Met246 and Asn250 in the D2 protein are essential for the operation of the quinone-Fe-acceptor complex of Photosystem II.

The chemical properties of the primary (QA) and secondary (QB) plastoquinone electron acceptors of Photosystem II (PS II) depend on their protein environments. The DE loop of the D2 protein (residues 222-262) contributes to the QA-binding site while the DE loop of the D1 protein (residues 233-266) contributes to the QB-binding environment. The roles of the invariant D2-Met246 and D2-Asn250 residues in the vicinity of the QA-binding site have been investigated in the cyanobacterium Synechocystis sp. PCC 6803 using mutants targeting both residues. The M246F strain was phenotypically similar to control cells; however, the M246A, N250A, and N250H strains had slowed photoautotrophic growth and were sensitive to high light and the addition of formate. In addition, the M246K and N250N strains were unable to assemble PS II. Chlorophyll a fluorescence measurements indicated electron transfer between QA and QB was modified in the M246A, N250A, and N250H strains, and the exchange of plastoquinol between the QB-binding site and the plastoquinone pool in the thylakoid membrane was impaired. Modified electron transfer in these mutants in the presence or absence of formate was restored by the addition of bicarbonate. In addition, thermoluminescence measurements showed a down shift in the redox midpoint potential of the QA/QA- couple in the N250A and N250H strains. These results demonstrate that Met246 and Asn250 play indispensable roles in the quinone-iron-acceptor complex, influencing both QA binding and the binding of the bicarbonate ligand to the non-heme iron that is located between QA and QB.