Replacement of the methionine axial ligand to the primary electron acceptor A0 slows the A0− reoxidation dynamics in Photosystem I

The recent crystal structure of photosystem I (PSI) from Thermosynechococcus elongatus shows two nearly symmetric branches of electron transfer cofactors including the primary electron donor, P700, and a sequence of electron acceptors, A, A0 and A1, bound to the PsaA and PsaB heterodimer. The central magnesium atoms of each of the putative primary electron acceptor chlorophylls, A0, are unusually coordinated by the sulfur atom of methionine 688 of PsaA and 668 of PsaB, respectively. We [Ramesh et al. (2004a) Biochemistry 43:1369–1375] have shown that the replacement of either methionine with histidine in the PSI of the unicellular green alga Chlamydomonas reinhardtii resulted in accumulation of A0− (in 300-ps time scale), suggesting that both the PsaA and PsaB branches are active. This is in contrast to cyanobacterial PSI where studies with methionine-to-leucine mutants show that electron transfer occurs predominantly along the PsaA branch. In this contribution we report that the change of methionine to either leucine or serine leads to a similar accumulation of A0− on both the PsaA and the PsaB branch of PSI from C. reinhardtii, as we reported earlier for histidine mutants. More importantly, we further demonstrate that for all the mutants under study, accumulation of A0− is transient, and that reoxidation of A0− occurs within 1–2 ns, two orders of magnitude slower than in wild type PSI, most likely via slow electron transfer to A1. This illustrates an indispensable role of methionine as an axial ligand to the primary acceptor A0 in optimizing the rate of charge stabilization in PSI. A simple energetic model for this reaction is proposed. Our findings support the model of equivalent electron transfer along both cofactor branches in Photosystem I.