NMR Structure Implications of Enhanced Efficacy of Obestatin Fragment Analogs.

Obestatin is a more recently discovered hormone
that is encoded by the ghrelin gene and produced in
the stomach and gut. We report NMR analysis on synthetic
Obestatin (OB23), a 23 residue peptide, along with three
overlapping fragments of the same in methanol solvent as a
first step towards structure activity relationship. Selective
substitutions on the promising N-terminal and middle
fragments of obestatin have been carried out in order to
improve the efficacy and potency. In the N-terminal fragment
two peptides were obtained by the replacement of Gly
(8) with a-aminoisobutyric acid (Aib, U) and Phe (F5) with
Cyclohexylalanine (Cha). In case of the middle fragment
both Gly (3) and Gly (8) were replaced with Aib residues.
The rationale being, these unusual amino acids could
provide protection from immediate degradation and aid
structure stabilization. Our previous studies showed that
the N-terminal and the middle fragment were unstructured
and hence this substitution would directly evaluate the
effect of structure on the activity of these fragment analogs.
Detailed NMR analysis clearly demonstrates formation of
helical secondary structure in all the peptide analogues and
provides justification for relative activities reported by our
group previously (Nagaraj et al. 2009).