Tuning of the Chl and Chl properties in photosystem II by site-directed mutagenesis of neighbouring amino acids.

Photosystem II is the water/plastoquinone photo-oxidoreductase of photosynthesis. The photochemistry and catalysis occur in a quasi-symmetrical heterodimer, D1D2, that evolved from a homodimeric ancestor. Here, we studied site-directed mutants in PSII from the thermophilic cyanobacterium Thermosynechoccocus elongatus, focusing on the primary electron donor chlorophyll a in D1, Chl, and on its symmetrical counterpart in D2, Chl, which does not play a direct photochemical role. The main conserved amino acid specific to Chl is D1/T179, which H-bonds the water ligand to its Mg, while its counterpart near Chl is the non-H-bonding D2/I178. The symmetrical-swapped mutants, D1/T179I and D2/I178T, and a second Chl mutant, D2/I178H, were studied. The D1 mutations affected the 686 nm absorption attributed to Chl, while the D2 mutations affected a 663 nm feature, tentatively attributed to Chl. The mutations had little effect on enzyme activity and forward electron transfer, reflecting the robustness of the overall enzyme function. In contrast, the mutations significantly affected photodamage and protective mechanisms, reflecting the importance of redox tuning in these processes. In D1/T179I, the radical pair recombination triplet on Chl was shared onto a pheophytin, presumably Phe and the detection of Phe supports the proposed mechanism for the anomalously short lifetime of Chl; e.g. electron transfer quenching by Q of Phe after triplet transfer from Chl. In D2/I178T, a charge separation could occur between Chl and Phe, a reaction that is thought to occur in ancestral precursors of PSII. These mutants help understand the evolution of asymmetry in PSII.