A 1H NMR study of human calcitonin in solution

Molecular Insights into the Effects of F16L and F19L Substitutions on the Conformation and Aggregation Dynamics of Human Calcitonin

Human calcitonin (hCT) regulates calcium-phosphorus metabolism, but its amyloid aggregation disrupts physiological activity, increases thyroid carcinoma risk, and hampers its clinical use for bone-related diseases like osteoporosis and Paget's disease. Improving hCT with targeted modifications to mitigate amyloid formation while maintaining its function holds promise as a strategy. Understanding how each residue in hCT's amyloidogenic core affects its structure and aggregation dynamics is crucial for designing effective analogues. Mutants F16L-hCT and F19L-hCT, where Phe residues in the core are replaced with Leu as in nonamyloidogenic salmon calcitonin, showed different aggregation kinetics. However, the molecular effects of these substitutions in hCT are still unclear. Here, we systematically investigated the folding and self-assembly conformational dynamics of hCT, F16L-hCT, and F19L-hCT through multiple long-time scale independent atomistic discrete molecular dynamics (DMD) simulations. Our results indicated that the hCT monomer primarily assumed unstructured conformations with dynamic helices around residues 4-12 and 14-21. During self-assembly, the amyloidogenic core of hCT14-21 converted from dynamic helices to β-sheets. However, substituting F16L did not induce significant conformational changes, as F16L-hCT exhibited characteristics similar to those of wild-type hCT in both monomeric and oligomeric states. In contrast, F19L-hCT exhibited substantially more helices and fewer β-sheets than did hCT, irrespective of their monomers or oligomers. The substitution of F19L significantly enhanced the stability of the helical conformation for hCT14-21, thereby suppressing the helix-to-β-sheet conformational conversion. Overall, our findings elucidate the molecular mechanisms underlying hCT aggregation and the effects of F16L and F19L substitutions on the conformational dynamics of hCT, highlighting the critical role of F19 as an important target in the design of amyloid-resistant hCT analogs for future clinical applications.

Tailoring the nuclear Overhauser effect for the study of small and medium-sized molecules by solvent viscosity manipulation

The nuclear Overhauser effect (NOE) is a consequence of cross-relaxation between nuclear spins mediated by dipolar coupling. Its sensitivity to internuclear distances has made it an increasingly important tool for the determination of through-space atom proximity relationships within molecules of sizes ranging from the smallest systems to large biopolymers. With the support of sophisticated FT-NMR techniques, the NOE plays an essential role in structure elucidation, conformational and dynamic investigations in liquid-state NMR. The efficiency of magnetization transfer by the NOE depends on the molecular rotational correlation time, whose value depends on solution viscosity. The magnitude of the NOE between ¹H nuclei varies from +50% when molecular tumbling is fast to -100% when it is slow, the latter case corresponding to the spin diffusion limit. In an intermediate tumbling regime, the NOE may be vanishingly small. Increasing the viscosity of the solution increases the motional correlation time, and as a result, otherwise unobservable NOEs may be revealed and brought close to the spin diffusion limit. The goal of this review is to report the resolution of structural problems that benefited from the manipulation of the negative NOE by means of viscous solvents, including examples of molecular structure determination, conformation elucidation and mixture analysis (the ViscY method).

The structure of secretin family GPCR peptide ligands: implications for receptor pharmacology and drug development

The secretin family G protein-coupled receptors, characterized by a large N-terminal extracellular domain and seven transmembrane helices, are drug targets in many diseases, including migraine, cardiovascular disease, diabetes, osteoporosis and inflammatory disorders. Their activating ligands are peptides with an average length of 30 amino acids. In this article we review the available structural data for these peptides and how this explains their activity. We emphasize how this information may be used to accelerate the development of new drugs against these receptors.

NMR characterization of monomeric and oligomeric conformations of human calcitonin and its interaction with EGCG

Calcitonin is a 32-residue peptide hormone known for its hypocalcemic effect and its inhibition of bone resorption. While calcitonin has been used in therapy for osteoporosis and Paget's disease for decades, human calcitonin (hCT) forms fibrils in aqueous solution that limit its therapeutic application. The molecular mechanism of fiber formation by calcitonin is not well understood. Here, high-resolution structures of hCT at concentrations of 0.3 mM and 1 mM have been investigated using NMR spectroscopy. Comparing the structures of hCT at different concentrations, we discovered that the peptide undergoes a conformational transition from an extended to a β-hairpin structure in the process of molecular association. This conformational transition locates the aromatic side chains of Tyr12 and Phe16 in a favorable way for intermolecular π-π stacking, which is proposed to be a crucial interaction for peptide association and fibrillation. One-dimensional (1)H NMR experiments confirm that oligomerization of hCT accompanies the conformational transition at 1 mM concentration. The effect of the polyphenol epigallocatechin 3-gallate (EGCG) on hCT fibrillation was also investigated by NMR and electron microscopy, which show that EGCG efficiently inhibits fibril formation of hCT by preventing the initial association of hCT before fiber formation. The NMR experiments also indicate that the interaction between aromatic rings of EGCG and the aromatic side chains of the peptide may play an important role in inhibiting fibril formation of hCT.

Calcitonin in a protochordate, Ciona intestinalis--the prototype of the vertebrate calcitonin/calcitonin gene-related peptide superfamily

The calcitonin (CT)/CT gene-related peptides (CGRPs) constitute a large peptide family in vertebrates. However, no CT/CGRP superfamily members have so far been identified in invertebrates, and the evolutionary process leading to the diverse vertebrate CT/CGRP superfamily members remains unclear. In this study, we have identified an authentic invertebrate CT, Ci-CT, in the ascidian Ciona intestinalis, which is the phylogenetically closest invertebrate chordate to vertebrates. The amino acid sequence of Ci-CT was shown to display high similarity to those of vertebrate CTs and to share CT consensus motifs, including the N-terminal circular region and C-terminal amidated proline. Furthermore, the Ci-CT gene was found to be the only Ciona CT/CGRP superfamily gene. Ci-CT also exhibited less potent, but significant, activation of the human CT receptor, as compared with salmon CT. Physiological analysis revealed that Ci-CT reduced the osteoclastic activity that is specific to vertebrate CTs. CD analysis demonstrated that Ci-CT weakly forms an alpha-helix structure. These results provide evidence that the CT/CGRP superfamily is essentially conserved in ascidians as well as in vertebrates, and indicate that Ci-CT is a prototype of vertebrate CT/CGRP superfamily members. Moreover, expression analysis demonstrated that Ci-CT is expressed in more organs than vertebrate CTs in the cognate organs, suggesting that an original CT/CGRP superfamily member gene was also expressed in multiple organs, and each CT/CGRP superfamily member acquired its current specific tissue distribution and physiological role concomitantly with diversification of the CT/CGRP superfamily during the evolution of chordates. This is the first report on a CT/CGRP superfamily member in invertebrates.

Amyloidogenesis of natively unfolded proteins

Aggregation and subsequent development of protein deposition diseases originate from conformational changes in corresponding amyloidogenic proteins. The accumulated data support the model where protein fibrillogenesis proceeds via the formation of a relatively unfolded amyloidogenic conformation, which shares many structural properties with the pre-molten globule state, a partially folded intermediate first found during the equilibrium and kinetic (un)folding studies of several globular proteins and later described as one of the structural forms of natively unfolded proteins. The flexibility of this structural form is essential for the conformational rearrangements driving the formation of the core cross-beta structure of the amyloid fibril. Obviously, molecular mechanisms describing amyloidogenesis of ordered and natively unfolded proteins are different. For ordered protein to fibrillate, its unique and rigid structure has to be destabilized and partially unfolded. On the other hand, fibrillogenesis of a natively unfolded protein involves the formation of partially folded conformation; i.e., partial folding rather than unfolding. In this review recent findings are surveyed to illustrate some unique features of the natively unfolded proteins amyloidogenesis.

Calcitonin-derived peptide carriers: mechanisms and application

Among the family of the so-called cell-penetrating peptides (CPP) sequences derived from the native peptide hormone human calcitonin (hCT) have recently proven to translocate different bioactive molecules across cellular membranes. Herein, we give an extensive summary of the development of hCT-derived carrier peptides, beginning with the therapeutic nasal administration of full-length hCT. Hence, N-terminally truncated hCT fragments were investigated and subsequently optimised to extend their field of application. The latest generation of hCT-derived carrier peptides are highly effective, branched peptides. The current state of the art is reviewed concerning the structural requirements, mechanistic assumptions and metabolic features of these peptides as well as experiments proving their excellent carrier potential.

Segment TM7 from the cytoplasmic hemi-channel from VO-H+-V-ATPase includes a flexible region that has a potential role in proton translocation

A 900-MHz NMR study is reported of peptide sMTM7 that mimics the cytoplasmic proton hemi-channel domain of the seventh transmembrane segment (TM7) from subunit a of H(+)-V-ATPase from Saccharomyces cerevisiae. The peptide encompasses the amino acid residues known to actively participate in proton translocation. In addition, peptide sMTM7 contains the amino acid residues that upon mutation cause V-ATPase to become resistant against the inhibitor bafilomycin. 2D TOCSY and NOESY (1)H-(1)H NMR spectra are obtained of sMTM7 dissolved in d(6)-DMSO and are used to calculate the three-dimensional structure of the peptide. The NMR-based structures and corresponding dynamical features of peptide sMTM7 show that sMTM7 is composed of two alpha-helical regions. These regions are separated by a flexible hinge of two residues. The hinge acts as a ball-and-joint socket and both helical segments move independently with respect to one another. This movement in TM7 is suggested to cause the opening and closing of the cytoplasmic proton hemi-channel and enables proton translocation.

Solid-state NMR as a method to reveal structure and membrane-interaction of amyloidogenic proteins and peptides

It is important to understand the Amyloid fibril formation in view of numerous medical and biochemical aspects. Structural determination of amyloid fibril has been extensively studied using electron microscopy. Subsequently, solid state NMR spectroscopy has been realized to be the most important means to determine not only microscopic molecular structure but also macroscopic molecular packing. Molecular structure of amyloid fibril was first predicted to be parallel beta-sheet structure, and subsequently, was further refined for Abeta(1-40) to be cross beta-sheet with double layered in register parallel beta-sheet structure by using solid state NMR spectroscopy. On the other hand, anti-parallel beta-sheet structure has been reported to short fragments of Abeta-amyloid and other amyloid forming peptides. Kinetic study of amyloid fibril formation has been studied using a variety of methods, and two-step autocatalytic reaction mechanism used to explain fibril formation. Recently, stable intermediates or proto-fibrils have been observed by electron microscope (EM) images. Some of the intermediates have the same microscopic structure as the matured fibril and subsequently change to matured fibrils. Another important study on amyloid fibril formation is determination of the interaction with lipid membranes, since amyloid peptide are cleaved from amyloid precursor proteins in the membrane interface, and it is reported that amyloid lipid interaction is related to the cytotoxicity. Finally it is discussed how amyloid fibril formation can be inhibited. Firstly, properly designed compounds are reported to have inhibition ability of amyloid fibril formation by interacting with amyloid peptide. Secondly, it is revealed that site directed mutation can inhibit amyloid fibril formation. These inhibitors were developed by knowing the fibril structure determined by solid state NMR.

Structure and localization of an essential transmembrane segment of the proton translocation channel of yeast H+-V-ATPase

Vacuolar (H+)-ATPase (V-ATPase) is a proton pump present in several compartments of eukaryotic cells to regulate physiological processes. From biochemical studies it is known that the interaction between arginine 735 present in the seventh transmembrane (TM7) segment from subunit a and specific glutamic acid residues in the subunit c assembly plays an essential role in proton translocation. To provide more detailed structural information about this protein domain, a peptide resembling TM7 (denoted peptide MTM7) from Saccharomyces cerevisiae (yeast) V-ATPase was synthesized and dissolved in two membrane-mimicking solvents: DMSO and SDS. For the first time the secondary structure of the putative TM7 segment from subunit a is obtained by the combined use of CD and NMR spectroscopy. SDS micelles reveal an alpha-helical conformation for peptide MTM7 and in DMSO three alpha-helical regions are identified by 2D 1H-NMR. Based on these conformational findings a new structural model is proposed for the putative TM7 in its natural environment. It is composed of 32 amino acid residues that span the membrane in an alpha-helical conformation. It starts at the cytoplasmic side at residue T719 and ends at the luminal side at residue W751. Both the luminal and cytoplasmatic regions of TM7 are stabilized by the neighboring hydrophobic transmembrane segments of subunit a and the subunit c assembly from V-ATPase.

Conformational analysis of human calcitonin in solution

The solution conformation of human calcitonin in a mixture of 60% water and 40% trifluoroethanol has been determined by the combined use of 1H NMR spectroscopy and distance geometry calculations with a distributed computing technique. 1H NMR spectroscopy provided 195 distance constraints and 13 hydrogen bond constraints. The 20 best converged structures exhibit atomic rmsd of 0.43 A for the backbone atoms from the averaged coordinate position in the region of Asn3-Phe22. The conformation is characterized by a nearly amphiphilic alpha-helix domain that extends from Leu4 in the cyclic region to His20. There are no significant differences observed among the overall structures of a series of calcitonins obtained from ultimobranchial bodies, including those that possess 20- to 50-fold greater activity. Three aromatic amino acid residues, Tyr12, Phe16 and Phe19, form a hydrophobic surface of human calcitonin. Bulky side chains on the surface could interfere with the ligand-receptor interaction thereby causing its low activity, relative to those of other species.

Structure of peptide solutions: a light scattering and numerical study

We investigated the interactions between protein molecules in solution, in particular for low salt concentrations and thus strong electrostatic interactions where a treatment based on the second virial coefficient is not sufficient. Static and dynamic light scattering experiments on solutions containing the peptide human calcitonin (hCT) were combined with calculations based on the Ornstein-Zernike equation with the hypernetted chain (HNC) closure and computer simulations within the primitive electrolyte model. The simulation illustrates the distribution of proteins in solution and the formation of (transient) protein aggregates. It furthermore allows us to predict the physical stability of hCT solutions in dependence of ionic strength, pH and hCT concentration.

The synthesis and study of side-chain lactam-bridged peptides

Side-chain lactam bridges linking amino acid residues that are spaced several residues apart in the linear sequence offer a convenient and flexible method for introducing conformational constraints into a peptide structure. The availability of a variety of selectively cleavable protecting groups for amines and carboxylic acids allows for several approaches to the synthesis of monocyclic, dicyclic, and bicyclic lactam-bridged peptides by solid-phase methods. Multicyclic structures are also accessible, but segment-condensation syntheses with solution-phase cyclizations are most likely to provide the best synthetic approach to these more complex constrained peptides. Lactam bridges linking (i, i + 3)-, (i, i + 4), and (i, i + 7)-spaced residue pairs have all proven useful for stabilization of alpha helices, and (i, i + 3)-linked residues have also been demonstrated to stabilize beta-turns. These structures are finding an increasing number of applications in protein biology, including studies of protein folding, protein aggregation, peptide ligand-receptor recognition, and the development of more potent peptide therapeutics. Defining the functional roles of the amphiphilic alpha-helices in medium-sized peptide hormones, and studying helix propagation from rigid, alpha-helix initiating bicyclic peptides are among the most exciting developments currently underway in this field.

Amyloid fibril formation by pentapeptide and tetrapeptide fragments of human calcitonin

The process of amyloid fibril formation by the human calcitonin hormone is associated with medullary thyroid carcinoma. Based on the effect of pH on the fibrillization of human calcitonin, the analysis of conformationally constrained analogues of the hormone, and our suggestion regarding the role of aromatic residues in the process of amyloid fibril formation, we studied the ability of a short aromatic charged peptide fragment of calcitonin (NH(2)-DFNKF-COOH) to form amyloid fibrils. Here, using structural and biophysical analysis, we clearly demonstrate the ability of this short peptide to form well ordered amyloid fibrils. A shorter truncated tetrapeptide, NH(2)-DFNK-COOH, also formed fibrils albeit less ordered than those formed by the pentapeptide. We could not detect amyloid fibril formation by the NH(2)-FNKF-COOH tetrapeptide, the NH(2)-DFN-COOH tripeptide, or the NH(2)-DANKA-COOH phenylalanine to the alanine analogue of the pentapeptide. The formation of amyloid fibrils by rather hydrophilic peptides is quite striking, because it was speculated that hydrophobic interactions might play a key role in amyloid formation. This is the first reported case of fibril formation by a peptide as short as a tetrapeptide and one of very few cases of amyloid formation by pentapeptides. Because the aromatic nature seems to be the only common property of the various very short amyloid-forming peptides, it further supports our hypothesis on the role of aromatic interactions in the process of amyloid fibril formation.

Conformationally constrained human calcitonin (hCt) analogues reveal a critical role of sequence 17-21 for the oligomerization state and bioactivity of hCt

Calcitonin (Ct) is a 32-residue peptide hormone that is mainly known for its hypocalcemic effect and the inhibition of bone resorption. Our previous studies have led to potent, side-chain lactam-bridged human Ct (hCt) analogues [Kapurniotu, A. Kayed, R., Taylor, J.W. & Voelter W. (1999) Eur. J. Biochem. 265, 606-618; Kapurniotu, A. & Taylor, J.W. (1995) J. Med. Chem. 38, 836-847]. We have hypothesized that a possibly type I beta turn/beta sheet conformation in the region 17-21 may play an important role in hCt bioactivity. To investigate this hypothesis, analogues of the potent hCt agonist cyclo17,21-[Asp17,Lys21]hCt (1) bearing type I (and II') or II beta turn-promoting substituents at positions 18 and 19 were designed, synthesized and their solution conformations, human Ct receptor binding affinities and in vivo hypocalcemic potencies were assessed. The novel analogues include cyclo17,21-[Asp17,D-Phe19, Lys21]hCt (2), cyclo17,21-[Asp17,Aib18,Lys21]hCt (3), cyclo17,21-[Asp17,D-Lys18,Lys21]hCt (4), corresponding partial sequence peptides containing the lactam-bridged region 16-22, and nonbridged control peptides. Only 1 showed a higher Ct receptor binding affinity than hCt, whereas analogues 2-4 had similar receptor affinities to hCt. In the in vivo hypocalcemic assay, 3 and 4 were as potent as 1, whereas 2 completely lost the high potency of 1, suggesting that type I (and II') beta turn-promoting substituents are fully compatible with in vivo bioactivity. CD spectroscopy showed that analogues 1-4 were markedly beta sheet-stabilized compared to hCt and indicated the presence of distinct beta turn conformeric populations in each of the analogues. Unexpectedly, the D-amino acid- or Aib-containing cyclic analogues 2-4 but not 1 or hCt self-associated into SDS denaturation-stable dimers. Our results demonstrate a crucial role of the conformational and topological features of the residues in sequence 17-21 and in particular of residues 18 and 19 for human Ct receptor binding and in vivo bioactivity and also for the self association state of hCt. These results may assist to delineate the structure-function relationships of hCt and to design novel hCt agonists for the treatment of osteoporosis and other bone-disorder-related diseases.

Structure of human salivary histatin 5 in aqueous and nonaqueous solutions

The solution structure of human salivary histatin 5 (D-S-H-A-K-R-H-H-G-Y-K-R-K-F-H-E-K-H-H-S-H-R-G-Y) was examined in water (pH 3.8) and dimethyl sulfoxide solutions using 500 MHz homo- and heteronuclear two-dimensional (2D) nmr. The resonance assignment of peptide backbone and side-chain protons was accomplished by 2D total correlated spectroscopy and nuclear Overhauser effect (NOE) spectroscopy. The high JNH-C alpha H values (> or = 7.4 Hz), absence of any characteristic NH-NH (i, i + 1) or C alpha H-C beta H (i, i + 3) NOE connectivities, high d delta/dT values (> or = 0.004 ppm K-1) and the fast 1H/2H amide exchange suggest that histatin 5 molecules remain unstructured in aqueous solution at pH 3.8. In contrast, histatin 5 prefers largely alpha-helical conformation in dimethyl sulfoxide solution as evident from the JNH-C alpha H values (< or = 6.4 Hz), slow 1H/2H exchange, low d delta/dT values (< or = 0.003 ppm K-1) observed for amide resonances of residues 6-24, and the characteristic NH-NH (i, i + 1) and C alpha H-C beta H (i, i + 3) NOE connectivities. All backbone amide 15N-1H connectivities fall within 6 ppm on the 15N scale in the 2D heteronuclear single quantum correlated spectrum, and the restrained structure calculations using DIANA suggest the prevalence of alpha-helical conformations stabilized by 19 (5-->1) intramolecular backbone amide hydrogen bonds in polar aprotic medium such as dimethyl sulfoxide. The interside-chain hydrogen bonding and salt-bridge type interactions that normally stabilize the helical structure of linear peptides in aqueous solutions are not observed. Histatin 5, unlike other naturally occurring antimicrobial polypeptides such as magainins, defensins, and tachyplesins, does not adopt amphiphilic structure, precluding its insertion into microbial membranes and formation of ion channels across membranes. Electrostatic (ionic type) and hydrogen bonding interactions of the positively charged and polar residues with the head groups of microbial membranes or with a membrane-bound receptor could be the initial step involved in the mechanism of antimicrobial activity of histatins.

Conformational transitions and fibrillation mechanism of human calcitonin as studied by high-resolution solid-state 13C NMR

Conformational transitions of human calcitonin (hCT) during fibril formation in the acidic and neutral conditions were investigated by high-resolution solid-state 13C NMR spectroscopy. In aqueous acetic acid solution (pH 3.3), a local alpha-helical form is present around Gly10 whereas a random coil form is dominant as viewed from Phe22, Ala26, and Ala31 in the monomer form on the basis of the 13C chemical shifts. On the other hand, a local beta-sheet form as viewed from Gly10 and Phe22, and both beta-sheet and random coil as viewed from Ala26 and Ala31 were detected in the fibril at pH 3.3. The results indicate that conformational transitions from alpha-helix to beta-sheet, and from random coil to beta-sheet forms occurred in the central and C-terminus regions, respectively, during the fibril formation. The increased 13C resonance intensities of fibrils after a certain delay time suggests that the fibrillation can be explained by a two-step reaction mechanism in which the first step is a homogeneous association to form a nucleus, and the second step is an autocatalytic heterogeneous fibrillation. In contrast to the fibril at pH 3.3, the fibril at pH 7.5 formed a local beta-sheet conformation at the central region and exhibited a random coil at the C-terminus region. Not only a hydrophobic interaction among the amphiphilic alpha-helices, but also an electrostatic interaction between charged side chains can play an important role for the fibril formation at pH 7.5 and 3.3 acting as electrostatically favorable and unfavorable interactions, respectively. These results suggest that hCT fibrils are formed by stacking antiparallel beta-sheets at pH 7.5 and a mixture of antiparallel and parallel beta-sheets at pH 3.3.

Solution stability of salmon calcitonin at high concentration for delivery in an implantable system

Salmon calcitonin solutions (50 mg/mL and 100 mg/mL) were placed on stability at 37 degrees C for 1 year in a variety of solvent systems including water, ethanol, glycerol, propylene glycol (PG) and dimethyl sulfoxide (DMSO). Calcitonin degradation was monitored by RP-HPLC and size-exclusion chromatography. DMSO and pH 3.3 solutions provided optimum stability. Conformational stability was also monitored by FTIR over the 1 year time course and compared with chemical and physical stability. After 12 months at 37 degrees C, four major conformations were observed: a beta-sheet conformation (pH 3.3, pH 5.0, 70% DMSO and 70% glycerol), an aggregate conformation (pH 7.0 water), a strong alpha-helical conformation (70% EtOH, 70% PG) and a weak alpha-helical conformation (100% DMSO). No correlation between structure and chemical stability was observed in which both the beta-sheet structure (pH 3.3, water) and a loose alpha-helical structure (100% DMSO) demonstrated good stability. However, some correlation was observed between structure and physical stability, where co-solvents inducing an alpha-helical structure resulted in a decrease in gelation. These two structural states associated with improved stability and minimal gelation, indicated that gelation can be reduced or eliminated by the use of pharmaceutically acceptable co-solvents. Finally, salmon calcitonin (50 mg/mL) was formulated in 100% DMSO and delivered from a DUROS implant over 4 months. Delivery at a target dose of 18 microg/day calcitonin at 37 degrees C was confirmed.

Rational design, conformational studies and bioactivity of highly potent conformationally constrained calcitonin analogues

Calcitonin is known for its hypocalcaemic effect and the inhibition of bone resorption, and is used therapeutically for the treatment of osteoporosis and Paget's disease. Our studies on the conformational features of human calcitonin (hCt) bioactivity have led to the conformationally constrained hCt analogue cyclo17,21-[Asp17, Lys21]hCt (1), which had a 5-10 times higher in vivo hypocalcaemic potency than hCt [Kapurniotu, A. & Taylor, J.W. (1995) J. Med. Chem. 38, 836-847]. We hypothesized that a stabilized, possibly type I beta turn/beta sheet conformation between residues 17 and 21 could play a crucial role in hCt bioactivity. Here, we designed, synthesized and studied the conformation and bioactivity of 19-member to 17-member ring-size analogues of 1 with the structure cyclo17,21-[Asp17,XX21]hCt with XX = Orn (2), Dab (3) and Dap (4), of the control peptide [Asp17,Orn21]hCt (5), and of the 19-member cyclo17,21-[Glu17,Dab21]hCt (6). Analyses of the far-UV CD spectra indicated increased type I beta turn and antiparallel beta sheet content in the bicyclic analogues compared with hCt. In the in vivo hypocalcaemic assay, cyclo17,21-[Asp17,Orn21]hCt (2) was found to have a 400-fold higher potency than hCt and was fourfold more potent than salmon calcitonin (sCt), which has been the most potent known Ct. Analogue 3 had a 30-fold higher potency than hCt, whereas the highly constrained analogue 4 was as potent as hCt. Bioactivity was not enhanced for the nonbridged compound [Asp17, Orn21]hCt (5), whereas cyclo17,21-[Glu17,Dab21]hCt (6) showed the same bioactivity as 1. This study identifies 2 as exhibiting the highest in vivo potency among currently known Cts, while it differs in only one amino acid residue from hCt, strongly suggesting that the introduced constraint may have served in 'freezing' hCt in a bioactive conformation. Our findings provide evidence for the first time that a beta turn/beta sheet conformation in region 17-21 of hCt and the topological features of the side chain of Asn17 are strongly associated with in vivo bioactivity, and offer a novel lead structure for a hCt-based drug for the treatment of osteoporosis and other bone-disorder-related diseases.

Phosphatydylglycerol promotes bilayer insertion of salmon calcitonin

Neutron diffraction from oriented multibilayers has been used to study the bilayer interaction of the amphipathic peptide salmon calcitonin. Penetration of calcitonin into bilayers composed of dioleoylphosphatidylcholine increases with the addition of 15% (mol) of the anionic phospholipid dioleoylphosphatidylglycerol. Neutron scattering profiles of water distribution in stacked bilayers show a continuous band of deuterons across each bilayer, consistent with the suggestion that the hormone forms transbilayer alpha-helixes under these conditions. These experiments add to the growing body of data on the role of phosphatidylglycerol in bilayer insertion of protein helices and suggests a possible evolutionary history for calcitonin.

Solution conformations of KNI-272, a tripeptide HIV protease inhibitor designed on the basis of substrate transition state: determined by NMR spectroscopy and simulated annealing calculations

KNI-272, a highly selective and potent HIV protease inhibitor containing allophenylnorstatine [(2S,3S)-3-amino-2-hydroxy-4-phenylbutyric acid], named Apns, has been studied in dimethylsulfoxide-d6 by NMR spectroscopy and simulated annealing calculations. 1H and 13C spectra showed the presence of two conformers characterized by the configuration of the imide bond between the Apns and Thz residues, i.e., trans and cis forms, respectively. Rotating frame Overhauser effect spectra revealed that the trans conformer is dominant. The solution structure calculated from the distance information resulting from nuclear Overhauser effects experiments is similar overall to those observed in the solid states, either as a single crystal or as complex with the protease. The results from both molecular dynamics simulations and experimental 13C longitudinal relaxation times indicate that the backbone of KNI-272 has a fairly rigid conformation.

Distinct conformational preferences of three cyclic beta-casomorphin-5 analogs determined using NMR spectroscopy and theoretical analysis

The conformational properties of three cyclic beta-casomorphin analogs based on the general formula H-Tyr-c[-D-Orn-2-Nal-D-Pro-Xaa-] (2-Nal = 2-naphthylalanine; Xaa = D-Ala, Sar or NMe-Ala) in DMSO solution were investigated using NMR spectroscopy in conjunction with molecular modeling techniques. The D-Ala5- and Sar5-analogs (compounds 1 and 2 respectively) are potent mixed mu-agonist/delta-antagonists with high mu- and delta-opioid receptor affinities, whereas the NMe-Ala5-analog (compound 3) is a potent mu-agonist and a weak partial delta-agonist. Distinct conformational differences emerged for the three compounds studied. Flexibility in the bare ring structures was found to increase in the order 3 < 2 < 1. The increased structural rigidity of 3 may be responsible for its low delta-receptor affinity as compared to the two other analogs. A low fractional population of conformers containing two cis peptide bonds was found for compound 2 but not for analog 1 or 3. Initial evidence for this observation was obtained from NMR differential line-broadening experiments and later confirmed by molecular mechanics simulations. Comparison of the temperature dependence of amide proton chemical shifts acquired for the three cyclic analogs indicate a large degree of intramolecular hydrogen bonding for 1 but not for the other two peptides.

Interfacial adsorption and aggregation associated changes in secondary structure of human calcitonin monitored by ATR-FTIR spectroscopy

The peptide hormone human calcitonin (hCT) has a marked tendency to aggregate in aqueous solutions, resulting in viscous and turbid dispersions consisting of long fibrils approximately 80 A in diameter. Both transmission (T-FTIR) and attenuated total reflection Fourier transform infrared (ATR-FTIR) experiments were applied on hCT adsorption and aggregation kinetics. By means of the surface sensitive ATR-FTIR spectroscopy at a hydrophobic/hydrophilic interface, early adsorption and aggregation steps of hCT could be followed in situ under real time conditions. Since the aggregation of hCT is associated with conformational changes, the secondary structure sensitive amide I'-band (D2O) could be used as a diagnostic marker. ATR-FTIR spectra recorded during the aggregation kinetics of hCT showed an increase of the amide I'-band intensity by a factor of 3.4, interpreted as pronounced adsorption of hCT molecules from bulk solution to the germanium plate. Furthermore, variations in the line shape of the amide I'-band were interpreted. At the beginning, hCT adopted a random coil conformation followed by distinct formations of alpha-helical and intermolecular parallel beta-sheet structures. Finally, the random coil content declined to 63%, whereas alpha and beta contents rose to 8% and 29%, respectively. From our kinetics results the alpha-structures were formed faster than the beta-structures. This was interpreted as an initial induction of amphiphilic helices during the adsorption process of hCT monomers. ATR-FTIR spectroscopy provides a sensitive analytical tool suggested to monitor interfacial adsorption and aggregation phenomena also of other peptides and proteins.

Conformational analysis of elcatonin in solution

The conformational characteristics of elcatonin, an analogue of eel calcitonin having a disulfide bond Cys1-Cys7 replaced by an ethylene linkage between residues 1 and 7, have been analyzed in aqueous trifluoroethanol solutions. Circular dichroic spectra of elcatonin and eel calcitonin itself reveal the presence of alpha-helices at trifluoroethanol concentrations above 15%. The spectral changes caused by the trifluoroethanol content of the solutions are interesting. An isosbetic point is detected for eel calcitonin indicating that a conformational transition occurs between two states, namely alpha-helical and random coil states. On the other hand, the CD curves of elcatonin at less than 15% trifluoroethanol deviate from the isosbetic point while those at higher concentration are similar to those of eCT. This can probably be attributed to the third element of the ordered structure of elcatonin which is formed in 15% trifluoroethanol. The solution conformation of elcatonin in a mixture of 60% water and 40% trifluoroethanol has been determined by the combined use of 1H-NMR spectroscopy and distance geometry calculations. The conformation is characterized by an amphiphilic alpha-helix between Thr6 and Thr21, which extends into the constrained cyclic portion of the molecule to Thr6. The third structural element of elcatonin found in the CD analysis is detected by some turn structures in the region between residue 1 and Ser5 in the calculated structure.

Solution conformation of salmon calcitonin in sodium dodecyl sulfate micelles as determined by two-dimensional NMR and distance geometry calculations

The 32 amino acid hormone salmon calcitonin was studied at pH 3.7 and 7.4 by two-dimensional NMR in sodium dodecyl sulfate (SDS) micelles at 310 K. The spectrum was fully assigned, and the secondary structure was obtained from nuclear Overhauser enhancement spectroscopy (NOESY), 3JHN alpha coupling constants, and slowly exchanging amide data. Three-dimensional structures consistent with NMR data were generated by using distance geometry calculations. A set of 260 interproton distances, derived from NOESY, and hydrogen-bond constraints, obtained from analysis of the amide exchange, were used. From the initial random conformations, 13 distance geometry structures with minimal violations were selected for further refinement with restrained energy minimization. In SDS, at both pHs, the main conformational feature of the hormone is an alpha-helix from Thr6 through Tyr22, thus including the amphipathic 8-22 segment and two residues of the Cys1-Cys7 N-terminal loop. The C-terminal decapeptide forms a loop folded back toward the helix. The biological significance of this conformation is discussed.