Theoretical studies on pro-leu-gly-nh2 conformation.

Abstract

Classical potential function calculations were carried out on the hypothalamic factor Pro-Leu-Gly-NH2. The results indicate that the proposed 1O-membered, hydrogen-bonded f-turn conformation of this tripeptide is a strongly preferred structure. Its stability appears to be inherent in the rather rigid backbone conformation of the leucine residue rather than the hydrogen bond between the carboxamide proton of glycinamide and the C=O of the proline moiety; the glycinamide has little influence on the 04 of the leucine backbone structure. The type II Mturn structure of the Pro-Leu-Gly-NH2 is preferred. Several years ago it was proposed that the hypothalamic factor Pro-Leu-Gly-NH2, enzymatically derived from its prohormone oxytocin, is an inhibiting factor for the release of pituitary melanotropin (melanocyte-stimulating hormone, MSH) (1). This biologically active tripeptide was subsequently isolated from bovine pituitary extracts (2) and it has since been found to possess additional, extrapituitary activities (3,4).On the basis of 1H nuclear magnetic resonance in dimethylsulfoxide and preliminary crystallographic data, Walter et al. proposed a preferred conformation of Pro-Leu-Gly-NH2, consisting of a 10-membered fl-turn (4 -1) closed by a hydrogen bond (5). This structure was confirmed by the x-ray crystallographic study of Reed and Johnson (6). Measurements of spin-lattice relaxation times of the carbon-13 nuclei of Pro-Leu-Gly-NH2 by Deslauriers et al. (7) showed this tripeptide to have a compact but flexible structure in aqueous medium. In addition, classical potential function calculations which incorporated electrostatic, torsion, and hydrogen-bond contributions revealed a large number of preferred conformations including the 4 -1 fl-turn (8).The present conformational energy calculations using the classical Lennard-Jones potential function, which includes an electrostatic term, represent a further attempt to ascertain whether the stability of the proposed structure of ProLeu-Gly-NH2 is primarily due to the hydrogen bonding energy or is actually inherent in the backbone of this tripeptide. A second goal of this study was to determine whether there was a preference for one of the two theoretically possible types of f-turn conformations (9-12). One is the type I, in which the leucine side chain would occupy an axial position, and the other is the type II with an equatorial position of the leucine side chain. Knowledge of the preferred type of f-turn of Pro-Leu-Gly-NH2 may be of particular interest when one considers the biological activity of this hypothalamic factor and its analogs. METHODS The Lennard-Jones 6-12 potential function was used, including an electrostatic term, and the procedure has been described in detail (13). The net charges in the Coulomb term were calculated by the INDO molecular orbital method. The molecular geometry of Pro-Leu-Gly-NH2 (Fig. 1) has been constructed using the reported x-ray crystallographic data (6) with the naturally occurring Pro(S)-Leu(S) absolute configuration. The...

Protein folding

“…A comparison of the conformations of dipeptide segments in the X-ray structures of 20 globular proteins with the calculated conformations of isolated dipeptides of the same amino acid sequences suggested that, for many dipeptide sequences in proteins, /?-bends are stabilized mainly by local interactions (Zimmerman et al 1977b). Ralston, De Coen & Walter (1974), Kang & Walter (1976), and Zimmerman & Scheraga (1977c) used energy calculations to determine stable conformations of H-Pro-L-Leu-Gly-NH 2 , the melanotropin release inhibiting factor. All three studies showed that a type II /?-bend was a low-energy structure, in agreement with the proposed structure in dimethylsulphoxide (Walter, Bernal & Johnson 1972) and with the X-ray crystal structure (Reed & Johnson, 1973).…”

Dopamine receptor modulation by conformationally constrained analogs of Pro-Leu-Gly-NH2

“…]amino]-2-oxo-1-piperidineacetamide (19). Boc-L-Pro-OH (1.37 g, 6.36 mmol) and lactam 18 (1.32 g, 6.36 mmol) were dissolved in DMF (10 mL), and the resulting solution was cooled to 0 °C.…”