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

Changes in the chemical structure of alpha-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 infrared (FTIR) difference spectroscopy in combination with specific [C-13]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 S-2/S-1 FTIR difference spectrum were affected by selective C-13 labeling of the alpha-carboxylate of Ala with L-[1-C-13] 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 alpha-methyl of Ala with L-[3-C-13] alanine had much smaller effects on the spectrum to induce isotopic bands due to a symmetric CH3 deformation coupled with the alpha-carboxylate. The isotopic bands for the alpha-carboxylate of Ala-344 showed characteristic changes during S-state cycling. The bands appeared prominently upon the S-1-to-S-2 transition and to a lesser extent upon the S-2-to-S-3 transition but reappeared at slightly upshifted frequencies with the opposite sign upon the S-3-to-S-0 transition. No obvious isotopic band appeared upon the S-0-to-S-1 transition. These results indicate that the alpha-carboxylate of C-terminal Ala-344 is structurally associated with a manganese ion that becomes oxidized upon the S-1-to-S-2 transition and reduced reversely upon the S-3-to-S-0 transition but is not associated with manganese ion(s) oxidized during the S-0-to-S-1 (and S-2-to-S-3) transition(s). Consistently, L-[1-C-13] alanine labeling also induced spectral changes in the low frequency (670-350 cm(-1)) S-2/S-1 FTIR difference spectrum.