Impact of replacement of D1 C-terminal alanine with glycine on structure and function of photosynthetic oxygen-evolving complex

Abstract

The C-terminal alanine 344 (Ala-344) in the D1 protein of photosystem II is conserved in all of the organisms performing oxygenic photosynthesis. A free α-COO- of Ala-344 has been proposed to be responsible for ligating the Mn cluster. Here, we constructed a mutant having D1 in which D1-Ala-344 was replaced with glycine (Gly) in cyanobacterium Synechocystis sp. PCC 6803. The effects of this minimal change in the side group from methyl to hydrogen on the properties of the oxygen-evolving complex were comprehensively investigated using purified core particles. The mutant grew photoautotrophically, and little change was observed in the protein composition of the oxygen-evolving core particles. The Gly-substituted oxygen-evolving complex showed small but normal S2 multiline and enhanced g = 4.1 electron spin resonance signals and S2-state thermoluminescence bands with slightly elevated peak temperature. The Gly substitution resulted in distinct but relatively small changes in a few bands arising from the putative carboxylate ligand for the Mn cluster in the mid-frequency (1800-1000 cm -1) S2/S1 Fourier transform infrared difference spectrum. In contrast, the low frequency (670-350 cm-1) S 2/S1 Fourier transform infrared difference spectrum was markedly changed by the substitution. The results indicate that the internal structure of the Mn cluster and/or the interaction between the Mn cluster and its ligand are considerably altered by a simple change in the side group, from methyl to hydrogen, at the C-terminal of the D1 protein.