Thymoquinone blocks pSer/pThr recognition by Plk1 Polo-box domain as a phosphate mimic.

Phosphorylation-dependent protein-protein interaction has rarely been targeted in medicinal chemistry. Thymoquinone, a naturally occurring antitumor agent, disrupts prephosphorylated substrate recognition by the polo-box domain of polo-like kinase 1, a key mitotic regulator responsible for various carcinogenesis when overexpressed. Here, crystallographic studies reveal that the phosphoserine/phosphothreonine recognition site of the polo-box domain is the binding pocket for thymoquinone and its analogue poloxime. Both small molecules displace phosphopeptides bound with the polo-box domain in a slow but noncovalent binding mode. A conserved water bridge and a cation-π interaction were found as their competition strategy against the phosphate group. This mechanism sheds light on small-molecule intervention of phospho-recognition by the polo-box domain of polo-like kinase 1 and other phospho-binding proteins in general.

Characterization and tissue-specific expression of two lepidopteran farnesyl diphosphate synthase homologs: implications for the biosynthesis of ethyl-substituted juvenile hormones.

The sesquiterpenoid juvenile hormone (JH) regulates insect development and reproduction. Most insects produce only one chemical form of JH, but the Lepidoptera produce four derivatives featuring ethyl branches. The biogenesis of these JHs requires the synthesis of ethyl-substituted farnesyl diphosphate (FPP) by FPP synthase (FPPS). To determine if there exist more than one lepidopteran FPPS, and whether one FPPS homolog is better adapted for binding the bulkier ethyl-branched substrates/products, we cloned three lepidopteran FPPS cDNAs, two from Choristoneura fumiferana and one from Pseudaletia unipuncta. Amino acid sequence comparisons among these and other eukaryotic FPPSs led to the recognition of two lepidopteran FPPS types. Type-I FPPSs display unique active site substitutions, including several in and near the first aspartate-rich motif, whereas type-II proteins have a more "conventional" catalytic cavity. In a yeast assay, a Drosophila FPPS clone provided full complementation of an FPPS mutation, but lepidopteran FPPS clones of either type yielded only partial complementation, suggesting unusual catalytic features and/or requirements of these enzymes. Although a structural analysis of lepidopteran FPPS active sites suggested that type-I enzymes are better suited than type-II for generating ethyl-substituted products, a quantitative real-time PCR assessment of their relative abundance in insect tissues indicated that type-I expression is ubiquitous whereas that of type-II is essentially confined to the JH-producing glands, where its transcripts are approximately 20 times more abundant than those of type-I. These results suggest that type-II FPPS plays a leading role in lepidopteran JH biosynthesis in spite of its apparently more conventional catalytic cavity.

Compact reduced thioredoxin structure from the thermophilic bacteria Thermus thermophilus.

The X-ray crystallographic structure of a thioredoxin from Thermus thermophilus was solved to 1.8 A resolution by molecular replacement. The crystals' space group was C2 with cell dimensions of a = 40.91, b = 95.44, c = 56.68 A, beta =91.41 degrees, with two molecules in the asymmetric unit. Unlike the reported thioredoxin structures, the biological unit of T. thermophilus thioredoxin is a dimer both in solution and in the crystal. The fold conforms to the "thioredoxin fold" that is common over a class of nine protein families including thioredoxin; however, the folded portion of this protein is much more compact than other thioredoxins previously solved by X-ray crystallography being reduced by one alpha-helix and one beta-strand. As with other thioredoxins, the active site is highly conserved even though the variation in sequence can be quite large. The T. thermophilus thioredoxin has some variability at the active site, especially compared with previously solved structures from bacterial sources.

Structure of a putative 2'-5' RNA ligase from Pyrococcus horikoshii.

Cyclic phosphodiesterase and 2'-5' RNA ligase are members of a superfamily of proteins which share structural similarities even though their homology may be very low. A putative 2'-5' RNA ligase from Pyrococcus horikoshii has been crystallized and its X-ray crystallographic structure determined to 2.4 A. The protein crystallized in the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 44.07, b = 45.47, c = 93.17 A and one protein monomer in the asymmetric unit. The molecular-replacement probe was a 2'-5' RNA ligase from Thermus thermophilus which shares 30% sequence identity. The P. horikoshii RNA ligase has some structural features that have more in common with a cyclic phosphodiesterase from Arabidopsis thaliana with which it has no significant homology, yet an examination of the electrostatic surface potential clearly defines its relationship to the T. thermophilus RNA ligase. However, the size of the active-site cleft is smaller and less positively charged than that of the T. thermophilus homologue, suggesting that the actual substrate may be smaller than that previously postulated for the latter.

Crystal structure of a purine/pyrimidine phosphoribosyltransferase-related protein from Thermus thermophilus HB8.

Adenine phosphoribosyltransferase (APRTase) is a widely distributed enzyme involved in the salvage of adenine to form an adenine nucleotide. We crystallized and determined the X-ray crystallographic structure of a purine/pyrimidine phosphoribosyltransferase-related protein from the thermophilic bacterium, Thermus thermophilus HB8. The crystal space group was C2 with unit cell dimensions of a = 167.42 A, b = 61.41 A, c = 102.39 A, beta = 94.0 degrees . Initial phases were determined to 2.6 A using the multiple wavelength anomalous dispersion method and selenomethionine substituted protein (Se-MAD), and refined using a 1.9 A "native" data set. The asymmetric unit contains two pairs of identical dimers, each related by noncrystallographic two-fold symmetry. The fifth monomer forms a similar dimer across a crystallographic two-fold axis. These dimers appear to be the biological unit with both monomers contributing to an unusual highly charged arginine-rich bridge region separating the two active sites. Comparison with distantly related APRTases reveal similarities and differences of the active site.

Structure of a closed-form uroporphyrinogen-III C-methyltransferase from Thermus thermophilus.

Uroporphyrinogen-III C-methyltransferase from Thermus thermophilus is a multifunctional protein responsible for two of the eight S-adenosyl-methionine-dependent methylations of the corrin ring during vitamin B(12) synthesis. The structure of this protein has been solved to 2.0 A resolution in both the apo and cofactor-bound form. The monomer consists of two domains, A and B, each consisting of a five-stranded beta-sheet and two or three alpha-helices, with the cofactor bound at the interface. The biological unit is the dimer found in the asymmetric unit. This dimer is related by a non-crystallographic twofold such that two B domains combine to form a long ten-stranded beta-sheet. When compared with solved related structures, this structure shows clear differences in the region involved in cofactor and substrate binding, affirming the role of several previously implicated residues and questioning others. The solved related structures are characterized by an exposed active site. The T. thermophilus structure has this site restricted by the interaction of a flexible loop structure with a highly conserved residue, suggesting a mechanistic role. This structure represents the ;closed' form of the protein.

Structure of the RNA-processing inhibitor RraA from Thermus thermophilis.

The menG gene product, thought to catalyze the final methylation in vitamin K(2) synthesis, has recently been shown to inhibit RNase E in Eschericha coli. The structure of the protein, since renamed RraA, has been solved to 2.3 A using the multiple-wavelength anomalous diffraction method and selenomethionine-substituted protein from Thermus thermophilus. The six molecules in the asymmetric unit are arranged as two similar trimers which have a degree of interaction, suggesting biological significance. The fold does not support the postulated methylation function. Genomic analysis, specifically a lack of an RNase E homologue in cases where homologues to RraA exist, indicates that the function is still obscure.

Crystallographic structure and biochemical analysis of the Thermus thermophilus osmotically inducible protein C.

The X-ray crystallographic structure of osmotically inducible Protein C from the thermophilic bacterium, Thermus thermophilus HB8, was solved to 1.6A using the multiple wavelength anomalous dispersion method and a selenomethionine incorporated protein (Se-MAD). The crystal space group was P1 with cell dimensions of a=37.58 A, b=40.95 A, c=48.14 A, alpha=76.9 degrees, beta=74.0 degrees and gamma=64.1 degrees. The two tightly interacting monomers in the asymmetric unit are related by a non-crystallographic 2-fold. The dimer structure is defined primarily by two very long anti-parallel, over-lapping alpha-helices at the core, with a further six-stranded anti-parallel beta-sheet on the outside of the structure. With respect to the beta-sheets, both A and B monomers contribute three strands each resulting in an intertwining of the structure. The active site consists of two cysteine residues from one monomer and an arginine and glutamic acid from the other. Enzymatic assays have revealed that T.thermophilus OsmC has a hydroperoxide peroxidase activity.

Expression, purification, crystallization and preliminary crystallographic analysis of osmotically inducible protein C.

Selenium-incorporated osmotically inducible protein C from the thermophilic bacterium Thermus thermophilus was overexpressed, purified and crystallized. The crystals belong to space group P1, with unit-cell parameters a = 37.58, b = 40.95, c = 48.14 A, alpha = 76.93, beta = 74.04, gamma = 64.05 degrees. Five data sets were collected from a single crystal to 1.6 A using synchrotron radiation for MAD phasing. Self-rotation functions and the Matthews coefficient are consistent with two molecules in the asymmetric unit.

Identifying androsterone (ADT) as a cognate substrate for human dehydroepiandrosterone sulfotransferase (DHEA-ST) important for steroid homeostasis: structure of the enzyme-ADT complex.

In steroid biosynthesis, human dehydroepiandrosterone sulfotransferase (DHEA-ST) in the adrenals has been reported to catalyze the transfer of the sulfonate group from 3'-phosphoadenosine-5'-phosphosulfate to dehydroepiandrosterone (DHEA). DHEA and its sulfate play roles as steroid precursors; however, the role of the enzyme in the catabolism of androgens is poorly understood. Androsterone sulfate is clinically recognized as one of the major androgen metabolites found in urine. Here it is demonstrated that this enzyme recognizes androsterone (ADT) as a cognate substrate with similar kinetics but a 2-fold specificity and stronger substrate inhibition than DHEA. The structure of human DHEA-ST in complex with ADT has been solved at 2.7 A resolution, confirming ADT recognition. Structural analysis has revealed the binding mode of ADT differs from that of DHEA, despite the similarity of the overall structure between the ADT and the DHEA binary complexes. Our results identify that this human enzyme is an ADT sulfotransferase as well as a DHEA sulfotransferase, implying an important role in steroid homeostasis for the adrenals and liver.

Crystal structure of human dehydroepiandrosterone sulphotransferase in complex with substrate.

Dehydroepiandrosterone sulphotransferase (DHEA-ST) is an enzyme that converts dehydroepiandrosterone (DHEA), and some other steroids, into their sulphonated forms. The enzyme catalyses the sulphonation of DHEA on the 3alpha-oxygen, with 3'-phosphoadenosine-5'-phosphosulphate contributing the sulphate. The structure of human DHEA-ST in complex with its preferred substrate DHEA has been solved here to 1.99 A using molecular replacement with oestradiol sulphotransferase (37% sequence identity) as a model. Two alternative substrate-binding orientations have been identified. The primary, catalytic, orientation has the DHEA 3alpha-oxygen and the highly conserved catalytic histidine in nearly identical positions as are seen for the related oestradiol sulphotransferase. The substrate, however, shows rotations of up to 30 degrees, and there is a corresponding rearrangement of the protein loops contributing to the active site. This may also reflect the low identity between the two enzymes. The second orientation penetrates further into the active site and can form a potential hydrogen bond with the desulphonated cofactor 3',5'-phosphoadenosine (PAP). This second site contains more van der Waal interactions with hydrophobic residues than the catalytic site and may also reflect the substrate-inhibition site. The PAP position was obtained from the previously solved structure of DHEA-ST co-crystallized with PAP. This latter structure, due to the arrangement of loops within the active site and monomer interactions, cannot bind substrate. The results presented here describe details of substrate binding to DHEA-ST and the potential relationship to substrate inhibition.