13C nuclear magnetic resonance studies of the conformation of the X-pro bond in the oligopeptide hormones, thyrotropin-releasing hormone, luteinizing hormone-releasing factor, angiotensin and melanocyte-stimulating hormone release-inhibiting factor

Conformational studies of N-Tyr-MIF-1 in aqueous solution by 1H nuclear magnetic resonance spectroscopy

N-Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) is an endogenous brain peptide with multiple effects on animal behavior. However, there have been no studies on the conformation of this tetrapeptide. In this report, we studied the conformation of N-Tyr-MIF-1 in aqueous solution by conventional one-dimensional and two-dimensional (COSY and NOESY) 1H nuclear magnetic resonance spectroscopy at 300 MHz. A complete set of assignments for the resolved resonances and approximate assignments for the overlapping resonances were made. The results demonstrate that N-Tyr-MIF-1 is in slow exchange between two conformers, most likely determined by the cis and trans states of the proline residue. The minor conformation represents 30 +/- 3% of the population over the temperature range from 3 degrees to 73 degrees. In the major conformation, the tyrosine aromatic ring appears to be close enough to interact directly with the proline pyrrolidine ring, as indicated by a strong temperature dependence of the proline C beta H, C delta H and C delta H' chemical shifts. In contrast, this interaction of the tyrosine and proline rings is not present in the minor conformation.

A second endogenous molecular form of mammalian hypothalamic luteinizing hormone-releasing hormone (LHRH), (hydroxyproline9)LHRH, releases luteinizing hormone and follicle-stimulating hormone in vitro and in vivo

In vitro and in vivo release of pituitary hormones were studied in the presence of (hydroxyproline9)LHRH ((Hyp)LHRH), a newly characterized endogenous molecular form of LHRH. Results were compared to those obtained with LHRH itself. (Hyp)LHRH, as LHRH, stimulated both luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release in a homothetic manner. The hydroxylated compound was, however, 24 times (in vitro) and 5 times (in vivo) less potent than LHRH. The lower activity of (Hyp)LHRH than of LHRH in the in vitro assay correlated well with a 28-fold lesser potency in a binding test using pituitary membrane preparations. The higher relative potency and the prolonged effect of (Hyp)LHRH in the in vivo test were related to a lesser susceptibility of the hydroxylated form to proteolytic degradation. Effects of LHRH and of (Hyp)LHRH were not additive, both peptides were equally able to desensitize gonadotrophs to a subsequent challenge by the other. Taken together, these observations suggest that both forms of LHRH act at the same receptor site. The lesser affinity of the hydroxylated compound is compensated to a certain extent by its higher resistance to enzymatic degradation. It is concluded that in spite of its lesser potency, (Hyp)LHRH may participate in the regulation of gonadotropins.

Conformation of proline residues in bacteriorhodopsin

Proline, noted as a hydrophilic residue with helix-breaking potential, nevertheless occurs widely in putatively alpha-helical transmembrane segments of many transport proteins. Ligand-activated or enzyme-assisted trans/cis isomerization of an X-proline peptide bond (where X = any amino acid)--a dynamic, reversible event which could alter the orientation of a transmembrane alpha-helix--may provide the molecular basis for a protein channel regulatory process. Further elucidation of such a function requires knowledge of the isomeric status of the X-Pro bonds in native conformations of membrane proteins. We have used 13C nuclear magnetic resonance (NMR) spectroscopy to examine the conformation of intramembranous X-Pro peptide bonds in biosynthetically-labelled samples of a model transport protein, bacteriorhodopsin (bR) (purple membrane). Spectra of 13C-Tyr-carbonyl labelled bR (in the solvent system CHCl3:CD3OD (1:1) + 0.1 M LiClO4) first established that all 11 bR Tyr residues were sufficiently mobile for their resonances to be detected and resolved, independent of their domain location within the bR sequence. By taking advantage of the known diagnostic chemical shifts of the isomers of Pro-C gamma carbon resonances, spectra of bR labelled with 13C gamma-Pro were then used to demonstrate that all 11 bR X-Pro peptide bonds--including those within the protein's membrane domain (Pro50, Pro91, Pro186)--are in the trans conformation in resting state bR.

Synthesis and conformational studies by 1H- and 13C-NMR spectroscopy of a novel, sterically constrained analogue of thyrotropin-releasing hormone

A novel thyrotropin-releasing hormone (TRH) analogue, [2,4-MePro3]-TRH (2,4-MePro: 2-carboxy-2,4-methanopyrrolidine), has been synthesized using a rapid solid phase peptide synthesis method, and its conformational properties investigated by one- and two-dimensional (2D) nmr spectroscopy and by proton Overhauser measurements. Following a published approach, calibrated interproton Overhauser effects were used together with distance geometry analysis to deduce that the single conformation of the His-2,4-MePro tertiary amide bond is trans in aqueous solution. This conclusion was corroborated by 2D dipolar-correlated (NOESY) spectroscopy. A preferentially extended conformation is indicated by the nmr data, similar to that of TRH. The phi, psi conformational space of 2,4-MePro is, however, significantly different from that of trans proline and the structural consequences of these differences at the C-terminus are discussed. The distribution of histidine side-chain conformations in the TRH analogue was deduced from coupling constants and from the short-range interaction between the imidazole ring and one of the prochiral faces of the 2,4-MePro side chain.

The space structure of a conformationally labile oligopeptide in solution: angiotensin

The paper describes a new approach to the problem of space structure description for conformationally labile molecules existing in solution as a set of different conformers in dynamic equilibrium. In such a case the "average" model derived exclusively from physico-chemical data represents a virtual structure devoid of physical sense. The proposed approach involves the selection of statistical weights wi for molecular conformers in solution by combined use of spectroscopic data and energy calculations (including the Monte-Carlo technique). Consequently, it appears possible to confine the entire region of all wi values only by those points (wi) that provide a reasonable agreement between the results of calculations and the experimental data. The approach was put to trial by using the linear octapeptide angiotensin, a well-known bioregulator with a wide spectrum of action. The 1H NMR and fluorescence spectroscopy were used as a source of experimental evidence concerning the space structure of the peptide in aqueous solution. The spin-lattice relaxation rates induced by the spin label allowed to estimate simultaneously several parameters characterizing the distance between the spin label and different functional groups in the angiotensin molecule. At least 5 types of angiotensin conformers were shown to be "indispensable" to achieve a good agreement between the results of energy calculations and 1H NMR spectroscopy data obtained in solution. The statistical weight estimates for angiotensin conformers permit to predict, with a high degree of accuracy, the value of singlet-singlet energy transfer between the Phe and Tyr aromatic chromophores of the molecule in aqueous solution. The proposed approach to the description of conformationally labile molecules can be actually regarded as stepwise refinement of statistical weight limits for sets of low-energy conformers in solution upon accumulation of new experimental evidence. The same appears to apply to conformationally labile molecules of non-peptide nature.

Pyrrolidine ring conformations in prolyl peptides from 13C spin-lattice relaxation times

A method was developed in the framework of a bistable jump model to obtain the pyrrolidine ring conformations in proline peptides from 13C spin-lattice relaxation times. Equations are presented expressing the ring torsions in terms of the 13C spin-lattice relaxation times of the ring carbons. This method was applied to 26 pyrrolidine ring systems and acceptable conformations were obtained.

Conformational energy analysis of melanostatin

Conformational energy calculations were carried out on the hypothalamic hormone melanostatin, a tripeptide with the primary structure H-L-Pro-L-Leu-Gly-NH2. The calculated lowest energy conformation was a type II beta bend, very similar to that reported in an X-ray crystal study. This conformation, however, was only one of 109 low-energy structures (less than or equal to 3 kcal/mol above the global minimum), indicating that the molecule in solution exists as an ensemble of conformations and is very flexible, in agreement with relaxation data from n.m.r. measurements. A statistical analysis yielded an average end-to-end distance of 6.8 A and a bend probability of 0.62, suggesting that, in nonpolar solvents, bend structures predominate within the statistical ensemble. The statistical analysis, however, also yielded a probability of only 0.11 for the occurrence of a 4 leads to 1 hydrogen bond. Hence, the calculations show that, although bend conformations predominate, bends would be difficult to observe in solution if the experiments were designed only to detect 4 leads to 1 hydrogen bonds.

Use of nuclear Overhauser effect in the study of peptides and proteins

The nuclear Overhauser effect (NOE) leads to changes in the intensity of signal(s) of a set of nuclei as a function of their respective distances. The use of NOE allows to obtain structural informations on peptides and proteins in solution as well as the study of interactions between small ligands and biomolecules. In this review, aspects of the basic theory of the NOE will be presented and the more recent applications of homonuclear and heteronuclear NOE's in biomolecules will be surveyed. Typical examples will be illustrated and limitations of the method will be discussed.

Correlation of the biological activity and solution conformation of [Asp1,Ile5]- and [Phe4,Tyr8]angiotensin II

Angiotensin II is known to undergo a reversible conformational transition and a change in potency in rat uterus in vitro with pK approximately 6.5. We have shown by carbon-13 NMR that the conformational transition involves all-trans to partly cis isomerization of the His6-Pro7 peptide bond. Isomerization from all-trans at pH 6.8 to approximately 16% cis at pH 8.0 is therefore correlated with a 10-fold increase in biological activity for [Asp1,Ile5]-angiotensin II in rat uterus in vitro. Isomerization from all-trans at pH 6.8 to approximately 16% cis at pH 8.0 in the competitive inhibitor [Phe4,Tyr8]angiotensin II is correlated with exhibition of virtually no agonist activity at low pH to full agonist activity at high pH. An angiotensin II conformation with Pro7 in the cis form may therefore be the conformation with maximal binding or biological activity at the cellular receptor.

13C nuclear magnetic resonance study of the cis-trans isomerism in X-Pro-Pro tripeptides

13C nuclear magnetic resonance has been used to characterize quantitatively the cis-trans isomerism about both peptide bonds in the tripeptides Ser-Pro-Pro and Arg-Pro-Pro. Detailed pH titration data indicate that the configuration about both peptide bonds is closely linked to titration of the terminal carboxyl group and, to a lesser extent, to titration of the terminal amino group. The Pro2 C-3 resonance has been found particularly useful for interpretation due to its sensitivity to the isomerization about both peptide bonds. Analysis of the probabilities of the trans-trans, cic-cis, cis-trans, and trans-cis isomers in aqueous solution indicates a stability decrease in the order given. Similarities in the isomerization behavior of the two peptides indicate that side chain interactions involving the first residue have very little effect on the observed cis/trans ratios. The sensitivity of the cis/trans ratio to titration of the terminal amino group is most readily explained on the basis of an indirect effect on carbonyl-carbonyl repulsion.

Quantitative 250 MHz proton magnetic resonance study of hydrogen-deuterium exchange. Angiotensin II hormone in trifluoroethanol

Hydrogen-deuterium exchange kinetics of (Asn1-Val5) angiotensin II has been investigated by proton magnetic resonance at 250 MHz in deuterated trifluoroethanol, as an approach to the "in situ" hormone conformation. An interactive program was specially developed to perform the data analysis on a computer similar to those used for spectroscopic data acquisition. Nine exchange sites are evidenced and characterized by their individual kinetic parameters. Three of them are assigned to peptide NH hydrogens, and the six remaining to slowly exchanging side chain protons. At 11 degrees C, more than three peptide hydrogens, sterically hindered or involved in hydrogen bonds, do not exchange. These results corroborate previous circular dichroism and infrared investigations performed in the same solvent, and suggest a family of well-folded conformations, stabilized in trifluoroethanol by internal hydrogen bonds, involving both the backbone and the side chain hydrogens.

13C-nuclear magnetic resonance study of [85% 13C-enriched proline]thyrotropin releasing factor: 13C-13C vicinal coupling constants and conformation of the proline residue

To understand fully interactions between peptides and cellular receptors, peptide side chain conformation must be defined. In many cases the complexity of proton nuclear magnetic resonance (NMR) prevents this but the present work demonstrates this problem can be solved by using 13C enrichment. Selective 13C enrichment of a natural peptide hormone has been achieved by preparing [85% 13C-enriched proline]thyrotropin releasing factor which was examined by 13C NMR spectroscopy at various pH values. Because of the 13C enrichment, one-bonded and three-bonded (vicinal) 13C-13C coupling constants have been determined. The latter vary from 0 to 5 Hz and show bond angle dependence. These data indicate that in this hormone the pyrrolidine ring is not free but fixed in the Cgamma-endo puckered conformation. It has also been possible to assign chemical shift values for a second order 13C NMR spectrum.

Conformational flexibility of luteinizing hormone-releasing hormone in aqueous solution. A carbon-13 spin-lattice relaxation time study

The carbon-13 nuclear magnetic resonance (13C NMR) spectra of luteinizing hormone-releasing hormone (LH-RH) and lower homologous peptides have been assigned in aqueous solutions at various pH values. 13C spin-lattice relaxation times (T1) have been measured for all proton-bearing carbons at 25.2 and 67.9 MHz. From the T1 data the rates of overall molecular motion and intramolecular motion of side chains have been estimated. LH-RH is a flexible molecule in solution, having segmental motion along the backbone as well as in the nonaromatic side chains.