Validation of a homology model of Mycobacterium tuberculosis DXS: rationalization of observed activities of thiamine derivatives as potent inhibitors of two orthologues of DXS.

The enzyme DXS catalyzes the first, rate-limiting step of the 2-C-methyl-d-erythritol-4-phosphate (MEP, 1) pathway using thiamine diphosphate (ThDP) as cofactor; the DXS-catalyzed reaction constitutes also the first step in vitamin B1 and B6 metabolism in bacteria. DXS is the least studied among the enzymes of this pathway in terms of crystallographic information, with only one complete crystal structure deposited in the Protein Data Bank (Deinococcus radiodurans DXS, PDB: ). We synthesized a series of thiamine and ThDP derivatives and tested them for their biochemical activity against two DXS orthologues, namely D. radiodurans DXS and Mycobacterium tuberculosis DXS. These experimental results, combined with advanced docking studies, led to the development and validation of a homology model of M. tuberculosis DXS, which, in turn, will guide medicinal chemists in rationally designing potential inhibitors for M. tuberculosis DXS.

Structures and reaction mechanisms of riboflavin synthases of eubacterial and archaeal origin.

The biosynthesis of one riboflavin molecule requires one molecule of GTP and two molecules of ribulose 5-phosphate as substrates. GTP is hydrolytically opened, converted into 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione by a sequence of deamination, side chain reduction and dephosphorylation. Condensation with 3,4-dihydroxy-2-butanone 4-phosphate obtained from ribulose 5-phosphate leads to 6,7-dimethyl-8-ribityllumazine. The dismutation of 6,7-dimethyl-8-ribityllumazine catalysed by riboflavin synthase produces riboflavin and 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione. A pentacyclic adduct of two 6,7-dimethyl-8-ribityllumazines has been identified earlier as a catalytically competent reaction intermediate of the Escherichia coli enzyme. Acid quenching of reaction mixtures of riboflavin synthase of Methanococcus jannaschii, devoid of similarity to riboflavin synthases of eubacteria and eukaryotes, afforded a compound whose optical absorption and NMR spectra resemble that of the pentacyclic E. coli riboflavin synthase intermediate, whereas the CD spectra of the two compounds have similar envelopes but opposite signs. Each of the compounds could serve as a catalytically competent intermediate for the enzyme by which it was produced, but not vice versa. All available data indicate that the respective pentacyclic intermediates of the M. jannaschii and E. coli enzymes are diastereomers. Whereas the riboflavin synthase of M. jannaschii is devoid of similarity with those of eubacteria and eukaryotes, it has significant sequence similarity with 6,7-dimethyl-8-ribityllumazine synthases catalysing the penultimate step of riboflavin biosynthesis. 6,7-Dimethyl-8-ribityllumazine synthase and the archaeal riboflavin synthase appear to have diverged early in the evolution of Archaea from a common ancestor.

Role of conservative residue Cys158 in the formation of an active photoprotein complex of obelin.

Using site directed mutagenesis, the conservative residue Cys158 of recombinant apoobelin was substituted for serine (C158S, S-mutant) or alanine (C158A, A-mutant). These point mutations resulted in significant changes in the apoobelin structure accompanied by slowing of photoprotein complex formation, decrease of its stability, and changing of its bioluminescence characteristics. The enzymatic properties of the photoprotein decreased in the series: wild-type protein > S-mutant > A-mutant. This is consistent with rank of nucleophilicity SH > OH > CH(3) of cysteine, serine, and alanine side chain functional groups, respectively. Possible mechanisms of the involvement of the apoobelin Cys158 SH-group in the formation of the enzyme-substrate complex are considered.

A pentacyclic reaction intermediate of riboflavin synthase.

The S41A mutant of riboflavin synthase from Escherichia coli catalyzes the formation of riboflavin from 6,7-dimethyl-8-ribityllumazine at a very low rate. Quenching of presteady-state reaction mixtures with trifluoroacetic acid afforded a compound with an absorption maximum at 412 nm (pH 1.0) that can be converted to a mixture of riboflavin and 6,7-dimethyl-8-ribityllumazine by treatment with wild-type riboflavin synthase. The compound was shown to qualify as a kinetically competent intermediate of the riboflavin synthase-catalyzed reaction. Multinuclear NMR spectroscopy, using various 13C- and 15N-labeled samples, revealed a pentacyclic structure arising by dimerization of 6,7-dimethyl-8-ribityllumazine. Enzyme-catalyzed fragmentation of this compound under formation of riboflavin can occur easily by a sequence of two elimination reactions.

Riboflavin synthase of Escherichia coli. Effect of single amino acid substitutions on reaction rate and ligand binding properties.

Conserved amino acid residues of riboflavin synthase from Escherichia coli were modified by site-directed mutagenesis. Replacement or deletion of phenylalanine 2 afforded catalytically inactive proteins. S41A and H102Q mutants had substantially reduced reaction velocities. Replacements of various other conserved polar residues had little impact on catalytic activity. (19)F NMR protein perturbation experiments using a fluorinated intermediate analog suggest that the N-terminal sequence motif MFTG is part of one of the substrate-binding sites of the protein.

Biosynthesis of riboflavin.

The biosynthesis of one riboflavin molecule requires one molecule of GTP and two molecules of ribulose 5-phosphate. The imidazole ring of GTP is hydrolytically opened, yielding a 4,5-diaminopyrimidine that is converted to 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione by a sequence of deamination, side chain reduction, and dephosphorylation. Condensation of 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione with 3,4-dihydroxy-2-butanone 4-phosphate obtained from ribulose 5-phosphate affords 6,7-dimethyl-8-ribityllumazine. Dismutation of the lumazine derivative yields riboflavin and 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione, which is recycled in the biosynthetic pathway. Two reaction steps in the biosynthetic pathway catalyzed by 3,4-dihydroxy-2-butanone 4-phosphate synthase and riboflavin synthase are mechanistically very complex. The enzymes of the riboflavin pathway are potential targets for antibacterial agents.

Obelin mRNA--a new tool for studies of translation in cell-free systems.

Obelin mRNA obtained in vitro with the aid of SP6 RNA polymerase was translated in a wheat germ cell-free system. Only the polypeptide with a molecular mass of about 20 kDa was synthesized. The activation of apoobelin with a synthetic coelenterazine revealed a luminescence activity initiated by calcium. The specific activity was 3.6 +/- 0.4 x 10(15)photons per mg of the in vitro synthesized obelin (k=6.9s(-1)). The luminescence of the obelin was in a good correlation with the protein concentration calculated by the incorporation of [14C]Leu. The determination of the amount of de novo synthesized obelin based on measurement of its luminescence is one-thousand times more sensitive than the approach based on the incorporation of labeled amino acid. Thus, obelin mRNA has some advantages for evaluating the efficiency of cell-free translation when compared with standard methods.

Sequence of the cDNA encoding the Ca(2+)-activated photoprotein obelin from the hydroid polyp Obelia longissima.

A cDNA clone encoding the Ca(2+)-activated photoprotein, obelin (Obl), from Obelia longissima was sequenced. The nucleotide (nt) sequence contained two long overlapping open reading frames (ORFs), one of which encoded apoobelin (apoObl). The deduced amino acid (aa) sequence of apoObl revealed that this 195-aa protein has three EF-hand structures that are characteristic for Ca(2+)-binding domains. Strong aa homology was shown among apoObl, apoaequorin and apoclytin. The second ORF present in the obl cDNA consists of 139 codons and encodes a very basic protein with a calculated pI of 10.56 and a molecular mass of 16,153 Da.

Mn(2+)-activated luminescence of the photoprotein obelin.

The light emission of obelin may be initiated by Mn2+ under alkaline conditions. The luminescence takes place in a pH range from 7 to 12 with a sharp optimum at 11.75. The first-order rate constant for Mn(2+)-activated luminescence decay is more than 9 s-1, while that for Ca(2+)-activated luminescence decay is only 6.9 s-1. The Mn2+ concentration-effect curve for obelin determined with simple dilutions of manganese salt is a sigmoid curve. The slope of the curve is moderately dependent on the pH and was not more than 1 within the pH range tested. The maximal light emission, which is initiated by 3.6 x 10(-5) M Mn2+ at pH 11.75 was about 10% of the maximal Ca(2+)-activated luminescence. Mg2+ ions inhibit the Mn(2+)-activated luminescence of obelin. The addition of OH. and O2- scavengers did not influence the Mn(2+)-activated luminescence, but when singlet oxygen quenchers were added, the Mn(2+)-dependent light emission was inhibited. This suggests that the 1O2 might be formed and itself be responsible for chromophore oxidation attended with light emission. NEM and Na2S2O4 inhibit the Mn(2+)-initiated light emission of obelin completely, showing that endogenous hydroperoxide and SH-group(s) of the photoprotein are essential for both Ca(2+)-activated and Mn(2+)-activated light emission of obelin.

Expression and properties of the recombinant lumazine (riboflavin) protein from Photobacterium leiognathi.

Photobacterium leiognathi lumazine protein has been expressed in Escherichia coli in high yield, 30 mg/l. The cloned gene was one previously reported by Illarionov (EMBL X56534), that had a similar sequence and was located in the same position as the lumazine protein gene in P. phosphoreum. This gene was placed downstream of the T7 gene 10 promoter of the plasmid pT7-7. When the E. coli are grown at 37 degrees C the protein accumulates in inclusion bodies but solubilization can be achieved in 6 M urea. By a simple procedure of dialysis in the presence of riboflavin and centrifugation, without any chromatography, the recombinant holoprotein is purified to 95% homogeneity. The spectral properties of this recombinant riboflavin protein are the same as those of a fluorescent riboflavin-bound protein produced by many strains of P. leiognathi. The absorption spectrum has the same maxima, 276, 386, 464 nm, the circular dichroism is also the same, and both absorption spectrum and CD are the same as that of apo-lumazine protein having riboflavin bound. The riboflavin on the recombinant can be easily replaced by 6,7-dimethyl-8-ribityllumazine. The absorption and fluorescence spectra, fluorescence yield, and bioluminescence properties of this rebound protein identify it as authentic lumazine protein.

Isolation of bioluminescent functions from Photobacterium leiognathi: analysis of luxA, luxB, luxG and neighboring genes.

Genes encoding luminescence of Photobacterium leiognathi have been cloned in Escherichia coli. The luminescent clones were readily apparent. Among them, a clone containing a recombinant plasmid with a 13.5-kb insertion was identified. This DNA fragment contained all of the luminescence-encoding genes. The luciferase-encoding genes (lux) in this DNA fragment were localized. We have sequenced a part of the cloned lux region and identified the luxA, luxB and luxG genes encoding the alpha and beta subunits of luciferase and a gamma protein with an Mr of 26,180, respectively. The analysis of deduced amino acid sequences and comparison with known luciferase sequences from Vibrio harveyi, indicate the common origin of these proteins.

[Nucleotide sequence of genes for alpha- and beta-subunits of luciferase from Photobacterium leiognathi]. / Nukleotidnaia posledovatel'nost' genov alpha- i beta-sub''edinits liutsiferazy Photobacterium leiognathi.

Nucleotide sequence of the Photobacterium leiognathi DNA containing genes of alpha and beta subunits of luciferase has been determined. We also deduced amino acid sequence and molecular mass of luciferase and localized luciferase genes in the sequenced DNA fragment.

[Cloning and expression of genes of the luminescence system in Photobacterium leiognathi]. / Klonirovanie i ékspressiia genov liuminestsentnoi sistemy Photobacterium leiognathi.

The genes of Photobacterium leiognathi luminescence system were cloned in plasmid pUC18. Escherichia coli cells harboring a recombinant plasmid pPHL1 are luminescent. pPHL1 contains luciferase genes and genes responsible for aldehyde biosynthesis. The luminescence of Escherichia coli is subject to autoinductor regulation similar to the one existing in luminescent bacteria. The 2.7 kb fragment of Photobacterium leiognathi DNA containing the genes for alpha- and beta-luciferase subunits were cloned in pUC19.