Publisher Summary Turns are a fundamental class of polypeptide structure and are defined as sites where the peptide chain reverses its overall direction. In the past 20 years, the peptide field has witnessed major development, stimulated by the discovery of a host of bioactive peptides. Turn structures have been proposed and implicated in the bioactivity of several of these naturally occurring peptides. In addition, many structural details of turns have been derived from conformational studies of model peptides. During this same period, more than 100 complete protein structures have been elucidated in single-crystal X-ray studies. These examples document the rich diversity of structural patterns in the chain folds of native proteins. Turns are intrinsically polar structures with backbone groups that pack together closely and side chains that project outward. Such an array of atoms may constitute a site for molecular recognition, and indeed, the literature abounds with suggestions that turns serve as loci for receptor binding, antibody recognition, and post-translational modification. In peptides, turns are the conformations of choice for simultaneously optimizing both backbone–chain compactness (intramolecular nonbonded contacts) and side-chain clustering (to facilitate intermolecular recognition). Presence of turns in bioactive conformations may in fact also reflect the lack of alternative conformational possibilities. The aim of this chapter is to examine structural and functional roles of turns in peptides and proteins.

Turns in peptides and proteins.

The acquisition of the native three-dimensional structure of proteins consists of sequential folding reactions with well-populated and well-defined structural intermediates. For small proteins successive stages in the folding have been resolved kinetically; these suggest that H-bonded elements of secondary structure are formed first, followed by folding steps to generate the complete tertiary structure.

Folding and association of proteins

Occurrence and role ofcis peptide bonds in protein structures

Carbon—hydrogen spin—spin coupling constants

Cis peptide bonds in proteins: residues involved, their conformations, interactions and locations.

Tetrapeptides with proline in position 2, asparagine or leucine in position 3, and glycine in positions 1 and 4, with end groups free or blocked on the N-terminal side, were studied in their various ionic states in 2H2O and in Me2SO-d6 by 1H- and 13C-nmr. In order to clarify or refine some details, successive substitutions of the residues in these peptides with amino acids enriched to 85% in 13C, or to 85% 13C plus 97% 2H were carried out. The 1H and 13C chemical shifts as well as the 1H-1H, 13C-13C, and 13C-1H coupling constants and the signal intensities show strong similarity of behavior between the tetrapeptides of asparagine and leucine. The main conformational characteristics are (1) the almost total stabilization of the trans conformer in the type I β-turn structure when the peptide is in the zwitterion state dissolved in Me2SO. This is deduced from the 3J and the 3J coupling constants, which both furnish a dihedral angle of ϕ3 = −90°, and from the positive value of the temperature coefficient of the glycine-4 amide protons, which suggests a type 4 1 hydrogen bond; (2) the evolution of cis and trans isomer fractions which change with the ionic state of the peptides in Me2SO, whereas they remain constant in aqueous solution; and (3) the conformation of the pyrrolidine ring as it follows the variations in cis:trans isomer populations together with the side-chain rotamer fractions of the residue in position 3. In the β-turn conformation the isomer cis is less abundant and the pyrrolidine ring is more flexible; this explains the perfect accommodation of the proline residue in position 2 of a bend. The interdependence of these phenomena where interactive forces play a predominant role underlines the importance of cooperative effects in the molecule. The results also suggest that the cis isomer of proline can adapt itself just as well as the trans isomer to position 2 of a type I β-turn.

NMR evidence for a type I β‐turn in (Pro2)‐tetrapeptides and interdependence of cis:Trans isomerism, ring flexibility, and backbone conformation

Proton NMR study of the conformation of bradykinin: pH titration.

We describe the synthesis and the conformational analysis by ir, CD, and proton-nmr spectroscopy of four model peptides of the type N-Ac-Tyr-X-His-NH2 with X = Val, Leu, Ala, Gly. These peptides represent the central sequence of the hormone angiotensin II and its position-5 analogs. We studied their conformational behavior in aqueous solution during pH titration and in organic solvents. For specific purposes of spectral analysis (ir band assignment, proton-nmr signal assignment, heteronuclear vicinal coupling constants), we synthesized three isotopically enriched homologs of the mother sequence, i.e., N-Ac-(15N-Tyr)-Val-His-NH2, N-Ac-(13C, 2H, Tyr)-Val-His-NH2, and N-Ac-Tyr-(13C, 2H, Val)-His-NH2. Results are summarized as follows: the tyrosine and the histidine side chains influence each other through space; this mutual influence is modulated by the nature of the side chain in position X and decreases in going from XVal to XGly as a consequence of two simultaneous events, changes in the side-chain rotamer distribution and changes in the φ and ψ angles of residue X. The decrease in the bulkiness of the side-chain X (Val Gly) leads to increased flexibility of the peptide backbone at this site, which is also reflected in the apparent ratio of C5, C7, and intermediate conformations present in equilibrium. The three spectroscopic techniques, in addition to the results of chymotryptic degradation experiments, show a high level of agreement, and all reflect the dynamic conformation of these peptides in a different manner.

Conformation of the central sequence of angiotensin II and analogs.

Angiotensin II and its competitive inhibitor [Sar1, Ile8]-angiotensin II, as well as several analogs of these two compounds specifically chosen for their well-defined pharmacological properties, were studied by circular dichroism and nuclear magnetic resonance methods at various pH values in aqueous solution and in d6-dimethylsulfoxide. The results were compared with their biological activities. This allowed us to establish relationships between conformation and pressor activity, explaining most of the properties of angiotensin II, its inhibitor, and the analogs successively substituted in positions 3 and 5.

Conformation and activity in angiotensin II peptides.

The conformation–biological activity relationships in a series of angiotensin II analogs substituted in position 5 were studied. Results indicated that only analogs with β-branched residue in position 5 possess spectral and biological properties identical to that of parent angiotensin II.

Karl Lintner

Papers

NMR evidence for a type I β‐turn in (Pro2)‐tetrapeptides and interdependence of cis:Trans isomerism, ring flexibility, and backbone conformation

Proton NMR study of the conformation of bradykinin: pH titration.

Conformation of the central sequence of angiotensin II and analogs.

Conformation and activity in angiotensin II peptides.

The key role of residue 5 in angiotensin II