Conformation gastrin

In a separate study, compounds 10A and 10B were incorporated into the gastrin peptide sequence, replacing the Tyr12-Gly13, to provide a potent analogue. 60 These data were used to imply an Alan-Trp14 p-turn in the bioactive conformation of gastrin. 

Two attempts have been reported to predict the conformation of a polypeptide hormone by assembling the appropriate residues in their predicted conformation (63, 90). In this manner, the amino acid sequence of bradykinin was predicted to exist in a random coil conformation, with variation around the glycine bond, and with no interaction predicted between phenyl groups. As yet no experimental evidence confirms this prediction however, existing experimental evidence suggests that the prediction is reasonable (67-69). In the second study the conformation of gastrin tetrapeptide was predicted (90). 

Schaschke, N., Fiori, S., Weyher, E. et al. Cyclodextrin as carrier of peptide hormones conformational and biological properties of (3-cyclodextrin/gastrin constructs. J. Am. Chem. Soc. 1998, 120, 7030-7038. 

Potential Bioactive Conformations of Hormones of the Gastrin Family... 

Irrespective of the validity of the hypothetical membrane-bound pathway great efforts have been made in the last few years to determine preferred conformations of peptide hormones in media mimicking the more hydrophobic environments of membranes and/or receptor binding clefts. Our contributions in this field of research are confined to a pair of closely related gastrointestinal hormones, i.e. to gastrin and cholecystoklnin (CCK), which were chosen as model bioactive peptides. 

Preferred Conformations of Gastrin in Solution and in Presence of Surfactant Micelles... 

Fig. 6. Preferred, conformation of [Nle15)-gastrin-[5-l 7) in aqueous TFE. A) ribbon drawing of the backbone B) space filling representation of the molecule.

As discussed in section 2.2., N-terminal extensions of the active site portion of the gastrin molecule leads to a remarkable stabilization of its potential bioactive conformation with concomitant increase of hormonal potency. This aspect has been analyzed in the case of CCK too, by using the series of peptides listed in table 2 (64). CD measurements revealed that N-terminal extensions of the CCK nv-lecule in sequence mode are affecting only marginally the conformational st s of the bioactive core. As expected from what was known for gastrin, the CD... 

CCK-peptides at high TFE concentrations are very similar to those of gastrin peptides of similar size both regarding the overall pattern and the intensities of the maxima. The latter spectra were attributed to 7-turns, and a folding of the C-terminal portion of gastrin into a helical structure in TFE had been confirmed by NMR analysis (67). The presence of the identical C-terminal pentapeptide in the CCK molecule makes it most reasonable to assume that the observed CD properties in aqueous TFE reflect a similar conformational state of this sequence portion in CCK, too. This would also compare well with the 3D structure determined for fIhr,Nle)-CCK-9 by NMR in aqueous DMSO since the relatively small conformational shift from a 3iq- to an a-helix could easily result from the strong a-helix inducing effect of TFE as solvent (80,81). 

Additional support for a folding of the CCK-peptides into a conformation similar to that of the C-terminal portion of gastrin with the N-terminal tail as flexible arm for a decisive electrostatic interaction with a receptor counterpart was obtained with immunological experiments (82,83). CCK-10, CCK-12 and CCK-13 were linked covalently to the cysteine residue 107 of iso-1-cytochrome c via the... 

Both aqueous organic solvent mixtures and differently charged micelles can mimic only roughly the environment of natural cell membranes. In order to analyze in more appropriate model systems possible interactions of gastrin and CCK with cell membranes and to determine their conformational states in lipid bilayers, we have recently investigated in detailed manner this aspect using liposomes. The similarity betwen liposomes and natural membranes is extensively exploited both in vitro and in vivo because of the ability of liposomes to mimic the behaviour of natural membranes. Moreover, the value of liposomes as model membrane systems derives from the fact that they can be constructed with natural constituents. In our approach, we selected as model membranes those formed with the zwitterionic lipids di-myristoylphosphatidylcholine (DMPC) and di-palmitoylphosphatidylcholine (DPPC) as these lipids constitute the major components of most cell membranes. Moreover, in order to operate with a simple system, small unilamellar vesicles (SUVs) were used, i.e. with a diameter between 25 and 250 nm as resulting by rod-type sonication or by extrusion (51). 

Fig. 18. Low-energy calcium binding sites of Nle15]-gastrin-[5-17] (A) and [Thr,Nle]-CCK-9 (B) as calculated with the GRID programme using the peptide conformations determined by NMR analysis in aqueous organic media.

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