Academic and University Center, Zamek 136,CZ-37333 Nove Hrady, Czech Republic
Non-covalent interactions between transmembrane (TM) helices of membrane- protein complexes may affect subunit association and domain flexibility. Conformational changes of flexible protein domains have been suggested to be involved in electron-transfer gating in photosynthetic reaction centers (RC). We set out to study the role of non-covalent inter-helical interactions between two subunits of photosystem II (PSII) in the gating mechanism of electron- transfer between the quinones QA and QB. The core of PSII RC is made of two protein subunits, D1 and D2. Two TM helices of these subunits are associated in their membranal region through a single hydrogen bond (H-bond) embedded within a common helix packing motif (GxxxSxxxG). A putative H-donor in this contact point, D1-Ser212, was mutated to all other amino acids in the cyanobacterium Synechocystis sp. PCC 6803. Thirteen mutations were found to support photoautotrophic growth excluding bulky residues that are positively charged (Arg, Lys), or aromatic (Phe, Trp, Tyr and His). In the photoautotrophic mutants, the effect of the mutations on forward electron transfer rates and charge recombination was evaluated following fluorescence decay and thermoluminescence as a function of temperature. The results showed that weakly polar residues such as Ser (wild type), Thr, Ala and Cys had similar rate constants (kAB) of electron transfer over a range of temperatures (10 – 40 °C). On the other hand, strong polar and/or bulkier residues, such as Gln, Asn, Glu and Asp, had lower kAB, which increased in a temperature-dependent manner. The temperature effect on kAB varied among the mutants suggesting that protein conformations influence electron transfer rates. Moreover, good correlations were found between the activation enthalpies of forward electron transfer dH‡ and the activation energy of charge recombination (EA), and between dH‡ and mean packing values, especially when comparing residues that can form H-bonds. These findings suggest that weak hydrogen bonding and polar interactions at the TM helical interface between the D1 and D2 subunits affect local protein conformations involved in the gating of electron-transfer from QA to QB in PSII.