Evidence from biosynthetically incorporated strontium and FTIR difference spectroscopy that the C-terminus of the D1 polypeptide of photosystem II does not ligate calcium

Recent FTIR studies have provided evidence that the C-terminal alpha-COO- group of the D1 polypeptide at D1-Ala344 is a unidentate ligand of a Mn ion in photosystem II [Chu, H.-A., Hiller, W., and Debus, R. J. (2004) Biochemistry 43, 3152-3166; Kimura, Y., Mizusawa, N., Yamanari, T., Ishii, A., and Ono, T.-A. (2005) J. Biol. Chem. 280, 2078-2083]. However, the FTIR data could not exclude Ca ligation. Furthermore, the recent similar to 3.5 A X-ray crystallographic structural model positions the (alpha-COO(-)group of D1-Ala344 near a Ca ion [Ferreira, K. N., Iverson, T. M., Maghlaoui, K., Barber, J., and Iwata, S. (2004) Science 303, 1831-1838]. Therefore, to conclusively establish whether the alpha-COO- group of D1-Ala344 ligates Mn or Ca, the symmetric carboxylate stretching mode of the alpha-COO- group of D1-Ala344 was identified in the S-2-minus-S-1 FTIR difference spectrum of PSII particles having Sr substituted for Ca. Cells of the cyanobacterium Synechocystis sp. PCC 6803 were propagated in media having Sr substituted for Ca and containing either L-[1-C-13]alanine or unlabeled (C-12) alanine. The S-2-minus-S-1 FTIR difference spectra of the purified PSII particles show that substituting Sr for Ca alters several carboxylate stretching modes, including some that may correspond to one or more metal ligands, but importantly does not alter the symmetric carboxylate stretching mode of the alpha-COO- group of D1-Ala344. In unlabeled PSII particles, this mode appears at similar to 1356 cm(-1) in the S, state and at either similar to 1337 or similar to 1320 cm(-1) in the S-2 state, irrespective of whether the PSII particles contain Ca or Sr. These data are inconsistent with Ca ligation and show, therefore, that the C-terminal alpha-COO- group of the D1 polypeptide ligates a Mn ion. These data also show that substituting Ca with the larger Sr ion perturbs other unidentified carboxylate groups, at least one of which may ligate the Mn-4 cluster.