The multicopper oxidase phenoxazinone synthase (PHS) catalyzes the penultimate step in the biosynthesis of the antibiotic actinomycin D by Streptomyces antibioticus. PHS exists in two oligomeric forms: a dimeric form and a hexameric form, with older actinomycin-producing cultures containing predominately the hexameric form. The structure of hexameric PHS has been determined using X-ray diffraction to a resolution limit of 2.30 A and is found to contain several unexpected and distinctive features. The structure forms a hexameric ring that is centered on a pseudo 6-fold axis and has an outer diameter of 185 A with a large central cavity that has a diameter of 50 A. This hexameric structure is stabilized by a long loop connecting two domains; bound to this long loop is a fifth copper atom that is present as a type 2 copper. This copper atom is not present in any other multicopper oxidase, and its presence appears to stabilize the hexameric structure.
Glutathione S-transferases are a family of multifunctional enzymes involved in the metabolism of drugs and xenobiotics. Two tyrosine residues, Tyr 7 and Tyr 111, in the active site of the enzyme play an important role in the binding and catalysis of substrate ligands. The crystal structures of Schistosoma japonicum glutathione S-transferase tyrosine 7 to phenylalanine mutant [SjGST(Y7F)] in complex with the substrate glutathione (GSH) and the competitive inhibitor S-octylglutathione (S-octyl-GSH) have been obtained. These new structural data combined with fluorescence spectroscopy and thermodynamic data, obtained by means of isothermal titration calorimetry, allow for detailed characterization of the ligand-binding process. The binding of S-octyl-GSH to SjGST(Y7F) is enthalpically and entropically driven at temperatures below 30 degrees C. The stoichiometry of the binding is one molecule of S-octyl-GSH per mutant dimer, whereas shorter alkyl derivatives bind with a stoichiometry of two molecules per mutant dimer. The SjGST(Y7F).GSH structure showed no major structural differences compared to the wild-type enzyme. In contrast, the structure of SjGST(Y7F).S-octyl-GSH showed asymmetric binding of S-octyl-GSH. This lack of symmetry is reflected in the lower symmetry space group of the SjGST(Y7F).S-octyl-GSH crystals (P6(3)) compared to that of the SjGST(Y7F).GSH crystals (P6(3)22). Moreover, the binding of S-octyl-GSH to the A subunit is accompanied by conformational changes that may be responsible for the lack of binding to the B subunit.
Phenoxazinone synthase, an oligomeric multicopper oxidase produced by Streptomyces antibioticus, is responsible for the six-electron oxidative coupling of two molecules of 4-methyl 3-hydroxyanthraniloyl pentapeptide to form the phenoxazinone chromophore of the antineoplastic agent actinomycin D. Spectroscopic studies have shown that the enzyme contains one type I (blue) and three to four type II copper centers. However, the exact arrangement of the copper centers in this multicopper oxidase is unknown. As a first step towards determining the three-dimensional structure of the enzyme, phenoxazinone synthase has been crystallized. The hexameric form of phenoxazinone synthase was purified from 72 h cultures of S. lividans containing the plasmid pIJ702. Purified hexamers were concentrated to 75 mg ml(-1) and used to grow two forms of crystals. Data collected from the two crystal forms were processed in two separate space groups. Crystals of both forms were grown at 288 K using the sitting-drop vapour-diffusion method. Native data sets extending to resolutions of 3.35 and 2.30 A have been collected and processed in space groups R32 and P1, respectively.
The Bacillus sphaericus binary toxin is expressed during the early stages of sporulation and is composed of two separately encoded polypeptides: BinA (41.9 kDa) and BinB (51.4 kDa). The binary toxin forms microcrystalline inclusions inside the mother cell that, once ingested, are solubilized in the alkaline pH of the larval gut. B. sphaericus cultures were grown to complete sporulation, harvested and washed. The binary toxin was solubilized with 50 mM NaOH, purified using column chromatography and crystallized. Crystals were grown using the sitting-drop vapor-diffusion method in a glycine-buffered solution containing 0.5 M NaCl and 19.5% PEG 4000 as the precipitant. Native data were collected to a resolution of 2.7 A.
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