3-DIMENSIONAL STRUCTURE OF GURMARIN, A SWEET TASTE-SUPPRESSING POLYPEPTIDE

The solution structure of gurmarin was studied by two-dimensional prot on NMR spectroscopy at 600 MHz. Gurmarin, a 35-amino acid residue poly peptide recently discovered in an Indian-originated tree Gymnema sylve stre, selectively suppresses the neural responses of rat to sweet tast e stimuli. Sequence-specific resonance assignments were obtained for a ll backbone protons and for most of the side-chain protons, The three- dimensional solution structure was determined by simulated-annealing c alculations on the basis of 135 interproton distance constraints deriv ed from NOEs, six distance constraints for three hydrogen bonds and 16 dihedral angle constraints derived from coupling constants. A total o f 10 structures folded into a well-defined structure with a triple-str anded antiparallel beta-sheet. The average rmsd values between any two structures were 1.65+/-0.39 Angstrom for the backbone atoms (N, C-alp ha, C) and 2.95+/-0.27 Angstrom for all heavy atoms. The positions of the three disulfide bridges, which could not be-determined chemically, were estimated to be Cys(3)-Cys(18), Cys(10)-Cys(23) and Cys(17)-Cys( 33) On the basis of the NMR distance constraints. This disulfide bridg e pattern in gurmarin turned out to be analogous to that in omega-cono toxin and Momordica charantia trypsin inhibitor-II, and the topology o f folding was the same as that in omega-conotoxin.