STRUCTURAL EFFECTS OF THE SELECTIVE REDUCTION OF AMIDE CARBONYL GROUPS IN MOTILIN 1-12 AS DETERMINED BY NUCLEAR-MAGNETIC-RESONANCE

Motilin is a 22-residue peptide stimulating stomach and intestinal mot ility. The motilin 1-12 fragment displays biological effects similar t o the native peptide. Selective reduction of the amide carbonyl groups to form CH2NH analogs leads to a significant reduction in activity fo r the first two N-terminal positions and to a complete loss of activit y for all other positions. The structures of motilin 1-12 and ten redu ced analogs were investigated using the temperature dependence of the amide NH chemical shifts. In all the analogs, the structure of the N-t erminal region (residues 1-5) was different from the structure of moti lin 1-12, which is characterized by hydrogen bonding between Phe(1) an d Ile(4). The structure of the C-terminal region of analogs was simila r to the structure of motilin 1-12 for the first two reduction positio ns only (1-2 and 2-3), indicating that the C-terminal portion of motil in 1-12 is more critical for biological activity. Complete structural characterizations of motilin 1-12, [CH2NH](1-2)- and [CH2NH](4-5)-moti lin 1-12 were performed by two-dimensional NMR spectroscopy and molecu lar modeling. The structural features observed confirm the differences based on the temperature dependence of the amide NH chemical shifts. These results demonstrate that conservation of the amide bond rigidity is essential for the activity of non-hydrolyzable analogs. (C) Munksg aard 1995.