Biomedical subjects

Michael S Chapman

Publications and source records attributed to Michael S Chapman.

24 records · Page 2Linked to original sources

PubMed2002

Refinement of the arginine kinase transition-state analogue complex at 1.2 A resolution: mechanistic insights.

The three-dimensional crystal structure of an arginine kinase transition-state analogue complex has been refined at 1.2 A resolution, with an overall R factor of 12.3%. The current model provides a unique opportunity to analyze the structure of a bimolecular (phosphagen kinase) enzyme in its transition state. This atomic resolution structure confirms in-line transfer of the phosphoryl group and the catalytic importance of the precise alignment of the substrates. The structure is consistent with a concerted proton transfer that has been proposed for an unrelated kinase. Refinement of anisotropic temperature factors and translation-libration-screw (TLS) analyses led to the identification of four rigid groups and their prevalent modes of motion in the transition state. The relative magnitudes of the mobility of rigid groups are consistent with their proposed roles in catalysis.

Animals↗

PubMed2002

The atomic structure of adeno-associated virus (AAV-2), a vector for human gene therapy.

The structure of the adeno-associated virus (AAV-2) has been determined to 3-A resolution by x-ray crystallography. AAV is being developed as a vector for gene therapy to treat diseases including hemophilia, cancer, and cystic fibrosis. As in the distantly related autonomous parvoviruses, the capsid protein has a beta-barrel fold, but long loops between the beta-strands share little structural homology with other parvoviruses, leading to unique surface features. Most prominent are groups of threefold-related peaks, each an intimate association of loops from two neighboring subunits. Mutations affecting cell entry and receptor binding are clustered near the positively charged side of each peak, implicating the region in attachment to the cellular receptor, heparan sulfate proteoglycan. Amino acids involved in antibody binding are in the same general vicinity. The structure will guide rational engineering of vector capsids to tailor cellular targeting and to avoid immediate neutralization by an immune system sensitized by prior exposure to AAV.

Amino Acid Sequence↗

PubMed2002

Simulated-annealing real-space refinement as a tool in model building.

Methods have been developed that further automate the building of macromolecular structures into electron-density maps. The software supports molecular-dynamics real-space refinement of an atomic model to local regions of a map within the context of a popular molecular-modeling program, O [Jones et al. (1991), Acta Cryst. A47, 110-119]. It is implemented as a module to the CNS refinement package [Brünger et al. (1998), Acta Cryst. D54, 905-921], controlled by a graphical user interface and commands executed directly through the molecular-graphics package. The method is illustrated with examples of the building and rebuilding of protein and nucleic acid models in which laborious manual adjustments are avoided. The resulting models show improved convergence during subsequent reciprocal-space refinement. The novel feature of the RSRef2000 software is the combination of simulated-annealing optimization with local real-space refinement, allowing several local minima to be explored quickly and automatically within the context of interactive model building.

Arginine Kinase↗

PubMed2002

Polylysine induces an antiparallel actin dimer that nucleates filament assembly: crystal structure at 3.5-A resolution.

An antiparallel actin dimer has been proposed to be an intermediate species during actin filament nucleation. We now show that latrunculin A, a marine natural product that inhibits actin polymerization, arrests polylysine-induced nucleation at the level of an antiparallel dimer, resulting in its accumulation. These dimers, when composed of pyrene-labeled actin subunits, give rise to a fluorescent excimer, permitting detection during polymerization in vitro. We report the crystallographic structure of the polylysine-actin-latrunculin A complex at 3.5-A resolution. The non-crystallographic contact is consistent with a dimeric structure and confirms the antiparallel orientation of its subunits. The crystallographic contacts reveal that the mobile DNase I binding loop of one subunit of a symmetry-related antiparallel actin dimer is partially stabilized in the interface between the two subunits of a second antiparallel dimer. These results provide a potential explanation for the paradoxical nucleation of actin filaments that have exclusively parallel subunits by a dimer containing antiparallel subunits.

Actins↗

PubMed2002

An improved hydrogen bond potential: impact on medium resolution protein structures.

A new semi-empirical force field has been developed to describe hydrogen-bonding interactions with a directional component. The hydrogen bond potential supports two alternative target angles, motivated by the observation that carbonyl hydrogen bond acceptor angles have a bimodal distribution. It has been implemented as a module for a macromolecular refinement package to be combined with other force field terms in the stereochemically restrained refinement of macromolecules. The parameters for the hydrogen bond potential were optimized to best fit crystallographic data from a number of protein structures. Refinement of medium-resolution structures with this additional restraint leads to improved structure, reducing both the free R-factor and over-fitting. However, the improvement is seen only when stringent hydrogen bond selection criteria are used. These findings highlight common misconceptions about hydrogen bonding in proteins, and provide explanations for why the explicit hydrogen bonding terms of some popular force field sets are often best switched off.

Animals↗

PubMedDate not supplied

Low-resolution structure refinement in electron microscopy.

A real-space structure refinement method, originally developed for macromolecular X-ray crystallography, has been applied to protein structure analysis by electron microscopy (EM). This method simultaneously optimizes the fit of an atomic model to a density map and the stereo-chemical properties of the model by minimizing an energy function. The performance of this method is characterized at different resolution and signal-to-noise ratio conditions typical for EM electron density maps. A multi-resolution scheme is devised to improve the convergence of the refinement on the global energy minimum. Applications of the method to various model systems are demonstrated here. The first case is the arrangement of FlgE molecules in the helical filament of flagellar hook, in which refinement with segmented rigid bodies improves the density correlation and reduces severe van der Waals contacts among the symmetry-related subunits. The second case is a conformational analysis of the NSF AAA ATPase in which a multi-conformer model is used in the refinement to investigate the arrangement of the two ATPase domains in the molecule. The third case is a docking simulation in which the crystal structure of actin and the NOE data from NMR experiments on the dematin headpiece are combined with a low-resolution EM density map to generate an atomic model of the F-actin-dematin headpiece structure.

Actins↗