Propensities Of Amino Acid Residues Research Articles

The relevance of short peptides for an understanding of unfolded and intrinsically disordered proteins.

Over the last thirty years the unfolded state of proteins has attracted considerable interest owing to the discovery of intrinsically disordered proteins which perform a plethora of functions despite resembling unfolded proteins to a significant extent. Research on both, unfolded and disordered proteins has revealed that their conformational properties can deviate locally from random coil behavior. In this context results from work on short oligopeptides suggest that individual amino acid residues sample the sterically allowed fraction of the Ramachandran plot to a different extent. Alanine has been found to exhibit a peculiarity in that it has a very high propensity for adopting polyproline II like conformations. This Perspectives article reviews work on short peptides aimed at exploring the Ramachandran distributions of amino acid residues in different contexts with experimental and computational means. Based on the thus provided overview the article discussed to what extent short peptides can serve as tools for exploring unfolded and disordered proteins and as benchmarks for the development of a molecular dynamics force field.

UnderstandingRamachandranplot for dipeptide: A density functional theory and information‐theoreticapproach study

AbstractThe Ramachandran plot in biochemistry visualizes the propensity of amino acid residues to assume different secondary structures. It can be obtained by different approaches such as bioinformatics and molecular dynamics. In this work, we systematically scan the two single bonds around the α‐carbon of a peptide to generate the three‐dimensional potential energy surface and then project it onto the coordinate plane to generate the Ramachandran plot. Using the two schemes of the total energy partition and information‐theoretic approach in density functional theory, we analyze the plot and find out that the dominant contributor to determine the local minima of secondary structures, especially β‐sheets, in the Ramachandran plot is the electrostatic interaction, whereas steric and exchange‐correlation contributions play minor yet indispensable roles, especially for determining α‐helixes. As the generalization of our prior studies for systems with one rotatable bond, our current results confirm what we yielded before from the viewpoint of energetic contributions, and at meanwhile provide an in‐depth understanding about the nature and origin of secondary structure propensities from the Ramachandran plot.

Translocation of non-interacting heteropolymer protein chains in terms of single helical propensity and size

In this work we present an analytical framework to calculate the average translocation time τ required for an ideal proteinogenic polypeptide chain to cross over a small pore on a membrane. Translocation is considered to proceed as a chain of non-interacting amino acid residues of sequence {Xj} diffuses through the pore against an energy barrier Δℱ, set by chain entropy and unfolding-folding energetics. We analyze the effect of sequence heterogeneity on the dynamics of translocation by means of helical propensity of amino acid residues. In our calculations we use sequences of fifteen well-known proteins that are translocated which span two orders of magnitude in size according to the number of residues N. Results show non-symmetric free energy barriers as a consequence of sequence heterogeneity, such asymmetry in energy may be useful in differentiated directions of translocation. For the fifteen polypeptide chains considered we found conditions when sequence heterogeneity has not a significant effect on the time scale of translocation leading to a scaling law τ ∝ Nν, where ν ∼ 1.6 is an exponent that holds for most ground state energies. We also identify conditions when sequence heterogeneity has a great impact on the time scale of translocation, in consequence, no more scaling laws for τ there exist.

Helix Propensities of Amino Acid Residues via Thioester Exchange.

We describe the use of thioester exchange equilibria to measure the propensities of amino acid residues to participate in helical secondary structure at room temperature in the absence of denaturants. Thermally or chemically induced unfolding has previously been employed to measure α-helix propensities among proteinogenic α-amino acid residues, and quantitative comparison with precedents indicates that the thioester exchange system is reliable for residues that lack side chain charge. This system allows the measurement of α-helix propensities for d-α-amino acid residues and propensities of residues with nonproteinogenic backbones, such as those derived from a β-amino acid, to participate in an α-helix-like secondary structure.

Probing the Conformation-Dependent Preferential Binding of Ethanol to Cationic Glycylalanylglycine in Water/Ethanol by Vibrational and NMR Spectroscopy.

The conformational propensity of amino acid residues is determined by an intricate balance of peptide-solvent and solvent-solvent interactions. To explore how the systematic replacement of water by a cosolvent affects the solvation of both the amino acid backbone and side chains, we performed a combined vibrational spectroscopy and NMR study of cationic glycylalanylglycine (GAG) in different ethanol/water mixtures of between 0 and 42 mol percent ethanol. Classical model peptide N'-methylacetamide was used as a reference system to probe solvent-induced spectroscopic changes. The alanine residue of GAG in water is known to exhibit a very high propensity for polyproline II (pPII). Adding up to 30 mol % ethanol at room temperature leads only to minor changes in the Ramachandran distribution of alanine, which mostly changes within the individual conformational subspaces. A further increase in the ethanol fractions leads to a destabilization of pPII and a stabilization of β-strand conformations. At higher temperatures, different degrees of enthalpy-entropy compensations lead to a much stronger influence of ethanol on the peptide's conformational distribution. Ethanol-induced changes in chemical shifts and amide I wavenumbers strongly suggest that ethanol replaces water preferentially in the solvation shell of the polar C-terminal peptide group and of the alanine side chain, whereas the N-terminal group remains mostly hydrated. Furthermore, we found that ethanol interacts more strongly with the peptide if the latter adopts β-strand conformations. This leads to an unusual positive temperature coefficient for the chemical shift of the C-terminal amide proton. Our data suggests a picture in which GAG eventually accumulates at water-ethanol interfaces if the ethanol fractions exceed 0.3, which explains why the further addition of ethanol eventually causes self-aggregation and the subsequent formation of a hydrogel.

Dihedral angle preferences of amino acid residues forming various non-local interactions in proteins.

In theory, a polypeptide chain can adopt a vast number of conformations, each corresponding to a set of backbone rotation angles. Many of these conformations are excluded due to steric overlaps. Ramachandran and coworkers were the first to look into this problem by plotting backbone dihedral angles in a two-dimensional plot. The conformational space in the Ramachandran map is further refined by considering the energetic contributions of various non-bonded interactions. Alternatively, the conformation adopted by a polypeptide chain may also be examined by investigating interactions between the residues. Since the Ramachandran map essentially focuses on local interactions (residues closer in sequence), out of interest, we have analyzed the dihedral angle preferences of residues that make non-local interactions (residues far away in sequence and closer in space) in the folded structures of proteins. The non-local interactions have been grouped into different types such as hydrogen bond, van der Waals interactions between hydrophobic groups, ion pairs (salt bridges), and ππ-stacking interactions. The results show the propensity of amino acid residues in proteins forming local and non-local interactions. Our results point to the vital role of different types of non-local interactions and their effect on dihedral angles in forming secondary and tertiary structural elements to adopt their native fold.

Effect of graphene oxide on the conformational transitions of amyloid beta peptide: A molecular dynamics simulation study

The interactions between nanomaterials (NMs) and amyloid proteins are central to the nanotechnology-based diagnostics and therapy in neurodegenerative disorders such as Alzheimer's and Parkinson's. Graphene oxide (GO) and its derivatives have shown to modulate the aggregation pattern of disease causing amyloid beta (Aβ) peptide. However, the mechanism is still not well understood. Using molecular dynamics simulations, the effect of graphene oxide (GO) and reduced graphene oxide (rGO) having carbon:oxygen ratio of 4:1 and 10:1, respectively, on the conformational transitions (alpha-helix to beta-sheet) and the dynamics of the peptide was investigated. GO and rGO decreased the beta-strand propensity of amino acid residues in Aβ. The peptide displayed different modes of adsorption on GO and rGO. The adsorption on GO was dominated by electrostatic interactions, whereas on rGO, both van der Waals and electrostatic interactions contributed in the adsorption of the peptide. Our study revealed that the slight increase in the hydrophobic patches on rGO made it more effective inhibitor of conformational transitions in the peptide. Alpha helix-beta sheet transition in Aβ peptide could be one of the plausible mechanism by which graphene oxide may inhibit amyloid fibrillation.

Structure based approach for understanding organism specific recognition of protein-RNA complexes.

BackgroundProtein-RNA interactions perform diverse functions within the cell. Understanding the recognition mechanism of protein-RNA complexes has been a challenging task in molecular and computational biology. In earlier works, the recognition mechanisms have been studied for a specific complex or using a set of non–redundant complexes. In this work, we have constructed 18 sets of same protein-RNA complexes belonging to different organisms from Protein Data Bank (PDB). The similarities and differences in each set of complexes have been revealed in terms of various sequence and structure based features such as root mean square deviation, sequence homology, propensity of binding site residues, variance, conservation at binding sites, binding segments, binding motifs of amino acid residues and nucleotides, preferred amino acid-nucleotide pairs and influence of neighboring residues for binding.ResultsWe found that the proteins of mesophilic organisms have more number of binding sites than thermophiles and the binding propensities of amino acid residues are distinct in E. coli, H. sapiens, S. cerevisiae, thermophiles and archaea. Proteins prefer to bind with RNA using a single residue segment in all the organisms while RNA prefers to use a stretch of up to six nucleotides for binding with proteins. We have developed amino acid residue-nucleotide pair potentials for different organisms, which could be used for predicting the binding specificity. Further, molecular dynamics simulation studies on aspartyl tRNA synthetase complexed with aspartyl tRNA showed specific modes of recognition in E. coli, T. thermophilus and S. cerevisiae.ConclusionBased on structural analysis and molecular dynamics simulations we suggest that the mode of recognition depends on the type of the organism in a protein-RNA complex.ReviewersThis article was reviewed by Sandor Pongor, Gajendra Raghava and Narayanaswamy Srinivasan.Electronic supplementary materialThe online version of this article (doi:10.1186/s13062-015-0039-8) contains supplementary material, which is available to authorized users.

Assessing backbone solvation effects in the conformational propensities of amino acid residues in unfolded peptides.

Conformational ensembles of individual amino acid residues within model GxG peptides (x representing different amino acid residues) are dominated by a mixture of polyproline II (pPII) and β-strand like conformations. We recently discovered rather substantial differences between the enthalpic and entropic contributions to this equilibrium for different amino acid residues. Isoleucine and valine exceed all other amino acid residues in terms of their rather large enthalpic stabilization and entropic destabilization of polyproline II. In order to shed light on these underlying physical mechanisms, we performed high-level DFT calculations to explore the energetics of four representative GxG peptides where x = alanine (A), leucine (L), valine (V), and isoleucine (I) in explicit water (10 H2O molecules with a polarizable continuum water model) and in vacuo. We found that the large energetic contributions to the stabilization of pPII result, to a major extent, from peptide-water, water-water interactions, and changes of the solvent self-energy. Differences between the peptide-solvent interaction energies of hydration in pPII and β-strand peptides are particularly important for the pPII ⇌ β equilibria of the more aliphatic peptides GIG and GLG. Furthermore, we performed a vibrational analysis of the four peptides in both conformations and discovered a rather substantial mixing between water motions and peptide vibrations below 700 cm(-1). We found that the respective vibrational entropies are substantially different for the considered conformations, and their contributions to the Gibbs/Helmholtz energy stabilize β-strand conformations. Taken together, our results underscore the notion of the solvent being the predominant determinant of peptide (and protein) conformations in the unfolded state.

Role of Enthalpy–Entropy Compensation Interactions in Determining the Conformational Propensities of Amino Acid Residues in Unfolded Peptides.

The driving forces governing the unique and restricted conformational preferences of amino acid residues in the unfolded state are still not well understood. In this study, we experimentally determine the individual thermodynamic components underlying intrinsic conformational propensities of these residues. Thermodynamic analysis of ultraviolet-circular dichroism (UV-CD) and (1)H NMR data for a series of glycine capped amino acid residues (i.e., G-x-G peptides) reveals the existence of a nearly exact enthalpy-entropy compensation for the polyproline II-β strand equilibrium for all investigated residues. The respective ΔHβ, ΔSβ values exhibit a nearly perfect linear relationship with an apparent compensation temperature of 295 ± 2 K. Moreover, we identified iso-equilibrium points for two subsets of residues at 297 and 305 K. Thus, our data suggest that within this temperature regime, which is only slightly below physiological temperatures, the conformational ensembles of amino acid residues in the unfolded state differ solely with respect to their capability to adopt turn-like conformations. Such iso-equilibria are rarely observed, and their existence herein indicates a common physical origin behind conformational preferences, which we are able to assign to side-chain dependent backbone solvation. Conformational effects such as differences between the number of sterically allowed side chain rotamers can contribute to enthalpy and entropy but not to the Gibbs energy associated with conformational preferences. Interestingly, we found that alanine, aspartic acid, and threonine are the only residues which do not share these iso-equilbiria. The enthalpy-entropy compensation discovered as well as the iso-equilbrium and thermodynamics obtained for each amino acid residue provide a new and informative way of identifying the determinants of amino acid propensities in unfolded and disordered states.

Nearest Neighbor Interactions Attenuate Intrinsic Amino Acid Conformational Preferences: A Combined Vibrational and NMR Study

Contrary to the isolated pair hypothesis, a pivotal ingredient of the random coil model, conformational distributions of amino acid residues in unfolded proteins cannot be considered as isolated from their nearest neighbors. To enable structural predictions of unfolded proteins or peptides (IDPs) knowledge about intrinsic propensities of amino acid residues must be complemented by information on nearest-neighbor interactions. To explore this influence, we preformed a joint vibrational and 2D-NMR spectroscopy study of selected “GxyG” host-guest peptides in aqueous solution: GDyG, GSyG, GxLG, GxVG, where x/y={A,K,LV}. The choice of D and S (L and V) for the x (y) position was motivated by their documented ability to drastically change the distribution of alanine in xAy tripeptides of truncated coil libraries. The observed spectral information were analyzed in terms of superpositions of 2D-Gaussian functions in the Ramachandran space, representing sub-ensembles of pPII-, β-strand-, helical-, and turn-like conformations. Results reveal that “turn forming residues” and hydrophobic residues have very strong effects on the conformation of their nearest neighbors. A direct comparison of the conformational distribution for S and D in GSAG ,GSLG, and GDAG with corresponding distributions in GxG peptiess reveals that hydrophobic neighbors cause drastic reductions of turn propensities. As a neighbor, K eliminates the asx turn propensity of D. In addition, insertion of these 'turn forming' residues (S, and D) causes conformational redistributions of L, A, and K from pPII to more extended β strand-like conformations while at the same time increasing the overall turn content. Concomitantly, the positions of pPII and particularly of β distributions are shifted to lower ψ values. Taken together, our results indicate that Dx and Sx motifs might act as conformational randomizers in proteins, attenuating intrinsic propensities of neighboring residues.

Disorder and order in unfolded and disordered peptides and proteins: A view derived from tripeptide conformational analysis. I. Tripeptides with long and predominantly hydrophobic side chains

We performed a conformational analysis of the central residues of three tripeptides glycyl-L-isoleucyl-glycine (GIG), glycyl-L-tyrosyl-glycine (GYG) and glycyl-L-arginyl-glycine (GRG) in aqueous solution, based on a global analysis of amide I' band profiles and NMR J-coupling constants. The results are compared with recently reported distributions of GVG, GFG and GEG. For GIG and GYG, we found that even though the polyproline II (pPII) fraction is below 0.5, it is still the most populated conformation, whereas GVG and GFG show both a larger β-strand fraction. For GRG, we observed a clear dominance of pPII over β-strand, reminiscent of observations for GEG and GKG. This finding indicates that terminal charges on otherwise hydrophobic residue side chains stabilize pPII over β-strand conformations. For all peptides investigated we found that a variety of compact and turn-like conformations constitute nearly 20 percent of their conformational distributions. Attempts to analyze our data with a simple two-state pPII-->/<--β model therefore do not yield any satisfactory reproduction of experimental results. A comparison of the obtained GxG ensembles with conformational distributions of GxG segments in truncated coil libraries (helices and sheets omitted) revealed a much larger fraction of type II β(i+2) and type III β like conformations for the latter. Thus, a comparison of conformational distributions of unfolded peptide segments in solution and in coil libraries reveal interesting information on how the interplay between intrinsic propensities of amino acid residues and non-local interactions in polypeptide chains determine the conformations of loop segments in proteins.

Insilico study of the A2AR–D2R kinetics and interfacial contact surface for heteromerization

G-protein-coupled receptors (GPCRs) are cell surface receptors. The dynamic property of receptor-receptor interactions in GPCRs modulates the kinetics of G-protein signaling and stability. In the present work, the structural and dynamic study of A(2A)R-D(2)R interactions was carried to acquire the understanding of the A(2A)R-D(2)R receptor activation and deactivation process, facilitating the design of novel drugs and therapeutic target for Parkinson's disease. The structure-based features (Alpha, Beta, SurfAlpha, and SurfBeta; GapIndex, Leakiness and Gap Volume) and slow mode model (ENM) facilitated the prediction of kinetics (K (off), K (on), and K (d)) of A(2A)R-D(2)R interactions. The results demonstrated the correlation coefficient 0.294 for K (d) and K (on) and the correlation coefficient 0.635 for K (d) and K (off), and indicated stable interfacial contacts in the formation of heterodimer. The coulombic interaction involving the C-terminal tails of the A(2A)R and intracellular loops (ICLs) of D(2)R led to the formation of interfacial contacts between A(2A)R-D(2)R. The properties of structural dynamics, ENM and KFC server-based hot-spot analysis illustrated the stoichiometry of A(2A)R-D(2)R contact interfaces as dimer. The propensity of amino acid residues involved in A(2A)R-D(2)R interaction revealed the presence of positively (R, H and K) and negatively (E and D) charged structural motif of TMs and ICL3 of A(2A)R and D(2)R at interface of dimer contact. Essentially, in silico structural and dynamic study of A(2A)R-D(2)R interactions will provide the basic understanding of the A(2A)R-D(2)R interfacial contact surface for activation and deactivation processes, and could be used as constructive model to recognize the protein-protein interactions in receptor assimilations.

PPII Propensity of Multiple-Guest Amino Acids in a Proline-Rich Environment

There has been considerable debate about the intrinsic PPII propensity of amino acid residues in denatured polypeptides. Experimentally, this scale is based on the behavior of guest amino acid residues placed in the middle of proline-based hosts. We have used classical molecular dynamics simulations combined with replica-exchange methods to carry out a comprehensive analysis of the conformational equilibria of proline-based host oligopeptides with multiple guest amino acids including alanine, glutamine, valine, and asparagine. The tracked structural characteristics include the secondary structural motifs based on the Ramachandran angles and the cis/trans isomerization of the prolyl bonds. In agreement with our recent study of single amino acid guests, we did not observe an intrinsic PPII propensity in any of the guest amino acids in a multiple-guest setting. Instead, the experimental results can be explained in terms of (i) the steric restrictions imposed on the C-terminal guest amino acid that is immediately followed by a proline residue and (ii) an increase in the trans content of the prolyl bonds due to the presence of guest residues. In terms of the latter, we found that the more guests added to the system, the larger the increase in the trans content of the prolyl bonds, which results in an effective increase in the PPII content of the peptide.

Solvent Dependence of Trialanine Conformers

Determining the conformational propensities of amino acid residues in short peptides is increasingly recognized as pivotal for obtaining a reliable picture of the unfolded states of peptides and proteins. In this context, ample experimental evidence indicates that alanine does not exhibit a statistical coil behavior, but rather shows a strong preference for sampling the polyproline II (PPII) region of the Ramachadran plot. Solvation in water has been proposed as the main reason for PPII stabilization. The mechanisms involved in the respective peptide-solvent interactions are still debated. Results from DFT-calculations on dialanine suggest that water stabilizes PPII conformers by forming hydrogen bond bridges between adjacent carbonyl and amide groups. Other models have invoked electrostatic interactions. To explore the solvent dependence of PPII stabilization, we used UV-CD and H-NMR spectroscopy to determine the conformations of cationic trialanine in water, as well as in binary water/glycerol water/ethanol mixtures. All spectra were recorded as a function of temperature ranging between 0°C and 90°C. The resultant dichrosim at 216nm and 3J(HαHN) coupling constants were subjected to a simplified two state PPII↔β strand thermodynamic analysis. The experimental results indicate that both alcohol co-solvents substantially lower the PPII propensity of alanine in each respective binary mixture. Glycerol seems to be most effective in replacing water in the solvation shell, causing large enthalpic decreases between the two conformers; from ΔH=18.15 kJ/mol in water, to ΔH=12.42 kJ/mol with only a 0.012 mole fraction admixture of glycerol.

Tri-Aspartic Acid Peptides in Water: A Suitable Model System for Determining the Structural Propensities of DxD Motifs in Unfolded Proteins

In the context of our ongoing investigations of the conformational propensities of amino acid residues in unfolded peptides we recently focused on the peculiar properties of aspartic acid (D). A detailed spectroscopic study on the tripeptide GDG has shown that D has an unusual preference for turn like structures. This is interesting since D has been shown to be involved in so called asx -turns which are stabilized by hydrogen bonding between the polar residue of e.g. D and the adjacent amide proton. D is also an essential ingredient of DxD motifs which are involved in the enzymatic activity and the Golgi glycosyltransferase GM2. To shed some light on how D-residues behave in D3-motifs we re-investigated the tripeptide D3 with IR, polarized Raman, vibrational circular dichroism, electronic circular dirchroism and NMR spectroscopy. The data are currently being analyzed. First results indicate that D3 has more PPII content than GDG at room temperature and that its actual structure depends on the protonation state of its residues. At high temperature the ECD spectra is indicative of a predominant sampling of right-handed helix or (type III β) turn like conformations, which is rather unexpected. Our results indicate that aspartic acid combines a high degree of plasticity with a preference for turn structures which might explain its role in turn forming segments in proteins and the functional properties of DxD motifs.

Sequence and structural features of binding site residues in protein-protein complexes: comparison with protein-nucleic acid complexes

BackgroundProtein-protein interactions are important for several cellular processes. Understanding the mechanism of protein-protein recognition and predicting the binding sites in protein-protein complexes are long standing goals in molecular and computational biology.MethodsWe have developed an energy based approach for identifying the binding site residues in protein–protein complexes. The binding site residues have been analyzed with sequence and structure based parameters such as binding propensity, neighboring residues in the vicinity of binding sites, conservation score and conformational switching.ResultsWe observed that the binding propensities of amino acid residues are specific for protein-protein complexes. Further, typical dipeptides and tripeptides showed high preference for binding, which is unique to protein-protein complexes. Most of the binding site residues are highly conserved among homologous sequences. Our analysis showed that 7% of residues changed their conformations upon protein-protein complex formation and it is 9.2% and 6.6% in the binding and non-binding sites, respectively. Specifically, the residues Glu, Lys, Leu and Ser changed their conformation from coil to helix/strand and from helix to coil/strand. Leu, Ser, Thr and Val prefer to change their conformation from strand to coil/helix.ConclusionsThe results obtained in this study will be helpful for understanding and predicting the binding sites in protein-protein complexes.

HORI: a web server to compute Higher Order Residue Interactions in protein structures

BackgroundFolding of a protein into its three dimensional structure is influenced by both local and global interactions within a protein. Higher order residue interactions, like pairwise, triplet and quadruplet ones, play a vital role in attaining the stable conformation of the protein structure. It is generally agreed that higher order interactions make significant contribution to the potential energy landscape of folded proteins and therefore it is important to identify them to estimate their contributions to overall stability of a protein structure.ResultsWe developed HORI [Higher order residue interactions in proteins], a web server for the calculation of global and local higher order interactions in protein structures. The basic algorithm of HORI is designed based on the classical concept of four-body nearest-neighbour propensities of amino-acid residues. It has been proved that higher order residue interactions up to the level of quadruple interactions plays a major role in the three-dimensional structure of proteins and is an important feature that can be used in protein structure analysis.ConclusionHORI server will be a useful resource for the structural bioinformatics community to perform analysis on protein structures based on higher order residue interactions. HORI server is a highly interactive web server designed in three modules that enables the user to analyse higher order residue interactions in protein structures. HORI server is available from the URL: http://caps.ncbs.res.in/hori

Mining α-Helix-Forming Molecular Recognition Features with Cross Species Sequence Alignments

Previously described algorithms for mining alpha-helix-forming molecular recognition elements (MoREs), described by Oldfield et al. (Oldfield, C. J., Cheng, Y., Cortese, M. S., Brown, C. J., Uversky, V. N., and Dunker, A. K. (2005) Comparing and combining predictors of mostly disordered proteins, Biochemistry 44, 1989-2000), also known as molecular recognition features (MoRFs) (Mohan, A., Oldfield, C. J., Radivojac, P., Vacic, V., Cortese, M. S., Dunker, A. K., and Uversky, V. N. (2006) Analysis of Molecular Recognition Features (MoRFs), J. Mol. Biol. 362, 1043-1059), revealed that regions undergoing disorder-to-order transition are involved in many molecular recognition events and are crucial for protein-protein interactions. However, these algorithms were developed using a training data set of a limited size. Here we propose to improve the prediction algorithms by (1) including additional alpha-MoRF examples and their cross species homologues in the positive training set, (2) carefully extracting monomer structure chains from the Protein Data Bank (PDB) as the negative training set, (3) including attributes from recently developed disorder predictors, secondary structure predictions, and amino acid indices, and (4) constructing neural network based predictors and performing validation. Over 50 regions which undergo disorder-to-order transition that were identified in the PDB together with a set of corresponding cross species homologues of each structure-based example were included in a new positive training set. Over 1500 attributes, including disorder predictions, secondary structure predictions, and amino acid indices, were evaluated by the conditional probability method. The top attributes, including VSL2 and VL3 disorder predictions and several physicochemical propensities of amino acid residues, were used to develop the feed forward neural networks. The sensitivity, specificity, and accuracy of the resulting predictor, alpha-MoRF-PredII, were 0.87 +/- 0.10, 0.87 +/- 0.11, and 0.87 +/- 0.08 over 10 cross validations, respectively. We present the results of these analyses and validation examples to discuss the potential improvement of the alpha-MoRF-PredII prediction accuracy.

Salmon Calcitonin and Amyloid β: Two Peptides with Amyloidogenic Capacity Adopt Different Conformational Manifolds in Their Unfolded States

The molecular conformations of salmon calcitonin in aqueous solution have been investigated by exploiting the different influences of excitonic coupling on the amide I band profile in the isotropic and anisotropic Raman, FTIR, and vibrational circular dichroism spectra of a polypeptide. The N-terminal loop, caused by a disulfide bridge between cysteines at positions 1 and 7, was modeled by performing a conformational search by molecular mechanics calculations. The remaining part of the peptide chain was modeled as a mixture of three sequences containing different fractions of residues adopting poly-l-proline II (PPII), extended beta-strand, and alpha-helix-like conformations. This yielded an excellent reproduction of the experimentally observed amide I' band profiles. A comparison with recent data on the beta-amyloid fragment Abeta(1)(-)(28) revealed a lower PPII content and more conformational heterogeneity for calcitonin. Thus, our results underscore the notion that individual structural propensities of amino acid residues give rise to structural differences between the unfolded states of even long peptide chains, at variance with expectations based on a random or statistical coil model.