Conformational energy calculation on the neurotensin c-terminal pentapeptide Arg-Pro-Tyr-Ile-Leu OH

Computer simulation of the conformational behaviour of angiotensinogen (6-13) renin substrate

The conformational behaviour of the biologically active angiotensinogen (6-13) fragment has been investigated by computer simulations. A large sample of conformers has been generated using the Monte-Carlo procedure, then analysed using classification and partition methods. Seven families can describe the conformational distribution. About 40% of conformers are fully extended, 28% are folded at the C-terminal His6-Pro7-Phe8-His9 level and the others are folded at different levels. The study highlights the extreme flexibility of the angiotensinogen fragment.

Crystal structure and conformational analysis of angiotensinogen fragments

The tripeptide acetyl-L-prolyl-L-phenylalanyl-L-histidine crystallizes in the orthorhombic space group P2(1)2(1)2(1) with eight molecules in a unit cell of dimensions a = 9.028(2), b = 140.54(6) and c = 42.41(1)A. The structure has been solved by direct methods and refined to an R value of 0.056 for 2904 observed reflections. The molecule exists as a zwitterion with terminal (His)CO2- and (imidazole)H+ as charged groups. The two peptide molecules in the structure adopt a type I beta-turn with Pro and Phe as the corner residues. The main conformational difference between the two crystallographically independent molecules is seen to be in the histidine side-chain orientations. The molecules arrange themselves in sheets perpendicular to the c axis. All hydrophobic side chains lie on one side of the sheets thus generated, whereas the hydrophilic groups are located on the other side. An interesting feature of the crystal structure is the existence of a water layer between adjacent peptide sheets. The conformational study of the isolated Ac-His-Pro-Phe-His-MA using energy calculations gives a rather limited number of stable conformers. The most stable corresponds to a type I beta-turn stabilized through two hydrogen bonds, followed by a less stable type II beta-turn (delta E = 2.0 kcal) and a partly helical structure (delta E = 2.6 kcal).

Analysis of tachykinin-binding site interactions using NMR and energy calculation data of potent cyclic analogues of substance P

The three-dimensional structures of [Cys3,6,Tyr8]-, [Gly2,Cys3,6,Tyr8]- and [DCys3,Cys6]substance P, designed as conformational analogues of substance P, have been studied by 1H-NMR (500 MHz) in different solvents and by energy calculations. As previously observed for substance P and physalaemin, two tachykinins acting via the NK-1 receptor, [Cys3,6,Tyr8]substance P presents an alpha-helical structure of the 4----8 sequence in methanol. This structure is stabilized by a beta-turn III via the formation of three hydrogen bonds involving the Cys-6, Phe-7 and Tyr-8 NH groups. In contrast to substance P, two of these hydrogen bonds are still present in dimethyl sulfoxide and in water the Cys-6 NH hydrogen bond is the only one remaining, such that a beta-turn structure inside the ring can be envisaged. In close agreement with the NMR data, the energy calculations lead to three types of folding for the core of [Cys3,6,Tyr8]substance P: a beta-turn III, a less stable beta-turn I (delta E = 3 kcal), and a beta-turn II (delta E = 4.6 kcal). The structure of Gly-Leu-Met-NH2 is strongly affected by changing the hydrophobicity of the medium. The most stable calculated conformation is the helix; however, numerous unrelated structures are destabilized by about 2-3 kcal/mol. These data are analyzed and discussed in connection with the high potency of [Cys3,6,Tyr8]substance P for both the NK-1 and NK-3 binding sites; that is the internal region of tachykinins (non-homologous amino acids) might present a similar three-dimensional structure when bound to the receptors (which may be at the origin of some lack of selectivity), whereas paradoxically the selectivity may be due to the common C-terminal sequence.

Conformational behaviour of some tachykinin C-terminal heptapeptides

The conformational behaviour of substance P, physalaemin and eledoisin C-terminal heptapeptides was investigated using empirical energy calculations. Although the conformational distributions of the three heptapeptides are somewhat different, they have a few common low energy conformations. Some of them, which satisfy the structure-activity relationships, may fit the well known SP-P receptors.

Conformational study of the neurotensin and substance P fragments: Pro-Arg-Arg-Pro and Arg-Pro-Lys-Pro

The conformational behaviour of the basic hydrophilic Pro-Arg-Arg-Pro and Arg-Pro-Lys-Pro peptides, neurotensin (NT) and Substance P fragments, has been taken up by semi-empirical calculations. The presence of two Pro residues prevents these peptides from giving any folded structure (alpha helix, beta turn . . .). In both peptides the most stable conformations are essentially relative to more or less stretched structures; structures involving one or more residues in a gamma turn form are often encountered in Pro-Arg-Arg-Pro peptide while mixed structures involving residues in very different conformations are found for the Arg-Pro-Lys-Pro-peptide. In both peptides, positively charged Lys and Arg side-chains most often point in opposite directions. The Pro-Arg-Arg-Pro peptide is part of the active NT (7-13) fragment where both Arg residues are necessary to the activity. A tentative study shows that the hydrophilic tetrapeptide induces NT (7-13) stretched conformations.