Collinearity of alpha-helices or beta strands in membrane proteins causes a characteristic peak centred on 4.9 Å resolution in diffraction intensity profiles, inducing higher diffraction anisotropy

Diffraction anisotropy is a phenomenon that impacts more specifically membrane proteins, compared to soluble ones, but the reasons for this discrepancy remained unclear. Often, it is referred to a difference in resolution limits between highest and lowest diffraction limits as a signature for anisotropy. We show in this article that there is no simple correlation between anisotropy and difference in resolution limits, with notably a substantial number of structures displaying various anisotropy with no difference in resolution limits. We further investigated diffraction intensity profiles, and observed a peak centred on 4.9Å resolution more predominant in membrane proteins. Since this peak is in the region corresponding to secondary structures, we investigated the influence of secondary structure ratio. We showed that secondary structure content has little influence on this profile, while secondary structure collinearity in membrane proteins correlate with a stronger peak. Finally, we could further show that the presence of this peak is linked to higher diffraction anisotropy.SynopsisMembrane protein diffraction anisotropy originates from a peak at 4.9 Å resolution in intensity profiles, due to secondary structure collinearity.

Molecular Dynamics Study of Insulin Mutants

Human insulin, a small protein hormone consisting of A-chain (21 residues) and B-chain (30 residues) linked by three disulfide bonds, is crucial for controlling the hyperglycemia in type I diabetes. In the present work molecular dynamics simulation (MD) with human insulin and its mutants was used to assess the influence of 10 point mutations (HisA8, ValA10, AspB10, GlnB17, AlaB17, GlnB18, AspB25, ThrB26, GluB27, AspB28), 6 double mutations (GluA13+GluB10, SerA13+GluB27, GluB1+GluB27, SerB2+AspB10, AspB9+GluB27, GluB16+GluB27) and one triple mutation (GluA15+AspA18+AspB3) in the protein sequence on the structure and dynamics of human insulin. A series of thermal unfolding MD simulations with wild type (WT) human insulin and its mutants was performed at 400 K with GROMACS software (version 5.1) using the CHARMM36m force field. The MD results have been analyzed in terms of the parameters characterizing both the global and local protein structure, such as the backbone root mean-square deviation, gyration radius, solvent accessible surface area, the root mean-square fluctuations and the secondary structure content. The MD simulation data showed that depending on time evolution of integral characteristics, the examined mutants can be tentatively divided into three groups: 1) the mutants HisA8, ValA10, AlaB17, AspB25, ThrB26, GluB27, GluA13+GluB10, GluB1+GluB27 and GluB16+GluB27, which exert stabilizing effect on the protein structure in comparison with wild type insulin; 2) the mutants GlnB17, AspB10, SerB2+AspB10 and GluA15+AspA18+AspB3 that did not significantly affect the dynamical properties of human insulin with a minimal stabilizing impact; 3) the mutants AspB28, AspB9+GluB27 and SerA13+GluB27, GlnB18, destabilizing the protein structure. Analysis of the secondary structure content provided evidence for the influence of AspB28, AspB9+GluB27 and SerA13+GluB27, GlnB18 on the insulin unfolding. Our MD results indicate that the replacement of superficial nonpolar residues in the insulin structure by hydrophilic ones gives rise to the increase in protein stability in comparison with the wild type protein.

2StrucCompare: a webserver for visualizing small but noteworthy differences between protein tertiary structures through interrogation of the secondary structure content

2StrucCompare is a webserver whose primary aim is to visualize subtle but functionally important differences between two related protein structures, either of the same protein or related homologues, with similar or functionally different tertiary structures. At the heart of the package is identifying and visualizing differences between conformations at the secondary structure and at the residue level, such as contact differences or side chain conformational differences found between two protein chains. The protein secondary structures are determined according to four established methods (DSSP, STRIDE, P-SEA and STICKS), and as each employs different assignment strategies, small conformational differences between the two structures can give rise to paired residues being denoted as having different secondary structure features with the different methods. 2StrucCompare captures both the large and more subtle differences found between structures, enabling visualization of these differences that could be key to an understanding of a proteins’ function. 2StrucCompare is freely accessible at http://2struccompare.cryst.bbk.ac.uk/index.php

Role of hydrophobic residues for the gaseous formation of helical motifs

The secondary structure content of proteins and their complexes may change significantly on passing from aqueous solution to the gas phase (as in mass spectrometry experiments).

Enhancement of exon skipping activity by reduction in the secondary structure content of LNA-based splice-switching oligonucleotides

LNA-based splice-switching oligonucleotides containing 7-deaza-2′-deoxyguanosine or 2′-deoxyinosine avoid secondary structure formation and showed higher exon skipping activities.

G-quadruplex secondary structure from circular dichroism spectroscopy

A curated library of circular dichroism spectra of 23 G-quadruplexes of known structure was built and analyzed. The goal of this study was to use this reference library to develop an algorithm to derive quantitative estimates of the secondary structure content of quadruplexes from their experimental CD spectra. Principle component analysis and singular value decomposition were used to characterize the reference spectral library. CD spectra were successfully fit to obtain estimates of the amounts of base steps in anti-anti, syn-anti or anti-syn conformations, in diagonal or lateral loops or in other conformations. The results show that CD spectra of nucleic acids can be analyzed to obtain quantitative structural information about secondary structure content in an analogous way to methods used to analyze protein CD spectra.

Development of advanced chemometric methods for analysis of deep-ultraviolet resonance Raman and circular dichroism spectroscopic data for protein secondary structure determination

Determination of protein secondary structure has become an area of great importance in biochemistry and biophysics as protein secondary structure is directly related to protein function and protein related diseases. While NMR and x-ray crystallography can predict placement of each atom in proteins to within an angstrom, optical methods are the preferred techniques for rapid evaluation of protein secondary structure content. Such techniques require calibration data to predict unknown protein secondary structure content where accuracy may be improved with the application of multivariate analysis. We compare protein secondary structure predictions obtained from multivariate analysis of ultraviolet resonance Raman (UVRR) and circular dichroism (CD) spectroscopic data using classical and partial least squares, and multivariate curve resolution-alternating least squares is made. Based on this analysis, the suggested best approach to rapid and accurate secondary structure determination is a combination of both CD and UVRR spectroscopy. While initial studies suggest that a complementary use of spectroscopic data from optical methods such as circular dichroism (CD), infrared (IR) and ultraviolet resonance Raman (UVRR) coupled with multivariate calibration techniques like multivariate curve resolution-alternating least squares (MCR-ALS) is the preferred route for real-time and accurate evaluation of protein secondary structure, further study presents a new strategy for the improvement of secondary structure determination of proteins by fusing CD and UVRR spectroscopic data. Also, a new method for determining the structural composition of each protein is employed, which is based on the relative abundance of the (phi,psi) dihedral angles of the peptide backbone as they correspond to each type of secondary structure. Comparison of the predicted protein secondary structures from MCR-ALS analysis of CD, UVRR and fused data with definitions obtained from dihedral angles of the peptide backbone, yields lower overall root mean squared errors of calibration for helical, sheet, poly-proline II type and total unfolded secondary structures with fused data. Considering that a disadvantage of multivariate calibration methods is the requirement of known concentration or spectral profiles, and second-order calibration methods, such as parallel factor analysis (PARAFAC), do not have such a requirement due to the "second-order advantage", PARAFAC was employed for analysis of UVRR data. An exceptional feature of UVRR spectroscopy is that UVRR spectra are also dependent on excitation wavelength as they are on secondary structure composition. Thus, higher order data can be created by combining protein UVRR spectra of several proteins collected at multiple excitation wavelengths. PARAFAC has been used to analyze UVRR data collected at multiple excitation wavelengths on several proteins to determine secondary structure content.