Two-component signal transduction systems are widespread in prokaryotes and control numerous cellular processes. Extensive investigation of sensor kinase and response regulator proteins from many two-component systems has established conserved sequence, structural, and mechanistic features within each family. In contrast, the phosphatases which catalyze hydrolysis of the response regulator phosphoryl group to terminate signal transduction are poorly understood. Here we present structural and functional characterization of a representative of the CheC/CheX/FliY phosphatase family. The X-ray crystal structure of Borrelia burgdorferi CheX complexed with its CheY3 substrate and the phosphoryl analogue reveals a binding orientation between a response regulator and an auxiliary protein different from that shared by every previously characterized example. The surface of CheY3 containing the phosphoryl group interacts directly with a long helix of CheX which bears the conserved (E - X2 - N) motif. Conserved CheX residues Glu96 and Asn99, separated by a single helical turn, insert into the CheY3 active site. Structural and functional data indicate that CheX Asn99 and CheY3 Thr81 orient a water molecule for hydrolytic attack. The catalytic residues of the CheX·CheY3 complex are virtually superimposable on those of the Escherichia coli CheZ phosphatase complexed with CheY, even though the active site helices of CheX and CheZ are oriented nearly perpendicular to one other. Thus, evolution has found two structural solutions to achieve the same catalytic mechanism through different helical spacing and side chain lengths of the conserved acid/amide residues in CheX and CheZ.

Summary In two-component regulatory systems, covalent phosphorylation typically activates the response regulator signaling protein and hydrolysis of the phosphoryl group reestablishes the inactive state. Despite highly conserved three-dimensional structures and active site features, the rates of catalytic autodephosphorylation for different response regulators vary by a factor of almost 106. Previous studies identified two variable active site residues, corresponding to Escherichia coli CheY residues 59 and 89, that modulate response regulator autodephosphorylation rates about 100-fold. Here, a set of five CheY mutants, which match other “model” response regulators (ArcA, CusR, DctD, FixJ, PhoB, or Spo0F) at variable active site positions corresponding to CheY residues 14, 59 and 89, were characterized functionally and structurally in an attempt to identify mechanisms that modulate autodephosphorylation rate. As expected, the autodephosphorylation rates of the CheY mutants were reduced six- to 40-fold relative to wild type CheY, but all still autodephosphorylated 12- to 80-fold faster than their respective model response regulators. Comparison of X-ray crystal structures of the five CheY mutants (complexed with the phosphoryl group analogue BeF3−) to wild type CheY or corresponding model response regulator structures gave strong evidence for steric obstruction of the phosphoryl group from the attacking water molecule as one mechanism to enhance phosphoryl group stability. Structural data also suggested that impeding the change of a response regulator from the active to inactive conformation might retard the autodephosphorylation reaction if the two processes are coupled, and that the residue at position '58' may contribute to rate modulation. A given combination of amino acids at positions '14', '59', and '89' adopted similar conformations regardless of protein context (CheY or model response regulator), suggesting that knowledge of residue identity may be sufficient to predict autodephosphorylation rate, and hence, the kinetics of the signaling response, in the response regulator family of proteins.

We have studied the structural elements that affect ligand exchange between the two high affinity biotinbinding proteins, egg white avidin and its bacterial analogue, streptavidin. For this purpose, we have developed a simple assay based on the antipodal behavior of the two proteins toward hydrolysis of biotinyl p-nitrophenyl ester (BNP). The assay provided the experimental basis for these studies. It was found that biotin migrates unidirectionally from streptavidin to avidin. Conversely, the biotin derivative, BNP, is transferred in the opposite direction, from avidin to streptavidin. A previous crystallographic study (Huberman, T., Eisenberg-Domovich, Y., Gitlin, G., Kulik, T., Bayer, E. A., Wilchek, M., and Livnah, O. (2001) J. Biol. Chem. 276, 32031-32039) provided insight into a plausible explanation for these results. These data revealed that the non-hydrolyzable BNP analogue, biotinyl p-nitroanilide, was almost completely sheltered in streptavidin as opposed to avidin in which the disordered conformation of a critical loop resulted in the loss of several hydrogen bonds and concomitant exposure of the analogue to the solvent. In order to determine the minimal modification of the biotin molecule required to cause the disordered loop conformation, the structures of avidin and streptavidin were determined with norbiotin, homobiotin, and a common long-chain biotin derivative, biotinyl ⑀-aminocaproic acid. Six new crystal structures of the avidin and streptavidin complexes with the latter biotin analogues and derivatives were thus elucidated. It was found that extending the biotin side chain by a single CH 2 group (i.e. homobiotin) is sufficient to result in this remarkable conformational change in the loop of avidin. These results bear significant biotechnological importance, suggesting that complexes containing biotinylated probes with streptavidin would be more stable than those with avidin. These findings should be heeded when developing new drugs based on lead compounds because it is difficult to predict the structural and conformational consequences on the resultant protein-ligand interactions.The interaction of egg white avidin and bacterial streptavidin with biotin has evolved into an indispensable tool for general use in the biological sciences and as a model for the study of the interaction of a ligand with a protein (1). Both avidin and streptavidin bind biotin with an essentially immeasurably high affinity constant (2).During the course of our studies on the avidin/streptavidinbiotin complex, we sought to examine whether biotin or its derivatives can be exchanged between the two proteins. Such a transfer can be of both practical and theoretical value. If practical application is desired, one can envision a situation where one of the proteins is first introduced into an experimental system followed by addition of the second biotin-binding protein. The question then arises whether one protein replaces the other or whether the effect will be cumulative. Such a situation may indeed occur upon in vivo ap...

The development of Tyrosinase inhibitors (TYRIs) could represent an efficacious strategy for pharmacological intervention on skin pathologies related to aberrant production of melanin. Based on in silico studies we designed and tested a library of twenty-four compounds bearing the 4-(4-fluorobenzyl) piperazin-1-yl]-fragment. As result, we identified several compounds with excellent inhibit effects at low micromolar concentration against TYR from Agaricus bisporus (TyM). Among them, compound 25 (IC 50 ¼ 0.96 mM) proved to be~20-fold more potent than the reference compound kojic acid (IC 50 ¼ 17.76 mM) having wide applications in the cosmetics and pharmaceutical industries. The mode of interaction of active inhibitor 25 was deciphered by means of crystallography as well as molecular docking and these results were consistent with kinetic experiments. Moreover, the identified compound 25 exhibited no considerable cytotoxicity and showed anti-melanogenic effects on B16F10 melanoma cells. Therefore, a combination of computational and biochemical approaches could represent a rational guidelines for further structural modification of this class of compounds as future anti-melanogenic agents.

The inhibition of tyrosinase (Ty, EC 1.14.18.1) represents an efficient strategy of decreasing melanogenesis and skin hyperpigmentation. A combination of crystallographic and docking studies on two different tyrosinases, that from Bacillus megaterium (TyBm) and that from a mushroom (TyM), has contributed to increasing our knowledge about their structural information and translating that information to the most druggable human Ty (TyH) isozyme. In particular, we designed and synthesized a series of 1-(4-fluorobenzyl)piperazine and 1-(4-fluorobenzyl)piperidine derivatives showing inhibitory activities on TyM at micromolar ranges and more potency than that of the reference compound, kojic acid. The crystal structures of TyBm with inhibitor 3 (IC value of 25.11 μM) and 16 (IC value of 5.25 μM) were solved, confirming the binding poses hypothesized by in silico studies and revealing the main molecular determinants for the binding recognition of the inhibitors.