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.
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, I1/2, is defined as the actinic light intensity that, at a given concentration of electron donro, renders the reaction-center pigment half oxidized and half reduced. To determine the role of a presumed reaction-center pigment, I1/2 is compared with another parameter, equivalent to I1/2, that is obtained independently of the reaciton-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.
D002736:Chloroplasts / Q000378:metabolism*; D003580:Cytochromes / Q000378:metabolism*; D003624:Darkness; D004579:Electron Transport; D007700:Kinetics; D008027:Light; D008433:Mathematics; D009249:NADP / Q000378:metabolism*; D010084:Oxidation-Reduction; D010788:Photosynthesis; D010944:Plants
D003580:Cytochromes; D009249:NADP