title: Ab initio study of NMR 15N chemical shift differences induced by Ca2+ binding to EF-hand proteins.
abstract: EF-hands are Ca(2+) binding motifs that are widely distributed throughout the entire living organism kingdom. At present, relatively little is known at a quantum mechanical level about the mechanisms that allow Ca(2+) to be recognized specifically by EF-hands and to induce a conformational switch from a compact ("closed") conformation to an "open" state that exposes a large patch of hydrophobic residues. Here, we present a study of NMR (15)N chemical shifts based on ab initio quantum mechanical calculations carried out on a minimalist model system linking both Ca(2+) binding sites across the beta-sheet of an EF-hand domain. Calculated and experimentally determined chemical shift changes are correlated with structural changes induced upon metal binding. The effect of Ca(2+) binding on these (15)N shifts can be dissected into two main contributions: one from pi-polarization of beta-sheet amide groups and the other from rotation of an isoleucine side chain. By correlating this description with experimental evidence, different polarization states for the beta-sheet amide groups were identified and linked to the overall conformation of different EF-hand domains. When all four beta-sheet amide groups are polarized, the ab initio calculations in our model indicate a cooperative stabilization effect due to the establishment of a circular network of donor-acceptor interactions connecting the two Ca(2+) ions across the beta-sheet. The emerging hypothesis from our analysis is that this cooperative network of interactions is essential for stabilizing the "open" conformation of an EF-hand domain.
authors.affiliation: Molecular Structure Division, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, United Kingdom. rbiekof@nimr.mrc.ac.uk
authors.forename: Rodolfo R
authors.initials: RR
authors.lastname: Biekofsky
authors.forename: Adrián G
authors.initials: AG
authors.lastname: Turjanski
authors.forename: Darío A
authors.initials: DA
authors.lastname: Estrin
authors.forename: James
authors.initials: J
authors.lastname: Feeney
authors.forename: Annalisa
authors.initials: A
authors.lastname: Pastore
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D002118:Calcium / Q000378:metabolism*; D002147:Calmodulin / Q000737:chemistry* / Q000378:metabolism*; D020035:Holoenzymes / Q000737:chemistry / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009587:Nitrogen Isotopes; D011487:Protein Conformation; D017434:Protein Structure, Tertiary
chemical_list: D002147:Calmodulin; D020035:Holoenzymes; D009587:Nitrogen Isotopes; D002118:Calcium