Comparison of the solution structures of the chimeric peptides galanin(1-12)-Ala-neuropeptide Y(25-36)amide and galanin(1-12)-Pro-neuropeptide Y(25-36)amide.
In a previous study [Arvidsson, K., Land, T., Langel, U., Bartfai, T. & Ehrenberg, A. (1993) Biochemistry 32, 7787-7798], the 25-amino-acid-residue chimeric peptide M32, galanin(1-12)-Pro-neuropeptide Y(25-36)amide, was found to bind very strongly to galanin receptors and also to have considerable affinity to neuropeptide Y receptors. The solution structure of M32 in 30% (by vol.) 1,1,1,3,3,3-hexafluoro-2-propanol, was determined from structural restraints obtained by two-dimensional 1H-NMR. Another peptide, M88, with Ala instead of Pro in position 13, was shown to bind with tenfold lower affinity to galanin receptors, i.e. with nearly the same affinity as galanin itself. The binding to neuropeptide Y receptors was altered in the opposite sense. The peptide M88 has now been examined by two-dimensional 1H-NMR under the same conditions as applied for M32. Using NMR-derived restraints, we have calculated structures of M88 by means of the programs CALIBA, HABAS and DIANA, and refined them by restrained energy minimisation and restrained molecular dynamics. The use of CALIBA and HABAS meant that the restraints were less restrictively formulated, than in the previous work. Hence, structures of M32 were recalculated in the same way as for M88, also including some additional restraints, possible to identify by comparison of the NMR spectra of the two peptides. The new structures of M32 confirm an alpha-helix starting at Pro13, but now continuing until Gln23, instead of ending at Ile20. Conversely, no secondary structure is now expressed in the N-terminal section. It is concluded that in the case of peptides the details of the calculated structures depend on how the restraints are calibrated. In M88, an alpha-helix is found over a long section, approximately Ser6-Gln23. Possible correlations between the binding affinities to receptors and the solution structures of the peptides M32 and M88 are discussed. This is done with particular reference to the length and stability of the alpha-helix and the flexibility of the N-terminal section and its bending direction versus the helix.