Photophosphorylation as Function of ADP Concentration at Varying Transmembrane Proton Gradients

Open Access
Publisher Website
Google Scholar
Abstract
Rates of photophosphorylation were measured at constant saturating phosphate concentration , varying ADP concentration , and varying light intensity. As the transmembrane proton gradient is decreased by phosphorylation to different extents depending on the concentration of ADP . rates of ATP formation obtained at the different ADP concentrations were plotted versus the actual steady state ΔpH (in the absence of ΔΨ) during the course of the reaction . ΔpH was monitored by the calibrated 9-aminoacridine fluorescence technique. In secondary plots phosphorylation as function of ADP concentration at different constant ΔpH values were obtained . The results indicate Michaelis-Menten kinetics. The true Km for ADP is virtually unaffected by ΔpH whereas Vmax (at ADP saturation ) strongly depends on ΔpH . The results are discussed in the framework of a simple enzyme kinetic model which considers the intrathylakoidal proton (at constant external pH ) as a third substrate for ATP formation. The model is capable o f explaining the reported results as well as a variety of important results from the literature.

This publication has 0 references indexed in Scilit: