D1-Asp170 is structurally coupled to the oxygen evolving complex in photosystem II as revealed by light-induced Fourier transform infrared difference spectroscopy

We report both mid-frequency (1800-1200 cm(-1)) and low-frequency (670-350 cm(-1)) S-2/S-1 FTIR difference spectra of photosystem II (PSII) particles isolated from wild-type* and D1-D170H mutant cells of the cyanobacterium Synechocystis sp. PCC 6803. Both mid- and low-frequency S-2/S-1 spectra of the Synechocystis wild-type* PSII particles closely resemble those from spinach PSII samples, which confirms an earlier result by Noguchi and co-workers [Noguchi, T., Inoue, Y., and Tang, X.-S. (1997) Biochemistry 36, 14705-14711] and indicates that the coordination environment of the oxygen evolving complex (OEC) in Synechocystis is very similar to that in spinach. We also found that there is no appreciable difference between the mid-frequency S-2/S-1 spectra of wild-type* and of D1-D170H mutant PSII particles, from which we conclude that D1-Asp170 does not undergo a significant structural change during the S-1 to S-2 transition. This result also suggests that, if D1-Asp170 ligates Mn, it does not ligate the Mn ion that is oxidized during the S-1 to S-2 State transition. Finally, we found that a mode at 606 cm(-1) in the low-frequency wild-type* S-2/S-1 spectrum shifts to 612 cm(-1) in the D1-D170H mutant spectrum. Because this 606 cm(-1) mode has been previously assigned to an Mn-O-Mn cluster mode of the OEC [Chu, H.-A., Sackett, H., and Babcock, G. T. (2000) Biochemistry 39, 14371-14376], we conclude that D1-Asp170 is structurally coupled to the Mn-O-Mn cluster structure that gives rise to this band. Our results suggest that D1-Asp170 either directly Ligates Mn or Ca2+ or participates in a hydrogen bond to the Mn4Ca2+ cluster. Our results demonstrate that combining FTIR difference spectroscopy with site-directed mutagenesis has the potential to provide insights into structural changes in Mn and Ca2+ coordination environments in the different S states of the OEC.