PubMed Health⌕ Search
Home/Archive/PubMed

PubMed · 7779327

X-ray microscopy.

D Sayre·H N Chapman
View original source ↗Export metadata ↓

Abstract

The subject of X-ray microscopy (high-resolution X-ray imaging of general nonperiodic structures), an area in which much progress has been made in recent years, is reviewed. The main techniques are briefly described. Achievable performance levels, which for many years were highly speculative, can now be understood with fair accuracy in terms of basic X-ray and specimen properties, and techniques have progressed to the point where actual results are nearing those levels. In terms of specimen size and imaging resolution, X-ray microscopies lie between electron and light microscopy, and are thus suited to imaging extremely large and complex structures; in addition, they demand little or no specimen preparation, and can be used to observe local composition and chemical state as well as structure. Thus X-rays, which have played the leading role in imaging crystallizable materials, may also prove to be highly valuable in the imaging of very large non-crystalline structures. Throughout the treatment, attention is paid to the relationships connecting the subject with X-ray crystallography.

Explore related subjects

Crystallography, X-RayDiagnostic ImagingHolographyMicroscopyMicroscopy, Phase-ContrastPhotonsRadiation DosageScattering, RadiationX-Ray DiffractionX-RaysJournal ArticleReview

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

D Sayre, H N Chapman. 1995-05-01. X-ray microscopy.. https://doi.org/10.1107/s0108767394011803

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related citations

PubMed1996

Crystal structures of complexes of a peptidic inhibitor with wild-type and two mutant HIV-1 proteases.

Crystal structures of the protease of human immunodeficiency virus type 1 (HIV-1) and two mutant proteases, V82D and V82N, have been determined. In all three cases the enzyme forms a complex with the peptidic inhibitor U-89360E. All structures have been determined to 2.3 A resolution and have satisfactory agreement factors: 0.173 for wild type, 0.175 for V82D, and 0.182 for V82N. Comparison of the three crystal structures provides explanations which are consistent with the known kinetic properties of these mutant enzymes with the U-89360E inhibitor [Lin, Y., Lin, X., Hong, L., Foundling, S., Heinrikson, R. L., Thaisrivongs, S., Leelamanit, W., Raterman, D., Shah, M., Dunn, B.M., & Tang, J. (1995) Biochemistry 34, 1143-1152]. Unfavorable van der Waals interactions between the inhibitor and the mutated side chains at position 82 are consistent with diminished affinity for the inhibitor by the mutant enzymes. If a mutation is potentially resistant to an inhibitor, the mutant enzyme should not only have an increased Ki for the inhibitor but should also preserve considerable catalytic capability. The V82D mutant possesses these qualities. In the V82D crystal structure, a water molecule, which connects the protease flap to the inhibitor, is missing or of low occupancy. Absence of this bridge may be important in determining catalytic capability. Moreover, mutation at position 82 induces change in two polypeptide backbone regions, 35-41 and 67-68, which may be related to protease flap mobility.

Crystallography, X-Ray↗
PubMed1996

Crystal structure of soybean lipoxygenase L-1 at 1.4 A resolution.

Lipoxygenases, which are widely distributed among plant and animal species, are Fe-containing dioxygenases that act on lipids containing (Z,Z)-pentadiene moieties in the synthesis of compounds with a variety of functions. Utilizing an improved strategy of data collection, low temperature, and synchrotron radiation of short wavelength, the structure of ferrous soybean lipoxygenase L-1, a single chain protein of 839 amino acid residues, has been determined by X-ray crystallography to a resolution of 1.4 A. The R-factor for the refined model is 19.7%. General features of the protein structure were found to be consistent with the results of prior crystallographic studies at lower (2.6 A) resolution. In contrast to the prior studies, the binding of a water molecule to the active site Fe was established. The octahedral coordination sphere of the Fe also includes the side chains of His499, His504, His690, and Asn694 as well as the terminal carboxylate of Ile839, which binds as a monodentate ligand. Asn694 is involved in a number of labile polar interactions with other protein groups, including an amide-aromatic hydrogen bond, and appears to be a weak ligand. Several possible access routes for dioxygen and fatty acids to the internal active site and substrate binding cavity are described. The protein structure restricts access to the Fe site such that the formation of an organo-Fe intermediate seems improbable. Structural restrictions pertinent to other proposed reaction intermediates, such as planar pentadienyl and nonplanar allyl radicals, are also discussed.

Crystallography, X-Ray↗
PubMed1996

2.3 A resolution X-ray crystal structure of the bisubstrate analogue inhibitor salicylhydroxamic acid bound to human myeloperoxidase: a model for a prereaction complex with hydrogen peroxide.

The X-ray crystal structure of a salicylhydroxamic acid (SHA) inhibitory complex with human myeloperoxidase (MPO) has been determined at 2.3 A resolution. The aromatic ring of the inhibitor binds to a hydrophobic region at the entrance to the distal heme pocket between heme pyrrole ring D and the side chain of Arg 239. The hydroxamic acid moiety is hydrogen bonded to both the distal histidine 95 and the adjacent glutamine 91 amide group but is not coordinated to the heme iron. SHA binding displaces three water molecules from the distal heme cavity and causes a small shift in the position of a fourth water molecule. Otherwise, there are no significant conformational differences between the active site regions of the complex and the native enzyme. The ability of the three SHA oxygen atoms to closely duplicate the hydrogen-bonding pattern of these three water molecules in the native enzyme is postulated to account for the strong binding of this inhibitor to MPO. The mode of binding of SHA to MPO provides information on the binding sites for aromatic peracid substrates that promote compound I formation as well as aromatic alcohols and amines that carry out single-electron reductions of compound I. Similarities in the hydrogen-bonding patterns of amino acid residues and water molecules in the distal heme pockets of myeloperoxidase and the nonhomologous cytochrome c peroxidase suggest that they may have similar mechanisms of compound I formation. A model is presented for a prereaction complex of myeloperoxidase in which hydrogen peroxide is hydrogen bonded to the distal histidine, as a prerequisite for deprotonation and subsequent binding at the sixth coordination site of the heme iron.

Crystallography, X-Ray↗