Electrons and Proton Transfer in Chloroplasts In Silico: 1. The Effect of Topological Factors on Energy Coupling in Chloroplasts with a Nonuniform Distribution of Protein Complexes

This paper presents a theoretical study of the effects of topological factors (density of thylakoid packing in grana) on the efficiency of energy coupling in chloroplasts. The study is based on a mathematical model of electron and proton transport processes coupled to ATP synthesis in chloroplasts. The model was developed by the authors earlier, and the nonuniform distribution of electron transport and ATP synthase complexes in the membranes of granal and intergranal thylakoids was taken into account in the model. The results of numerical experiments enabled the analysis of the distribution of lateral profiles of the transmembrane pH difference and the concentrations of mobile plastoquinone and plastocyanin electron transporters in granal and intergranal thylakoids and the dependence of this distribution on the metabolic state of class B chloroplasts (photosynthetic control state or the conditions of intensive ATP synthesis). Moreover, the influence of topological factors (the density of thylakoid packing in grana and the degree of thylakoid swelling) that affect the rate of diffusion of protons and mobile electron carriers in the intrathylakoid space and in the interthylakoidal gap was investigated. The results of numerical experiments that involved the variation of geometric parameters of the system revealed the influence of thylakoid thickness and the distance between the granal thylakoids on the lateral pH profiles inside the thylakoids (pH$_i$) and in the interthylakoidal gap (pH$_0$). Acidification of the intrathylakoid space characterized by the pH$_i$ value increased concomitantly to the increase of the width of the interthylakoidal gap lo and decreased concomitantly to the increase of the width of the intrathylakoidal space l$_i$.