Structural changes of D1 C-terminal α-carboxylate during S-state cycling in photosynthetic oxygen evolution

Abstract

Changes in the chemical structure of α-carboxylate of the D1 C-terminal Ala-344 during S-state cycling of photosynthetic oxygen-evolving complex were selectively measured using light-induced Fourier transform infra-red (FTIR) difference spectroscopy in combination with specific [ 13C]alanine labeling and site-directed mutagenesis in photosystem II core particles from Synechocystis sp. PCC 6803. Several bands for carboxylate symmetric stretching modes in an S2/S1 FTIR difference spectrum were affected by selective 13C labeling of the α-carboxylate of Ala with L-[1-13C]alanine, whereas most of the isotopic effects failed to be induced in a site-directed mutant in which Ala-344 was replaced with Gly. Labeling of the α-methyl of Ala with L-[3-13C]alanine had much smaller effects on the spectrum to induce isotopic bands due to a symmetric CH3 deformation coupled with the α-carboxylate. The isotopic bands for the α-carboxylate of Ala-344 showed characteristic changes during S-state cycling. The bands appeared prominently upon the S1-to-S2 transition and to a lesser extent upon the S2-to-S3 transition but reappeared at slightly upshifted frequencies with the opposite sign upon the S 3-to-S0 transition. No obvious isotopic band appeared upon the S0-to-S1 transition. These results indicate that the α-carboxylate of C-terminal Ala-344 is structurally associated with a manganese ion that becomes oxidized upon the S1-to-S2 transition and reduced reversely upon the S3-to-S0 transition but is not associated with manganese ion(s) oxidized during the S0-to-S1 (and S2-to-S3) transition(s). Consistently, L-[1-13C]alanine labeling also induced spectral changes in the low frequency (670-350 cm-1) S 2/S1 FTIR difference spectrum.