Chloramphenicol stereoisomers need to be distinguished: consequences observed from a proficiency test.

Although the use of chloramphenicol (CAP) as a veterinary drug is banned in the European Union and many other countries, monitoring for CAP residues in food is routine. Positive detections are few, but taken extremely seriously. European Union laboratories analysing for CAP should validate methods according to European Commission Decision 2002/657/EC, must be accredited to ISO 17025, and will generally participate in proficiency testing (PT) schemes, such as those offered by the Food Analysis Performance Assessment Scheme (FAPAS®). The FAPAS PTs aim to cover a wide range of relevant matrices including honey, prawns, fish, milk and kidney. Test materials are prepared either by animal dosing studies or by spiking raw matrix. The most common method reported by FAPAS participants used to screen for CAP residues is LC-MS/MS, but ELISA kits are increasingly being used. A recent PT round highlighted that the result obtained might be correlated with the type of analytical method being employed. Follow-up investigations have demonstrated that some of these variations in data are a function of the different stereoisomeric forms of CAP. This paper discusses the implication of this research on method validation requirements and European Union legislation.

Structural and electrostatic asymmetry at the active site in typical and atypical peroxiredoxin dimers.

The peroxiredoxins (Prx) are ubiquitous peroxidases involved in important biological processes; however, details of their enzymatic mechanism remain elusive. To probe potential dynamics-function relationships, molecular dynamics simulations and electrostatic calculations were performed on the atypical 2-cysteine thiol peroxidase (Tpx) from Streptococcus pneumoniae and results compared to a previous study of a typical 2-cysteine Prx from Trypanosoma cruzi. The analyses indicate a commonality between both typical and atypical Prx: dynamic asymmetry. Asymmetry is observed in structure, fluctuations, and active site electrostatics. Key residues, including Glu150 and Phe153, play roles in the developing asymmetry; furthermore, in the atypical 2-Cys Tpx, Glu150 exhibits conformation fluctuations suggesting involvement in a proton shuttle. The existence of a pathway of connected residues appears to propagate the asymmetry. The commonality of asymmetry and coupling pathways in both typical and atypical Prxs suggests a driving force toward dimer asymmetry as a common feature that plays a functional role in creating one active site with a lower cysteine pK(a).

Conformational and oligomeric effects on the cysteine pK(a) of tryparedoxin peroxidase.

Typical 2-Cys peroxiredoxins (Prxs) are peroxidases which regulate cell signaling pathways, apoptosis, and differentiation. These enzymes are obligate homodimers, and can form decamers in solution. During catalysis, Prxs exhibit cysteine-dependent reactivity which requires the deprotonation of the peroxidatic cysteine (C(p)) supported by a lowered pK(a) in the initial step. We present the results of molecular dynamics simulations combined with pKa calculations on the monomeric, dimeric and decameric forms of one typical 2-Cys Prx, the tryparedoxin peroxidase from Trypanosoma cruzi (PDB id, 1uul). The calculations indicate that C(p) (C52) pK(a) values are highly affected by oligomeric state; an unshifted C(p) pK(a) (approximately 8.3, comparable to the pK(a) of isolated cysteine) is calculated for the monomer. In the dimers, starting with essentially identical structures, the C(p)s evolve dynamically asymmetric pK(a)s during the simulations; one subunit's C(p) pK(a) is shifted downward at a time, while the other is unshifted. However, when averaged over time, or multiple simulations, the two subunits within a dimer exhibit the same C(p), showing no preference for a lowered pK(a) in either subunit. Two conserved pathways that communicate the asymmetric pK(a)s between C(p)s of different subunits can be identified. In the decamer, all the C(p) pK(a)s are shifted downward, with slight asymmetry in the dimers which form the decamers. Structural analyses implicate oligomerization effects as responsible for these oligomeric state-dependent C(p) pK(a) shifts. The intra-dimer and the inter-dimer subunit contacts in the decamer restrict the conformations of the side chains of several residues (T49, T54 and E55) calculated to be key in shifting the C(p) pK(a). In addition, the backbone fluctuations of a few residues (M46, D47 and F48) result in a different electrostatic environment for the C(p) in dimers relative to the monomers. These side chain and backbone interactions which contribute to pK(a) modulation indicate the importance of oligomerization to the function of the typical 2-Cys Prxs.

Using molecular dynamics to map interaction networks in an aminoacyl-tRNA synthetase.

Long-range functional communication is a hallmark of many enzymes that display allostery, or action-at-a-distance. Many aminoacyl-tRNA synthetases can be considered allosteric, in that their trinucleotide anticodons bind the enzyme at a site removed from their catalytic domains. Such is the case with E. coli methionyl-tRNA synthase (MetRS), which recognizes its cognate anticodon using a conserved tryptophan residue 50 A away from the site of tRNA aminoacylation. The lack of details regarding how MetRS and tRNA(Met) interact has limited efforts to deconvolute the long-range communication that occurs in this system. We have used molecular dynamics simulations to evaluate the mobility of wild-type MetRS and a Trp-461 variant shown previously by experiment to be deficient in tRNA aminoacylation. The simulations reveal that MetRS has significant mobility, particularly at structural motifs known to be involved in catalysis. Correlated motions are observed between residues in distant structural motifs, including the active site, zinc binding motif, and anticodon binding domain. Both mobility and correlated motions decrease significantly but not uniformly upon substitution at Trp-461. Mobility of some residues is essentially abolished upon removal of Trp-461, despite being tens of Angstroms away from the site of mutation and solvent exposed. This conserved residue does not simply participate in anticodon binding, as demonstrated experimentally, but appears to mediate the protein's distribution of structural ensembles. Finally, simulations of MetRS indicate that the ligand-free protein samples conformations similar to those observed in crystal structures with substrates and substrate analogs bound. Thus, there are low energetic barriers for MetRS to achieve the substrate-bound conformations previously determined by structural methods.

Insights into correlated motions and long-range interactions in CheY derived from molecular dynamics simulations.

CheY is a response regulator protein involved in bacterial chemotaxis. Much is known about its active and inactive conformations, but little is known about the mechanisms underlying long-range interactions or correlated motions. To investigate these events, molecular dynamics simulations were performed on the unphosphorylated, inactive structure from Salmonella typhimurium and the CheY-BeF(3)(-) active mimic structure (with BeF(3)(-) removed) from Escherichia coli. Simulations utilized both sequences in each conformation to discriminate sequence- and structure-specific behavior. The previously identified conformational differences between the inactive and active conformations of the strand-4-helix-4 loop, which are present in these simulations, arise from the structural, and not the sequence, differences. The simulations identify previously unreported structure-specific flexibility features in this loop and sequence-specific flexibility features in other regions of the protein. Both structure- and sequence-specific long-range interactions are observed in the active and inactive ensembles. In the inactive ensemble, two distinct mechanisms based on Thr-87 or Ile-95 rotameric forms, are observed for the previously identified g+ and g- rotamer sampling by Tyr-106. These molecular dynamics simulations have thus identified both sequence- and structure-specific differences in flexibility, long-range interactions, and rotameric form of key residues. Potential biological consequences of differential flexibility and long-range correlated motion are discussed.

Chemical and structural diversity in cyclooxygenase protein active sites.

A major pharmaceutical problem is designing diverse and selective lead compounds. The human genome sequence provides opportunities to discover compounds that are protein selective if we can develop methods to identify specificity determinants from sequence alone. We have analyzed sequence and structural diversity of sheep COX-1 and mouse COX-2 proteins by Active Site Profiling (ASP). Eleven residues that should serve as specificity determinants between COX-1 and COX-2 were identified; however, the literature suggests that only one has been utilized in structure-based discovery. ASP was used to create a position-specific scoring matrix, which was used to identify possible cross-reacting proteins from the human sequences. This method proved selective for cyclooxygenases, comparing well with results using BLAST. The methods identify a probable misannotation of a cyclooxygenase in which there is high sequence similarity scores using BLAST, but ASP shows it does not contain the residues necessary for cyclooxygenase function. ASP Analysis of human COX proteins suggests that some specificity determinants that distinguish COX-1 and COX-2 proteins are similar between sheep COX-1/mouse COX-2 and human COX-1/COX2; however, residue identities at those positions are not necessarily conserved. Our results lay groundwork for development of family-specific pattern recognition methods to selectively match compounds with proteins.

Synthesis and evaluation of analogues of Congo red as potential compounds against transmissible spongiform encephalopathies.

The synthesis of analogues of the amyloid stain Congo red (1) as potential compounds against transmissible spongiform encephalopathies (TSEs) is reported. Using the direct method, aniline (2) or diamines such as 4,4'-diaminodiphenylsulfone (dapsone, 9), 3,3'-diaminodiphenylsulfone (10), benzidine (11), 3,3'-dimethoxybenzidine (12) or 3,3'-dichlorobenzidine (13) were diazotised to afford the corresponding diazonium salts, which without isolation, were directly used for coupling with a range of aromatic sulfonic or carboxylic acids to provide the corresponding truncated dyes analogues of Congo red, 4, 6, 8, and the symmetrical bis azoic dyes 14-19, 21-22, 24 and 26-29 as their sodium salts. Compounds were assayed in a cellular model of scrapie, a sheep TSE. Some of the compounds were shown to have similar activity to the lead compound Congo red. Molecular modelling was carried out to investigate potential structure-activity relationships (SARs) relating to the size and shape of Congo Red analogues. Within the range of compounds tested no discernible SARs were found.