Angiotensin II conformations. Infrared and raman studies

Differentiation of advanced generation mutant wheat lines: Conventional techniques versus Raman spectroscopy

This research aimed to assess the feasibility of utilizing Raman spectroscopy in plant breeding programs. For this purpose, the evaluation of the mutant populations set up the application of 4 mM NaN₃ to the somatic embryos obtained from mature wheat (Triticum aestivum L. Adana-99 cv.) embryos. Advanced wheat mutant lines, which were brought up to the seventh generation with salt stress tolerance by following in vitro and in vivo environments constructed by mutated populations, were evaluated using conventional techniques [measurement of antioxidant enzyme activities (SOD, CAT, and POX), total chlorophyll, TBARS, and proline contents; measurement of the concentration of Na⁺ and K⁺ ions; and evaluation of gene expression by qPCR (TaHKT2;1, TaHKT1;5, TaSOS1, TaNa⁺/H⁺ vacuolar antiporter, TaV-PPase, TaV-ATPase, and TaP5CS)] and Raman spectroscopy. In this research, no significant difference was found in the increase of SOD, CAT, and POX antioxidant enzyme activities between the salt-treated and untreated experimental groups of the commercial cultivar, while there was a statistically significant increase in salt-treated advanced generation mutant lines as compared to control and the salt-treated commercial cultivar. Proline showed a statistically significant increase in all experimental groups compared to the untreated commercial cultivar. The degradation in the amount of chlorophyll was lower in the salt-treated advanced generation mutant lines than in the salt-treated commercial cultivar. According to gene expression studies, there were statistical differences at various levels in terms of Na⁺ and/or K⁺ uptake from soil to plant (TaHKT2;1, TaHKT1;5, and TaSOS1), and Na⁺ compartmentalizes into the cell vacuole (TaNa⁺/H⁺ vacuolar antiporter, Ta vacuolar pyrophosphatase, and Ta vacuolar H⁺-ATPase). The expression activity of TaP5CS, which is responsible for the transcription of proline, is similar to the content of proline in the current study. As a result of Raman spectroscopy, the differences in peaks represent the protein-related bands in mutant lines having a general decreasing trend in intensity when compared to the commercial cultivar. Amide-I (1,630 and 1,668 cm^-1), Histidine, Lysine, Arginine, and Leucine bands (823, 849, 1,241, 1,443, and 1,582 cm^-1) showed decreasing wavenumbers. Beta-carotene peaks at 1,153 and 1,519 cm^-1 showed increasing trends when the normalized Raman intensities of the mutant lines were compared.

Biochemical changes of the endothelium in the murine model of NO-deficient hypertension

Alterations in the α-helix and β-sheet content and the lipid-to-protein ratio are the most striking features of hypertension development in the vascular endothelium.

Some solution properties of pentagastrin and angiotensin: aggregation of pentagastrin

Pentagastrin and angiotensin have been studied in the presence and absence of urea by light-scattering, viscosity, dialysis, surface tension and other physicochemical techniques for evidence of peptide aggregation. Pentagastrin forms large linear, flexible aggregates (molecular weight 40 000) in phosphate buffer above a concentration of ∼0·1 g dl−1 and in solutions shows time-dependent flow behaviour. Angiotensin shows no evidence of aggregation beyond dimers. In 1m urea the pentagastrin is in the form of dimers.

Determination of the structure of lipid vesicle-bound angiotensin II and angiotensin I

A mass spectrometry (MS)-based strategy was developed to determine the structure of lipid vesicle-bound angiotensin II (AII) and angiotensin I (AI). It involves hydrogen-deuterium exchange (HDX), chemical modifications (e.g., nitration of tyrosine, acetylation of free amino group), and ladder sequencing. HDX is also combined with tandem mass spectrometry (MS/MS) to provide structural details at individual amino acid residues. It was observed that a major portion of both of these peptide hormones interacts with the phospholipid head groups on the surface of the vesicles and that Tyr residue is embedded in the vesicles. Both peptides have a U-shaped structure in the lipid environment.

The Angiotensin II AT1 Receptor Structure-Activity Correlations in the Light of Rhodopsin Structure

The most prevalent physiological effects of ANG II, the main product of the renin-angiotensin system, are mediated by the AT1 receptor, a rhodopsin-like AGPCR. Numerous studies of the cardiovascular effects of synthetic peptide analogs allowed a detailed mapping of ANG II's structural requirements for receptor binding and activation, which were complemented by site-directed mutagenesis studies on the AT1 receptor to investigate the role of its structure in ligand binding, signal transduction, phosphorylation, binding to arrestins, internalization, desensitization, tachyphylaxis, and other properties. The knowledge of the high-resolution structure of rhodopsin allowed homology modeling of the AT1 receptor. The models thus built and mutagenesis data indicate that physiological (agonist binding) or constitutive (mutated receptor) activation may involve different degrees of expansion of the receptor's central cavity. Residues in ANG II structure seem to control these conformational changes and to dictate the type of cytosolic event elicited during the activation. 1) Agonist aromatic residues (Phe8 and Tyr4) favor the coupling to G protein, and 2) absence of these residues can favor a mechanism leading directly to receptor internalization via phosphorylation by specific kinases of the receptor's COOH-terminal Ser and Thr residues, arrestin binding, and clathrin-dependent coated-pit vesicles. On the other hand, the NH2-terminal residues of the agonists ANG II and [Sar1]-ANG II were found to bind by two distinct modes to the AT1 receptor extracellular site flanked by the COOH-terminal segments of the EC-3 loop and the NH2-terminal domain. Since the [Sar1]-ligand is the most potent molecule to trigger tachyphylaxis in AT1 receptors, it was suggested that its corresponding binding mode might be associated with this special condition of receptors.

Molecular Dynamics Simulations of Angiotensin II in Aqueous and Dimethyl Sulfoxide Environments

Angiotensin II (Ang II) is an octapeptidic hormone, which plays an important role in the mechanisms of blood pressure control. In this work, extensive molecular dynamics (MD) simulations have been carried out on this peptide, both in aqueous and in dimethyl sulfoxide (DMSO) environments. Experimentally proposed models for the structure of angiotensin II in both environments are not consensual and the results obtained have provided some further insight about the structural properties of this hormone. In these simulations, the N-terminus of Ang II in the aqueous environment has been associated with a considerable larger flexibility than the correspondent C-terminus, but this was not found in the case of the DMSO environment. This is consistent with the assumption that the biological activity of Ang II is associated with its C-terminal residues embedded in a hydrophobic environment of its AT1 receptor. Other features detected in DMSO environment were an H(His6 imidazole)-O(Phe8 carboxylate) hydrogen bond and a salt-bridge structure involving the Asp1 and Arg2 side chains. An additional important conformational feature is the spatial proximity between Tyr4 and His6 in both water and DMSO environments. This molecular feature may trigger the interest for the synthetic chemists to apply rational design for the synthesis of novel AT1 antagonists.

Comparison of the solution structures of angiotensin I & II. Implication for structure-function relationship

Conformational analysis of angiotensin I (AI) and II (AII) peptides has been performed through 2D 1H-NMR spectroscopy in dimethylsulfoxide and 2,2,2-trifluoroethanol/H2O. The solution structural models of AI and AII have been determined in dimethylsulfoxide using NOE distance and 3JHNHalpha coupling constants. Finally, the AI family of models resulting from restrained energy minimization (REM) refinement, exhibits pairwise rmsd values for the family ensemble 0.26 +/- 0.13 A, 1.05 +/- 0.23 A, for backbone and heavy atoms, respectively, and the distance penalty function is calculated at 0.075 +/- 0.006 A2. Comparable results have been afforded for AII ensemble (rmsd values 0.30 +/- 0.22 A, 1.38 +/- 0.48 A for backbone and heavy atoms, respectively; distance penalty function is 0.029 +/- 0.003 A2). The two peptides demonstrate similar N-terminal and different C-terminal conformation as a consequence of the presence/absence of the His9-Leu10 dipeptide, which plays an important role in the different biological function of the two peptides. Other conformational variations focused on the side-chain orientation of aromatic residues, which constitute a biologically relevant hydrophobic core and whose inter-residue contacts are strong in dimethylsulfoxide and are retained even in mixed organic-aqueous media. Detailed analysis of the peptide structural features attempts to elucidate the conformational role of the C-terminal dipeptide to the different binding affinity of AI and AII towards the AT1 receptor and sets the basis for understanding the factors that might govern free- or bound-depended AII structural differentiation.

On the molecular basis of the recognition of angiotensin II (AII). NMR structure of AII in solution compared with the X-ray structure of AII bound to the mAb Fab131

The high-resolution 3D structure of the octapeptide hormone angiotensin II (AII) in aqueous solution has been obtained by simulated annealing calculations, using high-resolution NMR-derived restraints. After final refinement in explicit water, a family of 13 structures was obtained with a backbone RMSD of 0.73 +/- 0.23 A. AII adopts a fairly compact folded structure, with its C-terminus and N-terminus approaching to within approximately 7.2 A of each other. The side chains of Arg2, Tyr4, Ile5 and His6 are oriented on one side of a plane defined by the peptide backbone, and the Val3 and Pro7 are pointing in opposite directions. The stabilization of the folded conformation can be explained by the stacking of the Val3 side chain with the Pro7 ring and by a hydrophobic cluster formed by the Tyr4, Ile5 and His6 side chains. Comparison between the NMR-derived structure of AII in aqueous solution and the refined crystal structure of the complex of AII with a high-affinity mAb (Fab131) [Garcia, K.C., Ronco, P.M., Verroust, P.J., Brunger, A.T., Amzel, L.M. (1992) Science257, 502-507] provides important quantitative information on two common structural features: (a) a U-shaped structure of the Tyr4-Ile5-His6-Pro7 sequence, which is the most immunogenic epitope of the peptide, with the Asp1 side chain oriented towards the interior of the turn approaching the C-terminus; (b) an Asx-turn-like motif with the side chain aspartate carboxyl group hydrogen-bonded to the main chain NH group of Arg2. It can be concluded that small rearrangements of the epitope 4-7 in the solution structure of AII are required by a mean value of 0.76 +/- 0.03 A for structure alignment and approximately 1.27 +/- 0.02 A for sequence alignment with the X-ray structure of AII bound to the mAb Fab131. These data are interpreted in terms of a biological "nucleus" conformation of the hormone in solution, which requires a limited number of structural rearrangements for receptor-antigen recognition and binding.

UV resonance Raman study of angiotensin II conformation in nonaqueous environments: lipid micelles and acetonitrile

We used 206.5-nm excited resonance Raman measurements to examine the angiotensin II (AII) secondary structure in H(2)O in the presence of dodecylphosphocholine (DPC) micelles, sodium dodecylsulfate (SDS) monomers and micelles, and in a 70% acetonitrile (ACN-d)-30% water solution. Our AII-SDS titration absorption studies indicate the formation of a 1:2 AII:SDS complex in which two negatively charged SDS molecules attach to the AII positively charged N terminus and to Arg(2). Our 206.5-nm excited Raman results indicate that the 1:2 AII:SDS complexation increases the AII beta-turn composition. We also used 228.9-nm Raman excitation to probe the local solvent accessibility of Tyr(4) (AII) in DPC and SDS micelles. Our Tyr (AII) solvent accessibility studies suggest that the Tyr residue is more exposed to the aqueous environment in SDS micelles than in DPC micelles.

Conformational study of angiotensin II

Conformational free energy calculations using an empirical potential ECEPP/3 (Empirical Conformational Energy Program for Peptides, Version 3) were carried out on angiotensin II (AII) of sequence Asp-Arg-Val-Tyr-Ile-His-Pro-Phe to find the stable conformations of the free state in the unhydrated and the hydrated states. A conformational analysis of the unhydrated state was carried out using the buildup procedure. The free energy calculation using the hydration shell model was also carried out to obtain the stable conformation of the hydrated state. The calculated stable conformations of AII in both states have a partially right-handed alpha-helical structure stabilized by short- and medium-range interactions. The similarity between the lowest free energy conformations of the unhydrated and hydrated states suggests that the hydration might not be important to stabilize the overall conformation of AII in a free state. The absence of any intramolecular interaction of the Tyr side chain suggests the possible interaction of this residue with the receptor. In this study, we found that the low free energy conformations contain both the parallel-plate and the perpendicular-plate geometries of the His and Phe rings, suggesting the coexistence of both conformations.

Conformations in solution of angiotensin II, and its 1-7 and 1-6 fragments

High-resolution proton spectra at 620 MHz of human angiotensin II (1-8), angiotensin II (1-7), and angiotensin II (1-6) have been obtained in aqueous solution at acidic pH, and in dimethylsulfoxide solution. Complete chemical shift assignments for all three angiotensin peptides were made based on two-dimensional (2D) correlated spectroscopy and 2D-CA-MELSPIN spectra. Based on the measured values of 3JHNCH, the pattern of observed transverse Overhauser effects, and side-chain coupling constants, it is concluded that all three analogues exist in H2O or DMSO-d6 as a mixture of conformers that is largely extended, with negligible content of folded structures, such as beta-turns, gamma-turns, or helix content. The results fit well with those of Nikiforovich et al.

Proton NMR studies of angiotensin II and its analogs in aqueous solution

The 1H nuclear magnetic resonance (NMR) spectra of angiotensin II (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe) and five of its octapeptide analogs as well as angiotensin I (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu) and angiotensin III (Arg-Val-Tyr-Ile-His-Pro-Phe) in aqueous solutions (90% H2O/10% D2O) were completely assigned by two-dimensional COSY and ROESY experiments. All of the peptides give rise to two distinct sets of signals. The minor set accounts for about 5% of the total population below pH 5.5 and increases to 12-20% around pH 7.0. The two sets of signals result from a cis-trans isomerization of the His-Pro peptide bond with the major resonances arising from the trans isomer. One analog in which the Pro is replaced with a D-Pro displays a very different isomerization behavior. The measured coupling constants JNH-alpha CH, the temperature dependence of the amide proton shifts and the relative intensities of the intraresidue and sequential NH-alpha CH ROEs, are all indicative of an extended backbone conformation for ANGII. However, some evidence for the existence of conformers with local structure involving preferred sidechain positions for the Tyr, His, Phe, and the carboxyl group of the Phe was found, particularly in the ROESY and pH-titration experiments. Moreover, pH effects and the unusual amide exchange behavior of the Arg epsilon NH suggests the presence of interactions between the Asp and Arg sidechains of ANGII. At low temperatures the Arg guanidinium NH2 protons were detected as two broad peaks which are related by sizeable exchange peaks in ROESY experiments. This behavior could be useful as a general probe for the study of Arg sidechain mobility and accessibility in other peptides and proteins.

Normal modes of vibration of a model peptide adopting the C7 C5 structure. Application toi angiotensin II

A normal coordinate treatment was performed on the C7C5 conformation using a modified Urey-Bradley force field refined from previous studies on beta-turns. The predicted frequencies were compared with the experimental ones obtained on the peptide hormone human angiotensin II. The existence of both a beta-turn and a C7C5 conformation in solid and in aqueous solution is discussed.

Sequence-dependent conformations of short polypeptides in a hydrophobic environment

Surfactants, which provide a hydrophobic environment, may induce an ordered conformation in polypeptides and proteins that contain a sequence with helix-or beta-forming potential. This hypothesis has been illustrated in circular dichroic studies of oligopeptides and short polypeptides. These peptide-surfactant complexes can form (1) a helix, (2) a beta-form, (3) either form (depending on experimental conditions), or can remain in (4) an ordered form. The induced helix is stable in a surfactant solution below or above its critical micellar concentration, whereas the induced beta-form is usually converted back to an unordered form when the surfactant used is above its critical micellar concentration, or it is transformed into a helix in excess surfactant solution if the peptide has both the helix- and beta-forming potential. In most cases the observed conformations agree with those predicted from the amino acid sequences of the peptides. The induced conformation of a peptide can be destabilized by charges on the side groups having the same sign as that of surfactant ions. Disulfide bonds can inhibit the formation of induced conformation because of steric hindrance. The terminal effect can prevent a peptide from forming an ordered conformation near the NH2- and COOH-terminus.

Conformation of angiotensin II. Evidence for a specific hydrogen bonded conformation

The peptide amide hydrogen exchange rate of human angiotensin II in H2O have been measured at room temperature by the transfer of solvent saturation method. The data are consistent with the assumption of a highly motile dynamic equilibrium between folded and highly solvated conformations. The NH of His6 is observed to exchange more slowly than predicted, suggesting that it is a participant in an internal hydrogen bond. Several models previously suggested in the literature for the conformation of the peptide in aqueous solution are examined, and most are found to be inconsistent with the exchange data. Evidence in support of a structure for the Ile5-His6 fragment of the hormone involving a C7eq-C5 bend is presented.

The conformation of angiotensin II. II. The rates of peptide NH exchange with solvent for [Asn1, Val5]angiotensin II, angiotensin III and saralasin

The amide hydrogen exchange rates in H2O of two angiotensin agonists (angiotensinamide and angiotensin III) and one angiotensin antagonist (saralasin) have been measured at room temperature by the transfer of solvent saturation method. The NH of His6 is observed to exchange more slowly than predicted for all three peptides, suggesting that it is a participant in an intramolecular hydrogen bond. The NH-C alpha H 1H-NMR coupling constants are measured and found to be constant over the pH range of 5.0 to 6.5. The results are compared with those previously obtained for human angiotensin II and interpreted in terms of a dominant three-dimensional structure common to all four molecules. Two models for this structure are evaluated using the observed NH-C alpha H coupling constants and the reported activity of conformationally restrained derivatives.

Determination of the average end-to-end distance of two angiotensin II analogs by resonance energy transfer

The angiotensin II analogs H . Tyr-Arg-Val-Phe-Val-His-Pro-Trp . OH (I) and H . Trp-Arg-Val-Phe-Val-His-Pro-Tyr . OH (II) were synthesized and their conformations in dilute aqueous solution (3 X 10(-5) M) were studied by fluorescence techniques. Evaluation of singlet-singlet energy transfer between tyrosine (donor) and tryptophan (acceptor) in the biologically active analog I resulted in a low transfer efficiency (E approximately 0.1) Since transposition of the tyrosyl and tryptophanyl residues (analog II) produced a transfer efficiency similar to that observed in compound I, the orientation factor did not present a serious problem for the determination of the intramolecular Tyr-Trp distances in these peptides on the basis of the Förster equation. Similar average Tyr-Trp separations above 15 A were obtained for analogs I and II at pH 5.2 and no drastic titration effects on the distance were observed with compound I in the pH range 1.5-8.5. The observed end-to-end distances indicate that the conformations of analogs I and II are not quite as compact as some of the models which have been proposed for angiotensin II. Furthermore, the results exclude an electrostatic head-to-tail interaction between the terminal NH3+- and COO- groups as well as several proposed beta- and gamma-turn models.

A fluorescence study of the binding of calcium and terbium ions to angiotensin

The interactions of angiotensin II and a synthetic analogue, [Asn1, Val5] angiotensin II, with Ca2+ and Tb3+ have been monitored using the intrinsic fluorescence of the tyrosine residue at position 4 in both molecules. The data indicate that angiotensin II binds both metals with a dissociation constant of approximately 1 X 10(-4) M-1, while no significant binding was observed with the amide analogue. This suggests that the side chain carboxyl group of aspartic acid forms part of the binding site. Since the value of the dissociation constant suggests chelation of the metals by the hormone, the terminal carboxyl group of the peptide is also probably involved in metal binding. The fact that energy transfer was observed between Tb3+ and the tyrosine of angiotensin places the hydroxl or carbonyl group of the tyrosine close to the metal binding site.

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.

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.

Analysis of the conformation of polypeptides : the combined use of energy computations and nuclear magnetic resonance studies

This review surveys current approaches to the problem of determining the solution conformation of polypeptides. The basic principles of energy computations are described. The utility, problems, and limitations of various theoretical methods are summarized: conformational energy mapping, energy minimization, scanning of selected local conformations, statistical predictive schemes. The need for combining the calculations with experimental studies is pointed out. The information content of various physico-chemical methods is compared for this purpose. The analysis of nmr coupling constants is discussed in more detail. In combination with energy computations, it can furnish specific information on local aspects of the conformation. Examples of such combined studies on small peptides are summarized.

Conformational flexibility of angiotensin II. A carbon-13 spin-lattice relaxation study

Carbon-13 spin-lattice relaxation times (T1) have been determined for the carbon in the octapeptide hormone [5-isoleucine]-angiotensin II in aqueous solution. Two possible models for molecular motion are considered: isotropic overall motion of the hormone with internal motion of some residues and anisotropic overall molecular motion. The data are interpreted in detail using the former model. The alpha carbons of the peptide backbone are all equally restricted in their motion. The correlation time for overall molecular reorientation, calculated from an everage T1 value of 95 msec for the alpha carbons in the peptide backbone, is ca. 5 times 10-10 sec. The carbons in the side chains are more mobile than those in the peptide backbone, with the exception of the side chain of the Tyr residue which does not undergo rapid segmental motion. We propose that [5-isoleucine]-angiotensin II has a restricted backbone conformation and that the alpha carbons of the N- and C-terminal residues are constrained to nearly the same extent as the remaining alpha carbons in the peptide backbone. Chemical shift data indicate that the Pro residue adopts the trans conformation about the His-Pro bond and that the imidazole ring of His has a strong preference for the N-tau -H tautomer.