title: Modest stabilization by most hydrogen-bonded side-chain interactions in membrane proteins.
abstract: Understanding the energetics of molecular interactions is fundamental to all of the central quests of structural biology including structure prediction and design, mapping evolutionary pathways, learning how mutations cause disease, drug design, and relating structure to function. Hydrogen-bonding is widely regarded as an important force in a membrane environment because of the low dielectric constant of membranes and a lack of competition from water. Indeed, polar residue substitutions are the most common disease-causing mutations in membrane proteins. Because of limited structural information and technical challenges, however, there have been few quantitative tests of hydrogen-bond strength in the context of large membrane proteins. Here we show, by using a double-mutant cycle analysis, that the average contribution of eight interhelical side-chain hydrogen-bonding interactions throughout bacteriorhodopsin is only 0.6 kcal mol(-1). In agreement with these experiments, we find that 4% of polar atoms in the non-polar core regions of membrane proteins have no hydrogen-bond partner and the lengths of buried hydrogen bonds in soluble proteins and membrane protein transmembrane regions are statistically identical. Our results indicate that most hydrogen-bond interactions in membrane proteins are only modestly stabilizing. Weak hydrogen-bonding should be reflected in considerations of membrane protein folding, dynamics, design, evolution and function.
authors.affiliation: Department of Chemistry and Biochemistry, UCLA-DOE Center for Genomics and Proteomics, Molecular Biology Institute, USA.
authors.forename: Nathan Hyunjoong
authors.initials: NH
authors.lastname: Joh
authors.forename: Andrew
authors.initials: A
authors.lastname: Min
authors.forename: Salem
authors.initials: S
authors.lastname: Faham
authors.forename: Julian P
authors.initials: JP
authors.lastname: Whitelegge
authors.forename: Duan
authors.initials: D
authors.lastname: Yang
authors.forename: Virgil L
authors.initials: VL
authors.lastname: Woods
authors.forename: James U
authors.initials: JU
authors.lastname: Bowie
mesh_terms: D001436:Bacteriorhodopsins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D018360:Crystallography, X-Ray; D041961:Deuterium Exchange Measurement; D006860:Hydrogen Bonding; D008565:Membrane Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D009154:Mutation / Q000235:genetics; D017510:Protein Folding; D012995:Solubility; D013816:Thermodynamics
chemical_list: D008565:Membrane Proteins; D001436:Bacteriorhodopsins