Studies on the Intrinsic Membrane Proteins of Photosystem II from Spinach

Abstract

AbstractThe key reactions of photosynthetic water oxidation by visible light take place in photosystem II. Here we report on our recent results about functional studies on a transmembrane pigment‐protein complex of photosystem II. Using O2‐evolving PSII membrane fragments from spinach we isolated the intrinsic membrane proteins and investigated the effect of solubilization with three different non‐ionic detergents, β‐dodecylmaltoside (β‐DM), Triton X‐100 and β‐N‐octylglycopyranoside (β‐NOGP), on two parameters: 77 K fluorescence emission and II. PSII electron transport (DPC → DCIP). The following results were obtained: All the detergents dramatically modified the 77 K fluorescence emission spectra at/or near their critical micelle concentrations (CMC). The yield per flash or rate of DCIP‐reduction as an indicator of electron transport activity markedly depended on the type and concentration of the detergent. β‐NOGP caused a decrease of the electron transfer already below its CMC, whereas β‐DM and Triton X‐100 did not reveal a distinct modification at submicellar concentrations. In the case of β‐NOGP the half‐maximum activity is observed at about 20 mM. On the basis of a simple model assuming an apparent equilibrium between only two stable states (non‐solubilized and β‐NOGP‐solubilized PSII intrinsic membrane proteins) we calculated an apparent equilibrium constant Keq to 48.2 and a free solubilization energy of ΔGsol = −9.6 kJ/mol. The results are discussed with special emphasis on two questions: a) To what extent have the polypeptide subunits and the pigment and redox group arrangement the same structure and configuration in detergent‐solubilized state as in the thylakoid membrane and PSII membrane fragments, respectively? b) How can we interprete the solubilization properties of the different detergents on the basis of their physical and chemical features and the structural architecture of the PSII membrane fragments?