Correlation of redox levels of component electron carriers with total electron flux in an electron-transport system. P-700 and the photoreduction of NADP + in chloroplast fragments

Abstract

A mathematical analysis is described which measures the effects of actinic light intensity and concentration of an artificial electron donor on the steady-state light-induced redox level of a reaction-center pigment (e.g. P-700) and on the overall light-induced electron flux (e.g. reduction of NADP +). The analysis led to a formulation (somewhat similar to the Michaelis-Menten equation for enzyme kinetics) in which a parameter, I 1 2 , is defined as the actinic light intensity that, at a given concentration of electron donor, renders the reaction-center pigment half oxidized and half reduced. To determine the role of a presumed reaction-center pigment, I 1 2 is compared with another parameter, equivalent to I 1 2 , that is obtained independently of the reaction-center pigment by measuring the effect of actinic light intensity and concentration of electron donor on the overall electron flow. The theory was tested and validated in a model system with spinach Photosystem I chloroplast fragments by measurements of photooxidation of P-700 and light-induced reduction of NADP + by reduced 2,6-dichlorophenolindophenol. A possible extension of this mathematical analysis to more general electron-transport systems is discussed.