Contribution of Interhelical Weak Interactions to the Regulation of Protein-Gated Electron Transfer in the Membrane Milieu

Ilan Samish1, Haim J. Wolfson2, Avigdor Scherz, Weizmann Institute of Science; 2, Tel Aviv University

Protein conformational flexibility and reversible changes between active and inactive protein conformations, defined as protein gating, play key roles in protein systems. Although functionally distinct in many catalytic cycles, the underlying mechanisms remain enigmatic. We investigated protein motifs involved in the protein-gated electron transfer (ET) between two quinones (QA and QB) in Type II photosynthetic reaction centers. These protein complexes convert light energy into electrochemical potential by means of transmembrane ET through several cofactors that are held by two protein subunits. Multiple structural alignment (MUSTA algorithm) of all reaction centers located a conserved high-packing sub-region between the two subunits involving high-packing sequence motifs such as [S/C/A]xxxG. The region contained a conserved intersubunit hydrogen bond (HB), acting both via side-chains and via a C-alpha hydrogen donor, although the participating residues may vary. Rotamer-library-based combinatorial mutagenesis of the HB donor (D1-212) suggested that different polar residues might participate in the bond. In vivo combinatorial mutagenesis of the HB donor changed the kinetics and thermodynamics of ET (S. Kerner, I. Samish, D. Kaftan, H. Kless, A. Scherz, In publication). The change of enthalpy of activation was in correlation with the polarity and packing of the mutated residues. We suggest that reversible dissociation of this intersubunit HB combined with a strong packing motif plays a regulatory role in the final steps of photosynthetic charge separation. This mechanism of protein-gated ET by HB dissociation may be general and is probably relevant to additional membrane complexes.