Chapter 16: Functional Pattern of Photosystem II

This chapter reviews our current knowledge on the energetics, kinetics, and mechanism of electron transfer in Photosystem II (PS II), excluding oxidative water splitting, which is outlined in Chapter 17. Similarities of PS II with the reaction centers of anoxygenic (non-oxygen evolving) purple bacteria in the general functional and structural organization of charge separation and quinol formation are outlined. The striking differences are discussed that emerge from the thermodynamic requirements for water oxidation to molecular oxygen and four protons, i.e., the generation of electron holes of sufficiently strong oxidizing power. An understanding of the nature and the properties of the photoactive component P680 as the site of photochemical hole formation is of central relevance. The unique properties of P680 are best described by assigning P680 to a special multichromophoric unit (Chl a)₄ Pheo_x with x = 0, 1 or 2 (the value of x is a matter of controversy), rather than a single chromophore or special pair. The possible electronic structures of ¹P680*, ³P680 and P680^+˙ are discussed. It is emphasized that the surrounding protein matrix forms an integral and decisive part of the functional properties of P680, in particular for its extraordinarily high reduction potential. Evidence is presented that in the first electron transfer event of charge separation a “monomeric” type Chl a within P680 transfers an electron from its excited singlet state to an associated pheophytin (Pheo) a molecule, which acts as the primary electron acceptor. This event is followed by rapid spin redistribution, leading to predominant localization of the electron hole on a Chl a in P680^+˙ designated as P_D1, which is part of a “dimeric” structural motif termed P_D1P_D2 and is in close proximity to the redox-active tyrosine Y_Z. The subsequent reactions of P680^+˙reduction by Y_Z and of PQH₂ formation via a two-step, one-electron reaction sequence with Q$A−˙$ as reductant are described with special emphasis on the role of the protein dynamics for energetics and kinetics.