Lateral heterogeneity of the proton potential along the thylakoid membranes of chloroplasts.

This work is devoted to critical analysis and reexamination of the problem of lateral heterogeneity of the trans-thylakoid pH difference (ΔpH=pH-pH) in thylakoid membranes of chloroplasts. Correct measurements of ΔpH may be complicated by nonuniform partitioning of the protons pumped into the lumen of granal (stacked) and stroma-exposed thylakoids. We have compared results of ΔpH estimations in isolated bean chloroplasts by two different methods. One of the methods is based on the use of pH-sensitive spin-probes. Another approach relies on the analysis of pH-dependent post-illumination reduction of P - oxidized reaction center of photosystem I (PSI). Both methods lead to virtually the same values of ΔpH, as measured in the state of photosynthetic control when the ATP synthase complexes are inactive (ΔpH2.3-2.5 at pH8). Under the photophosphorylation conditions (metabolic state 3), ΔpH decreases due to the proton drain from the lumen to stroma via active ATP synthases (ΔpH1-2 at pH8). In this state, ΔpH values derived from kinetic data are smaller than ΔpH measured with the pH-probing amines. Such a discrepancy can be explained by the coexistence of thylakoids with different pH established in granal and stromal thylakoids. The kinetic method is equivalent to the use of a "local pH-meter", which is sensitive to less significant decrease in pH inside the stroma-exposed thylakoids. Otherwise, pH-indicating probes give information on pH values averaged out at both granal and stromal thylakoids. Experimental results have been analyzed within the framework of our mathematical model developed for simulation of electron and proton transport processes in laterally heterogeneous thylakoids. The model provides a reasonable description of experimental data, supporting the notion that the long-range diffusion of protons within the lumen and obstructed diffusion of mobile electron carriers (PQH and Pc) influence the lateral profiles of pH along the thylakoid membranes. The model predicts significant alkalization of the inter-thylakoid gap and the establishment of nonuniform lateral profiles of ΔpH under the photophosphorylation conditions. These results are discussed in the context of the problem of energy coupling in laterally heterogeneous lamellar system of chloroplasts.