Conformational studies of corticotropin1-32 and constitutive peptides by circular dichroism

Solvent Perturbation of Protein Structures - A Review Study with Lectins

Use of organic molecules as co-solvent with water, the ubiquitous biological solvent, to perturb the structure of proteins is popular in the research area of protein structure and folding. These organic co-solvents are believed to somehow mimic the environment near the cell membrane. Apart from that they induce non-native states which can be present in the protein folding pathway or those states also may be representative of the off pathway structures leading to amyloid formation, responsible for various fatal diseases. In this review, we shall focus on organic co-solvent induced structure perturbation of various members of lectin family. Lectins are excellent model systems for protein folding study because of its wide occurrence, diverse structure and versatile biological functions. Lectins were mainly perturbed by two fluoroalcohols - 2,2,2- trifluoroethanol and 1,1,1,3,3,3-hexafluoroisopropanol whereas glycerol, ethylene glycol and polyethylene glycols were used in some cases. Overall, all native lectins were denatured by alcohols and most of the denatured lectins have predominant helical secondary structure. But characterization of the helical states and the transition pathway for various lectins revealed diverse result.

Folding of intrinsically disordered plant LEA proteins is driven by glycerol-induced crowding and the presence of membranes

Late embryogenesis abundant (LEA) proteins are related to cellular dehydration tolerance. Most LEA proteins are predicted to have no stable secondary structure in solution, i.e., to be intrinsically disordered proteins (IDPs), but they may acquire α-helical structure upon drying. In the model plant Arabidopsis thaliana, the LEA proteins COR15A and COR15B are highly induced upon cold treatment and are necessary for the plants to attain full freezing tolerance. Freezing leads to increased intracellular crowding due to dehydration by extracellular ice crystals. In vitro, crowding by high glycerol concentrations induced partial folding of COR15 proteins. Here, we have extended these investigations to two related proteins, LEA11 and LEA25. LEA25 is much longer than LEA11 and COR15A, but shares a conserved central sequence domain with the other two proteins. We have created two truncated versions of LEA25 (2H and 4H) to elucidate the structural and functional significance of this domain. Light scattering and CD spectroscopy showed that all five proteins were largely unstructured and monomeric in dilute solution. They folded in the presence of increasing concentrations of trifluoroethanol and glycerol. Additional folding was observed in the presence of glycerol and membranes. Fourier transform infra red spectroscopy revealed an interaction of the LEA proteins with membranes in the dry state leading to a depression in the gel to liquid-crystalline phase transition temperature. Liposome stability assays revealed a cryoprotective function of the proteins. The C- and N-terminal extensions of LEA25 were important in cryoprotection, as the central domain itself (2H, 4H) only provided a low level of protection.

Melanocortin MC(4) receptor-mediated feeding and grooming in rodents

Decades ago it was recognized that the pharmacological profile of melanocortin ligands that stimulated grooming behavior in rats was strikingly similar to that of Xenopus laevis melanophore pigment dispersion. After cloning of the melanocortin MC1 receptor, expressed in melanocytes, and the melanocortin MC4 receptor, expressed mainly in brain, the pharmacological profiles of these receptors appeared to be very similar and it was demonstrated that these receptors mediate melanocortin-induced pigmentation and grooming respectively. Grooming is a low priority behavior that is concerned with care of body surface. Activation of central melanocortin MC4 receptors is also associated with meal termination, and continued postprandial stimulation of melanocortin MC4 receptors may stimulate natural postprandial grooming behavior as part of the behavioral satiety sequence. Indeed, melanocortins fail to suppress food intake or induce grooming behavior in melanocortin MC4 receptor-deficient rats. This review will focus on how melanocortins affect grooming behavior through the melanocortin MC4 receptor, and how melanocortin MC4 receptors mediate feeding behavior. This review also illustrates how melanocortins were the most likely candidates to mediate grooming and feeding based on the natural behaviors they induced.

Ligands, their receptors and ... plasma membranes

Ligand-receptor interactions are customarily described by equations that apply to solutes. Yet, most receptors are present in cell membranes so that sufficiently lipophilic ligands could reach the receptor by a two-dimensional approach within the membrane. As summarized in this review, this may affect the ligand-receptor interaction in many ways. Biophysicians calculated that, compared to a three-dimensional approach from the liquid phase, such approach could alter the time the ligands need to find a receptor. Biochemists found that ligand incorporation in lipid bilayers modifies their conformation. This, along with the depth at which the ligands reside in the bilayer, will affect the probability of successful receptor interaction. Novel mechanisms were also introduced, including "exosite" binding and ligand translocation between the receptor's alpha-helical transmembrane domains. Pharmacologists focused attention at ligand concentrations in membrane, their adsorption and release rates and the effects thereof on ligand potency and residence time at the receptor.

Conformation induction in melanotropic peptides by trifluoroethanol: fluorescence and circular dichroism study

Conformation induction in the two related peptides, alpha-melanocyte stimulating hormone (alpha-MSH) and delta-melanocyte stimulating hormone (delta-MSH), have been studied in solvent media containing varying percentages of the membrane-mimetic solvent 2,2,2-trifluoroethanol (TFE) using fluorescence and circular dichroism (CD) spectroscopy. Singular value decomposition (SVD) analysis of the CD spectra at different TFE concentrations showed that these spectra can be described as linear combinations of only two distinct basis spectra, corresponding to the peptides in the random-coil and 'folded' conformations. For alpha-MSH the spectrum of the folded state is very similar to the standard spectrum of the alpha-helix, while that for delta-MSH has partial resemblance to the helical spectrum. Fitting the data on ellipticity (at 222 nm) as a function of TFE volume fraction to an equation based on a two-state model describing TFE-induced conformation induction in the peptides gave values of (1.1 +/- 0.4) and (4.2 +/- 0.5) kcal mol-1 for alpha-MSH and delta-MSH, respectively, for the free energy of equilibrium between the helix and coil forms in water. Measurement of fluorescence emission parameters (emission maximum, quantum yield, steady-state anisotropy and mean excited-state lifetime) indicated that the microenvironment around the single tryptophan residues of both peptides changes in like manner with increasing concentration of TFE in the solvent. The similarity of fluorescence behaviour of the peptides suggests that their Trp fluorophores do not participate in secondary structure formation in TFE.

Studies of the binding and structure of adrenocorticotropin peptides in membrane mimics by NMR spectroscopy and pulsed-field gradient diffusion

The partition and structure of three adrenocorticotropic hormone peptides ACTH(1-10), ACTH(1-24), and ACTH(11-24) in water and in sodium dodecylsulfate (SDS) and dodecylphosphocholine (DPC) micelles were studied by 2D NMR and NMR gradient diffusion measurements. The diffusion rates, the NH chemical shifts, and the nuclear Overhauser effect patterns provided a coherent picture of binding of these peptides. All three peptides are significantly partitioned in the negatively charged SDS micelles and possess definite secondary structure, as opposed to random structures in water. For ACTH (1-24), the hydrophobic 1-10 segment is partitioned in DPC micelles, but the charged 11-24 segment prefers to remain in the aqueous region. ACTH(11-24) does not bind significantly to the DPC micelles. The binding of the ACTH peptides in these two widely used "membrane mimics" are substantially different from that in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers obtained by attenuated total reflection infrared spectroscopy and from our preliminary diffusion studies of the same peptides in POPC vesicles. This study showed that, in a given micellar medium, all corresponding segments of these peptides are located in the same membrane environment in the system, regardless of whether these segments exist by themselves or are attached to other segments. This result may contradict the membrane-compartments concept of Schwyzer, which suggests that ACTH(1-10) and ACTH(1-24) are located in different membrane compartments because they have different address segments, and consequently, bind to different receptors. The present results also suggest that the assumption that micelles are good membrane mimics should be carefully examined.

Folding of the presequence of yeast pAPI into an amphipathic helix determines transport of the protein from the cytosol to the vacuole

To investigate the role of the 17 residues long presequence (p17) in the transport of the precursor of yeast API (pAPI) from the cytosol to the vacuole we have studied the effects of point mutations upon its conformation and on the process of transport. 1H NMR analysis of p17 indicates that in aqueous solution 26% of the molecules have the 4-12 segment folded into an helix. The hydrophobic environment provided by SDS micelles promotes the folding of 54% of the p17 molecules into a 5-16 amphipathic alpha-helix. Both Schiffer-Edmunson helical wheel analysis of segment 4-12 and residue hydrophobic moments calculated considering all possible side-chain orientations between 80 and 120 degrees, indicate the amphipathic character of the helixes assembled in water and detergent. Charge interactions between the dipole pairs N-Glu2Glu3 and C-Lys12Lys13 are essential for helix stability and condition pAPI transport. Substitution of either Pro2Pro3 or Lys2Lys3 for Glu2Glu3, results in moderate destabilization of the helix, decreases protein targeting to the vacuolar membrane and partly inhibits translocation of the protein to the vacuolar lumen. Replacement of either Pro12Pro13 or Glu12Glu13 for Lys12Lys13, causes a major disruption of the helix, decreases protein targeting and blocks completely the translocation of the protein to the vacuolar lumen. Replacement of Gly7 for Ile7, a substitution which is known to destabilize alpha-helixes in peptides and proteins as a result of the peptide bond to the solvent at Gly residues, produces similar effects as the substitutions for the K12K13 pair. The effects of Gly7 on helix stability and protein transport are partly reversed by introduction of Asp residues at positions 2 and 3 and Ala at position 4. Replacements such as Arg2 for Glu2, or Arg6 for Glu6, which change the net and local charges of the presequence without altering its conformation, have no effect on the protein transport. These results provide direct evidence of the involvement of the presequence in the transport of pAPI from the cytosol to the vacuole. They show that folding of the pAPI presequence is conditioned by the physical/chemical properties of the environment and is critical for targeting the protein to the vacuolar membrane and for its translocation to the vacuolar lumen.

The basic subdomain of the c-Jun oncoprotein. A joint CD, Fourier-transform infrared and NMR study

The structural properties of the basic subdomain of the basic zipper (bZIP) protein c-Jun were examined by joint means of 1H-NMR, CD and Fourier-transform infrared (FTIR) spectroscopies. The basic subdomain (residues 252-281 in c-Jun) is responsible for sequence-specific recognition of DNA. A modified basic subdomain bSD (residues 1-35) and its N-terminal part and C-terminal part fragments (NP, residues 1-19; and, CP, residues 16-35) were prepared by solid-phase synthesis and purified by HPLC. In aqueous solution, in the absence of DNA, bSD behaved mostly as an unstructured peptide characterized by only 5% alpha helix. However, upon mixing bSD and a specific DNA fragment, i.e. a CRE(cAMP-responsive element)-containing hexadecanucleotide, the alpha helix was stabilized to an extent of 20% at 20 degrees C or 35% at 2 degrees C. At the same time, no significant change could be detected in the DNA spectra. Addition of trifluoroethanol to an aqueous bSD sample resulted in an increase of the alpha-helix content so that about 60% of alpha helix was found at a ratio of 75% trifluoroethanol (20 degrees C). These effects were reflected in both CD and FTIR measurements. Changes shown by the CD spectra during the process suggested a mechanism dominated by a two-state helix/unordered transition. NMR data, namely alpha H chemical shifts, NOE cross-peaks and NH temperature coefficients provided indications for extended or nascent helix structures within four short stretches dispersed along the sequence for c-Jun bSD, contrasting with the unique and continuous stretch reported for Gcn4 (yeast general control protein 4) bSD in aqueous solution. Trifluoroethanol stabilized the alpha-helix structure mainly at these four sites. The malleability of the basic subdomain of c-Jun was emphasized in relation to its ability to fit the DNA helix in adopting an alpha-helix structure. The complex formation apparently requires substantial conformational change from the peptide and only little from the oligonucleotide.

Structural properties of the myelin-associated glycoprotein ectodomain

Myelin-associated glycoprotein (MAG) has been proposed to mediate adhesive interactions during myelin development. We have used the baculovirus expression system to produce a truncated form of this molecule [soluble extracellular domain of MAG (sMAG)] consisting of the complete extracellular ectodomain. Spectroscopic studies indicate a high beta-sheet content, consistent with the prediction of Ig-like structure. Hydrodynamic studies indicate an asymmetric monomer, with a Stokes radius of 4.1-4.6 nm, a sedimentation coefficient of 3.6S, and a frictional ratio of approximately 1.6. We postulate that the outer two Ig-like domains form a unit that folds back over the rest of the molecule. Fluorescence quenching studies indicate that sMAG interacts with divalent cations and may have a functional lectin domain.

Conformation and orientation of regulatory peptides on lipid membranes. Key to the molecular mechanism of receptor selection

The reaction of regulatory peptides with their membrane-bound receptors often occurs via a membrane-associated state of the peptide. From infrared studies on thin lipid films, we have shown that several ligands of the opioid kappa receptor and the neurokinin NK-1 receptor insert their message segments as an alpha-helix, more or less perpendicularly, into the membrane. The binding parameters for these membrane-associated states were determined from the capacitance minimization potential of lipid bilayers. A theory has been developed to account for the observed binding constants and the preferred conformation and orientation of these peptides. In contrast to the kappa and NK-1 receptors, ligands of the opioid mu and delta, and the neurokinin NK-2 and NK-3 receptors, are predicted not to form the inserted alpha-helical structure. A selection between the mu and delta (or NK-2 and NK-3) receptors appears to be made on the basis of an electrostatic gradient near the membrane surface. The molecular mechanism of receptor selection thus appears to be based to a large extent on the membrane-induced compartmentalization of ligands for the different receptors.

Biological and physical properties of a model calcitonin containing a glutamate residue interrupting the hydrophobic face of the idealized amphiphilic alpha-helical region

2A new calcitonin analogue, model calcitonin III (MCt-III), has been synthesized, and its biological and physical characteristics have been studied. This analogue has an idealized alpha-helix from residue 8-22 with glutamate at position 15 interrupting an otherwise continuous surface of aliphatic side chains (those of leucine residues) on the hydrophobic face of the helix. MCt-III differs from a previous model, MCt-II, only by the substitution Leu15----Glu and is here compared with salmon calcitonin I (sCt-I) and MCt-II to elucidate further the role of the putative amphiphilic alpha-helix in determining biological and physical properties of the hormone. MCt-III shows physical properties intermediate between those of sCt-I and MCt-II, demonstrating the influence of appropriately positioned single residues on properties of amphiphilic structures. In our two biological assays, a brain-binding assay and an in vivo hypocalcemic assay, MCt-III reproduces the sigmoidal dose-response curves of sCt-I; this contrasts with the behavior of MCt-II, which demonstrated unusual dose-response curves in these two assays. MCt-III is almost three times more potent than sCt-I in our hypocalcemic assay; this activity groups MCt-III among the most potent known analogues of sCt-I.

Coordinative interactions in the separation of insulin and its derivatives by high-performance liquid chromatography

The effects of various inorganic cations and anions on the chromatographic behaviour of insulin and its derivatives have been investigated. It was shown that on an octadecylsilica column at a given eluent composition the retention of insulins depends primarily on the nature of the anions. The cations have a secondary effect, depending on their size and coordination ability. The mechanism of the effect of the ions is complex. It includes the formation of ion pairs, their influence on dissociation processes, a silanophilic effect, coordination processes and changes in the structures of the insulins.

Conformational aspects and rotational dynamics of synthetic adrenocorticotropin-(1-24) and glucagon in reverse micelles

The tryptophan (Trp) rotational dynamics and the secondary structure of the peptide hormones adrenocorticotropin-(1-24) [ACTH(1-24)]--the fully active N-terminal fragment of adrenocorticotropin-(1-39)--and glucagon were studied in aqueous solutions and in reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/water/isooctane, a system selected to mimic the membrane-water interface. In aqueous solutions, the total fluorescence intensity decays of their single Trp residue [Trp-9 and Trp-25 for ACTH(1-24) and glucagon, respectively] are multiexponential. This is also the case for ACTH(5-10), a fragment of the adrenocorticotropin "message" region. Time-resolved fluorescence anisotropy data evidence a high degree of rotational freedom of the single Trp residue. Transfer of these peptides from water to the aqueous core of reverse micelles induces severe restrictions of the Trp internal motion and of its local environment. The results indicate that the Trp-9 residue in ACTH(1-24 is maintained in the close neighborhood of the water-AOT molecular interface where the water molecules are strongly immobilized. By contrast, the Trp residues in ACTH(5-10) and glucagon are likely to be located closer to the center of the micellar aqueous core where the water molecules are in a more mobile state. Furthermore, the above location of Trp can be extended to the peptide chains themselves as evidenced by the overall correlation time values of the peptide-containing micelles. Nevertheless, in all peptides, the indole ring remains susceptible to oxidation by N-bromosuccinimide. Circular dichroism measurements evidence the induction in glucagon of alpha-helices remaining unaffected by the micellar water content. Conversely, beta-sheet structures are favored in ACTH(1-24) at low water-to-surfactant molar ratios (w0) but are disrupted by subsequent additions of water. These results are discussed in terms of the possible role of the micellar interfaces in selecting the preferred peptide dynamical conformation(s)

Comparison of structural requirements of alpha-MSH and ACTH for inducing excessive grooming and pigment dispersion

alpha-MSH and ACTH-like peptides are known to play an important role in the adaptation of many vertebrates to a new environment. These peptides induce pigment dispersion in amphibian melanophores through a receptor-mediated mechanism. In this study we compared the structural requirements of these peptides for melanotropic activity on Xenopus laevis melanophores with those for inducing excessive grooming in the rat. With the exception of ACTH1-24 there is a close resemblance in structure-activity relationships of the fragments and analogs tested in the two bioassays. [Nle4,-D-Phe7]-alpha-MSH is extremely active in both assays. Weak agonists such as [Leu9]-alpha-MSH did not possess antagonistic properties either in the melanophore assay or in the excessive grooming test. The data suggest that the mechanism of action of alpha-MSH-like peptides in rat brain is receptor-mediated like their action on melanophores.

Antiparallel beta-sheets in the crystal structure of the heptapeptide Met-Glu-His-Phe-Arg-Trp-Gly (ACTH 4-10)

The conformation of the molecules in ACTH 4-10 has been determined as part of a study of the conformations of the biologically active N-terminal fragments of the adrenocorticotropic hormone (ACTH). ACTH 4-10 crystallizes in two different superstructures. The substructure considered in the present work, is monoclinic, space group C2, a = 25.75(1) A, b = 27.78(1) A, c = 20.35(1) A, beta = 114.0(1) degrees, Z = 8 molecules ACTH 4-10 plus 22 weight per cent solvent. The crystals contain antiparallel beta-sheets, the orientations of the side groups are not found, because of disorder. The present crystal structure and those of other linear oligopeptides emphasize that antiparallel beta-sheets are energetically favourable. It is very unlikely, however, that the ACTH 4-10 crystals contain the molecules in their biologically active form.

Conformational changes of adrenocorticotropin peptides upon interaction with lipid membranes revealed by infrared attenuated total reflection spectroscopy

Infrared attenuated total reflection (IR-ATR) spectroscopy was used to study conformational and topological aspects of the interaction between two adrenocorticotropin fragments and dioleoylphosphatidylcholine membranes. Corticotropin-(1-10)-decapeptide, ACTH1-10, was found to exist as a rigid antiparallel pleated sheet structure in dry membranes. In aqueous environment, it completely escaped from the lipid. This dominant preference for the aqueous phase is a possible explanation for the very low biological potency of ACTH1-10 in some assays. On the other hand, the very potent corticotropin-(1-24)-tetracosapeptide, ACTH1-24, was firmly incorporated into dry and wet membranes. Aqueous environment even promoted the peptide-lipid interaction. Under these latter conditions, part of the molecule entered the bilayer and adopted a helical structure with the axis oriented perpendicularly to the bilayer plane. Contact of a 0.1 mM solution of ACTH1-24 in liquid deuterium oxide with the pure lipid membrane system resulted in measurable adsorption of the peptide to the membrane with the same conformational and topological characteristics as described above (perpendicularly oriented helix entering the bilayer). The helical part of the ACTH1-24 molecule entering the bilayer was the quite hydrophobic N-terminal decapeptide unit ("message" segment). The adjacent hydrophilic C-terminal tetradecapeptide unit ("address" segment) remained on the membrane surface. As the message region is essential for triggering corticotropin receptors, its intrusion into the membrane and its adoption of an oriented, helical conformation may facilitate receptor stimulation.

Conformation of corticotropin. An infrared spectrometry study of hydrogen exchange kinetics

1H--2H exchange kinetics of the peptide hydrogens in corticotropin have been examined in 2H2O and CF3C2H2O2H solutions by means of infrared absorption measurements. In aqueous solution, around pH 3, the experimental data suggest a partially ordered structure, since in the two corticotropins 1--24 and 1--32 about 6 slowly exchanging peptide protons are numbered. These might belong to the N-terminal part of the molecule. The C-terminal 25--32 octapeptide segment appears to be unordered and slightly destabilizes the overall hormone conformation. For corticotropin1--24 in CF3C2H2O2H, the qualitative interpretation of infrared spectra and the quantitative analysis of exchange data give evidence of a strong stabilization: a predominantly alpha-helical structure is induced by trifluoroethanol.

A proton NMR investigation of proline-24 cis-trans isomerism in corticotropin 1-32 and related peptides

250 MHz 1H-NMR studies performed on aqueous solutions of corticotropin1-32, corticotropin1-24, corticotropin15-32, corticotropin20-32 and corticotropin15-24 have allowed the location and the subsequent assignment of the signals of Tyrosine-23 aromatic protons and valine-22 methyl protons. These signals are sensitive to the geometry of proline-24, clearly transcribe its isomerism and yield the ratio of the cis-trans conformers. It is concluded that for large peptides in specific cases, the proton signals of side chains can be used to probe the backbone conformation.

Conformational features of bradykinin. A circular dichroism study of the aromatic side-chains

The circular dichroism (CD) of the peptide hormone bradykinin and its analogues, [Phe(H4)5]-bradykinin, [Phe(H4)8]bradykinin, [Phe(H4)5,8]bradykinin, [TyrOMe5]bradykinin, [TyrOMe8]bradykinin and [TyrOMe5.8]bradykinin, is described. The comparison of the CD spectra of these analogues with each other, recorded under a variety of conditions (pH, solvent, temperature), allows the monitoring of the behaviour of the aromatic side-chains (phenylalanine, tyrosine) and an estimation of their respective spectral contributions in both spectral regions (320-250 nm, 250-190 nm) with good precision. Conformational non-equivalence of the residues Phe-5 and Phe-8 together with some overall conformational features of bradykinin are thus established.

Influence of hydrogen bonding on the rotamer distribution of the histidine side chain in peptides: 1H NMR and CD studies

Both 1H NMR and circular dichroism pH titration studies on histidine, His-Gly, Gly-His and Gly-His-Gly indicate that the side-chain spatial orientation depends strongly on the vicinal charges. The arrangement of the imidazole side-chain (rotamer population) is shown by the histidine beta and beta' and the glycine methylene proton chemical shifts as well as the vicinal 1H-1H coupling constants 3JCalpha-H-beta-H, beta'-H. For His-Gly and Gly-His-Gly a good correlation can be found between the ionization of the glycine COOH group and the increase of rotamer III (g-g) which is also visualized by circular dichroism through an enhancement of the ellipticity at 212 nm. In these two peptides a hydrogen bond between the imidazolium and the carboxylate group is supposed to stabilize rotamer III at pH 4-5.

Coordination-chemistry study of polypeptides. III. protonation-deprotonation equilibrium study of synthetic alphaH -corticotropin1-32. Data on the pH-dependent conformation of corticotropin

By potentiometric equilibrium measurements and the computer evaluation of experimental data, the protonation equilibrium constants of four fragments of corticotropin (ACTH), ACTH1-32. ACTH1-28, ACTH1-14 and ACTH1-4, were determined and assigned to the corresponding functional groups. From the dependence of the protonation constants on the length of the peptide chain, it was established which functional groups participate in the formation of intramolecular hydrogen-bonds in aqueous solutions at various pH. These results indicated a pH-dependent conformation of the molecule.