title: Interactions between cytochrome c2 and the photosynthetic reaction center from Rhodobacter sphaeroides: the cation-pi interaction.
abstract: The cation-pi interaction between positively charged and aromatic groups is a common feature of many proteins and protein complexes. The structure of the complex between cytochrome c(2) (cyt c(2)) and the photosynthetic reaction center (RC) from Rhodobacter sphaeroides exhibits a cation-pi complex formed between Arg-C32 on cyt c(2) and Tyr-M295 on the RC [Axelrod, H. L., et al. (2002) J. Mol. Biol. 319, 501-515]. The importance of the cation-pi interaction for binding and electron transfer was studied by mutating Tyr-M295 and Arg-C32. The first- and second-order rates for electron transfer were not affected by mutating Tyr-M295 to Ala, indicating that the cation-pi complex does not greatly affect the association process or structure of the state active in electron transfer. The dissociation constant K(D) showed a greater increase when Try-M295 was replaced with nonaromatic Ala (3-fold) as opposed to aromatic Phe (1.2-fold), which is characteristic of a cation-pi interaction. Replacement of Arg-C32 with Ala increased K(D) (80-fold) largely due to removal of electrostatic interactions with negatively charged residues on the RC. Replacement with Lys increased K(D) (6-fold), indicating that Lys does not form a cation-pi complex. This specificity for Arg may be due to a solvation effect. Double mutant analysis indicates an interaction energy between Tyr-M295 and Arg-C32 of approximately -24 meV (-0.6 kcal/mol). This energy is surprisingly small considering the widespread occurrence of cation-pi complexes and may be due to the tradeoff between the favorable cation-pi binding energy and the unfavorable desolvation energy needed to bury Arg-C32 in the short-range contact region between the two proteins.
authors: Department of Physics, University of California at San Diego, 9500 Gilman Drive, La Jolla, California 92093, USA.
authors: M L
authors: ML
authors: Paddock
authors: K H
authors: KH
authors: Weber
authors: C
authors: C
authors: Chang
authors: M Y
authors: MY
authors: Okamura
mesh_terms: D024322:Amino Acids, Aromatic / Q000737:chemistry* / Q000235:genetics; D001120:Arginine / Q000235:genetics; D001665:Binding Sites / Q000235:genetics; D002412:Cations; D045362:Cytochromes c2 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D004579:Electron Transport / Q000235:genetics; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D007700:Kinetics; D016297:Mutagenesis, Site-Directed; D010782:Photolysis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding / Q000235:genetics; D012242:Rhodobacter sphaeroides / Q000201:enzymology* / Q000235:genetics; D013057:Spectrum Analysis; D013816:Thermodynamics; D014443:Tyrosine / Q000235:genetics
chemical_list: D024322:Amino Acids, Aromatic; D002412:Cations; D045322:Photosynthetic Reaction Center Complex Proteins; D014443:Tyrosine; D045362:Cytochromes c2; D001120:Arginine