Determination of helix orientations in proteins.
ABSTRACT Accurate description of helices, including curvature and bending, and determination of interhelical angles are essential for analysis of the three-dimensional fold and functionally important conformational changes in helical proteins. Here, a new computational method is presented that allows determination of angles between any helical stretches, the radius of curvature of curved helices, bending angle of bent helices, as well as symmetry relations within the protein molecule, using main chain atom coordinates. The method has been applied to describe changes in interhelical angles in calmodulin upon interaction with a target peptide, which reveals the conformational changes at a higher precision. Because subtle changes in helix-to-helix packing and interhelical angles often underlie significant functional transitions in proteins, this approach can serve as a useful tool for characterization of such conformational changes at an exceedingly high accuracy and thus provide detailed insight into the structure-function relationship of proteins.
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ABSTRACT: Conformational changes in the substrate access channel have been observed for several forms of cytochrome P450, but the extent of conformational plasticity exhibited by a given isozyme has not been completely characterized. Here we present crystal structures of P450cam bound to a library of 12 active site probes containing a substrate analogue tethered to a variable linker. The structures provide a unique view of the range of protein conformations accessible during substrate binding. Principal component analysis of a total of 30 structures reveals three discrete clusters of conformations: closed (P450cam-C), intermediate (P450cam-I), and fully open (P450cam-O). Relative to P450cam-C, the P450cam-I state results predominantly from a retraction of helix F, while both helices F and G move in concert to reach the fully open P450cam-O state. Both P450cam-C and P450cam-I are well-defined states, while P450cam-O shows evidence of a somewhat broader distribution of conformations and includes the open form recently seen in the absence of substrate. The observed clustering of protein conformations over a wide range of ligand variants suggests a multistep closure of the enzyme around the substrate that begins by conformational selection from an ensemble of open conformations and proceeds through a well-defined intermediate, P450cam-I, before full closure to the P450cam-C state in the presence of small substrates. This multistep pathway may have significant implications for a full understanding of substrate specificity, kinetics, and coupling of substrate binding to P450 function.Biochemistry 02/2011; 50(5):693-703. · 3.38 Impact Factor
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ABSTRACT: We have solved the crystal structure of a segment of nonerythroid alpha-spectrin (alphaII) consisting of the first 147 residues to a resolution of 2.3 A. We find that the structure of this segment is generally similar to a corresponding segment from erythroid alpha-spectrin (alphaI) but exhibits unique differences with functional significance. Specific features include the following: (i) an irregular and frayed first helix (Helix C'); (ii) a helical conformation in the junction region connecting Helix C' with the first structural domain (D1); (iii) a long A(1)B(1) loop in D1; and (iv) specific inter-helix hydrogen bonds/salt bridges that stabilize D1. Our findings suggest that the hydrogen bond networks contribute to structural domain stability, and thus rigidity, in alphaII, and the lack of such hydrogen bond networks in alphaI leads to flexibility in alphaI. We have previously shown the junction region connecting Helix C' to D1 to be unstructured in alphaI (Park, S., Caffrey, M. S., Johnson, M. E., and Fung, L. W. (2003) J. Biol. Chem. 278, 21837-21844) and now find it to be helical in alphaII, an important difference for alpha-spectrin association with beta-spectrin in forming tetramers. Homology modeling and molecular dynamics simulation studies of the structure of the tetramerization site, a triple helical bundle of partial domain helices, show that mutations in alpha-spectrin will affect Helix C' structural flexibility and/or the junction region conformation and may alter the equilibrium between spectrin dimers and tetramers in cells. Mutations leading to reduced levels of functional tetramers in cells may potentially lead to abnormal neuronal functions.Journal of Biological Chemistry 03/2010; 285(19):14572-84. · 4.65 Impact Factor
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ABSTRACT: A new method, dubbed "HAXIS" is introduced to describe local and global shape properties of a protein helix via its axis. HAXIS is based on coarse-graining and spline-fitting of the helix backbone. At each Cα anchor point of the backbone, a Frenet frame is calculated, which directly provides the local vector presentation of the helix. After cubic spline-fitting of the axis line, its curvature and torsion are calculated. This makes a rapid comparison of different helix forms and the determination of helix similarity possible. Distortions of the helix caused by individual residues are projected onto the helix axis and presented either by the rise parameter per residue or by the local curvature of the axis. From a non-redundant set of 2,017 proteins, 15,068 helices were investigated in this way. Helix start and helix end as well as bending and kinking of the helix are accurately described. The global properties of the helix are assessed by a polynomial fit of the helix axis and the determination of its overall curving and twisting. Long helices are more regular shaped and linear whereas short helices are often strongly bent and twisted. The distribution of different helix forms as a function of helix length is analyzed.Journal of Molecular Modeling 03/2013; · 1.98 Impact Factor