Lateral Diffusion of Plastocyanin and Plastoquinol in Thylakoid Membranes

Abstract

In thylakoid membranes of higher plants the electron transport complexes are heterogeneously distributed with Photosystem (PS) II in appressed and PS I in non-appressed thylakoid membranes [1] in contrast to cytochrome (cyt) bt₆/f complex, which is uniformly distributed throughout these membrane regions [2, 3]. This organization requires a long-range electron transport from PS II to PS I that is not fully understood [4]. Two electron carriers could mediate this long-range electron transport, the hydrophobic plastoquinol (PQH₂) and the hydrophilic plastocyanin. Plastoquinol diffuses in the lipid bilayer and transfers electrons from PS II to cyt b₆/f. Cyt b₆/f in stroma regions can only function efficiently in linear electron transport if PQH₂ reaches these complexes within a time considerably faster than that of the rate-limiting step. The rate, at which the PQH₂ molecule transfers electrons from PS II to the cyt b₆/f complex is determined by its diffusion coefficient and the rate of binding at the cyt b₆/f complex. Estimates of 10⁻⁸ and 10⁻⁶ cm²s⁻¹ have been made for the diffusion coefficient of PQH₂ that would be consistent with the times of electron transfer observed and the average distance between the two thylakoid regions [4–6]. Several lines of evidence indicate that the half time of diffusion must be considerably faster than that of the oxidation of PQH₂ of about 13 ms which is the slowest step in the chain [7,8].