Protein crystal structure solution by fast incorporation of negatively and positively charged anomalous scatterers

The preparation of derivatives by the traditional methods of soaking is one of the most time-consuming steps in protein crystal structure solution by X-ray diffraction techniques. The `quick cryosoaking' procedure for derivatization with halides (monovalent anions) offers the possibility of significantly speeding up this process [Dauter et al. (2000), Acta Cryst. D56, 232-237]. In the present work, an extension of this technique is proposed and the use of two different classes of compounds (monovalent and polyvalent cations) that can be successfully utilized in the quick cryosoaking procedure for the derivatization and phasing of protein crystals is described. This approach has been tested on hen egg-white lysozyme and has been successfully used to solve the structure of a novel trypsin inhibitor. The possibility of using cations in the fast cryosoaking procedure gives additional flexibility in the process of derivatization and increases the chances of success in phase determination. This method can be applied to high-throughput crystallographic projects.577996100

The structures of Micrococcus lysodeikticus catalase, its ferryl intermediate (compound II) and NADPH complex

The crystal structure of the bacterial catalase from Micrococcus lysodeikticus has been refined using the gene-derived sequence both at 0.88 Angstrom resolution using data recorded at 110 K and at 1.5 Angstrom resolution with room-temperature data. The atomic resolution structure has been refined with individual anisotropic atomic thermal parameters. This has revealed the geometry of the haem and surrounding protein, including many of the H atoms, with unprecedented accuracy and has characterized functionally important hydrogen-bond interactions in the active site. The positions of the H atoms are consistent with the enzymatic mechanism previously suggested for beef liver catalase. The structure reveals that a 25 Angstrom long channel leading to the haem is filled by partially occupied water molecules, suggesting an inherent facile access to the active site. In addition, the structures of the ferryl intermediate of the catalase, the so-called compound II, at 1.96 Angstrom resolution and the catalase complex with NADPH at 1.83 Angstrom resolution have been determined. Comparison of compound II and the resting state of the enzyme shows that the binding of the O atom to the iron (bond length 1.87 Angstrom) is associated with increased haem bending and is accompanied by a distal movement of the iron and the side chain of the proximal tyrosine. Finally, the structure of the NADPH complex shows that the cofactor is bound to the molecule in an equivalent position to that found in beef liver catalase, but that only the adenine part of NADPH is visible in the present structure

Structure of Mycobacterium smegmatis single-stranded DNA-binding protein and a comparative study involving homologus SSBs: biological implications of structural plasticity and variability in quaternary association

The structure of Mycobacterium smegmatis single-stranded DNA-binding protein (SSB) has been determined using three data sets collected from related crystals. The structure is similar to that of its homologue from Mycobacterium tuberculosis, indicating that the clamp arrangement that stabilizes the dimer and the ellipsoidal shape of the tetramer are characteristic features of mycobacterial SSBs. The central OB fold is conserved in mycobacterial SSBs as well as those from Escherichia coli, Deinococcus radiodurans and human mitochondria. However, the quaternary structure exhibits considerable variability. The observed plasticity of the subunit is related to this variability. The crystal structures and modelling provide a rationale for the variability. The strand involved in the clamp mechanism, which leads to higher stability of the tetramer, appears to occur in all high-G+C Gram-positive bacteria. The higher stability is perhaps required by these organisms. The mode of DNA binding of mycobacterial SSBs is different from that of E. coli SSB partly on account of the difference in the shape of the tetramers. Another difference between the two modes is that the former contains additional ionic interactions and is more susceptible to salt concentration

Get Phases from Arsenic Anomalous Scattering: de novo SAD Phasing of Two Protein Structures Crystallized in Cacodylate Buffer

The crystal structures of two proteins, a putative pyrazinamidase/nicotinamidase from the dental pathogen Streptococcus mutans (SmPncA) and the human caspase-6 (Casp6), were solved by de novo arsenic single-wavelength anomalous diffraction (As-SAD) phasing method. Arsenic (As), an uncommonly used element in SAD phasing, was covalently introduced into proteins by cacodylic acid, the buffering agent in the crystallization reservoirs. In SmPncA, the only cysteine was bound to dimethylarsinoyl, which is a pentavalent arsenic group (As (V)). This arsenic atom and a protein-bound zinc atom both generated anomalous signals. The predominant contribution, however, was from the As anomalous signals, which were sufficient to phase the SmPncA structure alone. In Casp6, four cysteines were found to bind cacodyl, a trivalent arsenic group (As (III)), in the presence of the reducing agent, dithiothreitol (DTT), and arsenic atoms were the only anomalous scatterers for SAD phasing. Analyses and discussion of these two As-SAD phasing examples and comparison of As with other traditional heavy atoms that generate anomalous signals, together with a few arsenic-based de novo phasing cases reported previously strongly suggest that As is an ideal anomalous scatterer for SAD phasing in protein crystallography

Folding, Design and Determination of Interaction Potentials Using Off-Lattice Dynamics of Model Heteropolymers

We present the results of a self-consistent, unified molecular dynamics study of simple model heteropolymers in the continuum with emphasis on folding, sequence design and the determination of the interaction parameters of the effective potential between the amino acids from the knowledge of the native states of the designed sequences.Comment: 8 pages, 3 Postscript figures, uses RevTeX. Submitted to Physical Review Letter