3-DIMENSIONAL STRUCTURE OF A GAMMA-CARBOXYGLUTAMIC ACID-CONTAINING CONOTOXIN, CONANTOKIN-G, FROM THE MARINE SNAIL CONUS-GEOGRAPHUS - THE METAL-FREE CONFORMER

Conantokin G is a gamma-carboxyglutamic acid-containing conotoxin from the venom of the marine cone snail Conus geographus. The 17-residue p eptide, which contains five gamma-carboxyglutamic acid (Gla) residues and an amidated C-terminal asparagine amide, was synthesized chemicall y in a form identical to the natural conantokin G. To gain insight int o the role of gamma-carboxyglutamic acid in the structure of this pept ide, we determined the three-dimensional structure of conantokin G by H-1 NMR and compared its structure to other conotoxins and to the gamm a-carboxyglutamic acid-containing regions of the vitamin K-dependent b lood-clotting proteins. Complete resonance assignments were made by tw o-dimensional H-1 NMR spectroscopy in the absence of metal ions. NOE c ross-peaks d(alpha N), d(NN), and d(beta N) provided interproton dista nce information, and vicinal spin-spin coupling constants (3)J(HN alph a), were used to calculate phi torsion angles. Distance geometry and s imulated annealing methods were used to derive 20 convergent structure s from a set of 227 interproton distance restraints and 13 torsion ang le measurements. The backbone rmsd to the geometric average for 20 fin al structures is 0.8 +/- 0.1 Angstrom. Conantokin G consists of a stru ctured region commencing at Gla 3 and extending through arginine 13. T his structure includes a partial loop centered around Gla 3 and Gla 4, a distorted type I turn between glutamine 6 and glutamine 9, and two type I turns involving Gla 10, leucine 11, and isoleucine 12 and argin ine 13. Together, these two turns define approximately 1.6 turns of a distorted 3(10) helix. The observed structure possesses structural ele ments similar to those seen in the disulfide-linked conotoxins.