator y elements which confer a degree of flexibility to regulation. The pat h to stability can be attained through any one of many combinations of enzyme activation states and metabolite levels. Consequently, there i s no unique solution to achieving a given flux rate. Understanding reg ulation involves the study of the time-dependent process by which stab ility is achieved. As a result of metabolic flexibility, patterns of m etabolite levels and enzyme activities will display some basic similar ities, but will also show marked differences between experiments.