Fluorescence in situ hybridization of Microcystis strains producing microcystin using specific mRNA probes

Cyanobacteria are ubiquitous micro-organisms that can produce toxic compounds, the cyanotoxins. The monitoring of such producers in the environment is of prime importance for human health. An attractive technology for such monitoring is fluorescence in situ hybridization (FISH), which allows the detection and enumeration of environmental micro-organisms. We present here the application of tyramide signal amplification fluorescence in situ hybridization (TSA-FISH) to the detection of microcystin-producing Microcystis strains. We used a 16S rRNA-specific probe, MICR3, to specifically label and observe by epifluorescence microscopy Microcystis aeruginosa strains. Using confocal laser scanning microscopy and a specific probe, MCYA, targeting the mcyA mRNA we have labelled M. aeruginosa PCC 7806, which produces microcystins. Microcystis aeruginosa PCC 7005 which does not produce microcystins is not labelled by this probe. Furthermore, we show here that this specific mRNA labelling in M. aeruginosa PCC 7806 is enhanced in cells illuminated for 1 h just after a dark period of cultivation of 24 h, conditions in which the mcyA gene is up regulated. The data presented here might be applicable to the monitoring of toxic Microcystis strains in the environment.

SIGNIFICANCE AND IMPACT OF THE STUDY

Cyanobacteria producing toxic compounds (cyanotoxins) are present in the environment and in water bodies. Their presence poses a threat on human and animal health. It is thus important to detect, identify and enumerate these toxic Cyanobacteria. Using tyramide signal amplification fluorescence in situ hybridization (TSA-FISH) and specific probes, with confocal laser scanning microscopy, we have specifically detected Microcystis strains producing microcystin toxins. The data presented here might be applied to the monitoring of water bodies at early stages and all along the formation of Microcystis blooms.

Inhibition of 7,8-diaminopelargonic acid aminotransferase by amiclenomycin and analogues

Cis and trans stereoisomers of amiclenomycin, a natural L-amino acid antibiotic, have been prepared using unequivocal routes. By using 1H NMR spectroscopy, the configuration of the six-membered ring of natural amiclenomycin was shown to be cis and not trans as originally proposed. Amiclenomycin and some synthetic analogues with the cis configuration irreversibly inactivate DAPA AT (7,8-diaminopelargonic acid aminotransferase), an enzyme involved in biotin biosynthesis, by forming an aromatic PLP (pyridoxal-5'-phosphate)-inhibitor adduct that is tightly bound to the active site. The following kinetic parameters for the inactivation of Escherichia coli DAPA AT by amiclenomycin were derived: K(I)=2 microM and k(inact)=0.4 min(-1). The structure of the aromatic adduct formed upon inactivation was confirmed by UV-visible spectroscopy, X-ray crystal structure determination and MS. Because Mycobacterium tuberculosis DAPA AT is a potential drug target, this enzyme was cloned, overexpressed and purified to homogeneity for biochemical characterization.

Slow-binding and competitive inhibition of 8-amino-7-oxopelargonate synthase, a pyridoxal-5'-phosphate-dependent enzyme involved in biotin biosynthesis, by substrate and intermediate analogs. Kinetic and binding studies

8-Amino-7-oxopelargonate synthase catalyzes the first committed step of biotin biosynthesis in micro-organisms and plants. Because inhibitors of this pathway might lead to antibacterials or herbicides, we have undertaken an inhibition study on 8-amino-7-oxopelargonate synthase using six different compounds. d-Alanine, the enantiomer of the substrate of this pyridoxal-5'-phosphate-dependent enzyme was found to be a competitive inhibitor with respect to l-alanine with a Ki of 0.59 mm. The fact that this inhibition constant was four times lower than the Km for l-alanine was interpreted as the consequence of the inversion-retention stereochemistry of the catalyzed reaction. Schiff base formation between l or d-alanine and pyridoxal-5'-phosphate, in the active site of the enzyme, was studied using ultraviolet/visible spectroscopy. It was found that l and d-alanine form an external aldimine with equilibrium constants K = 4.1 mm and K = 37.8 mm, respectively. However, the equilibrium constant for d-alanine aldimine formation dramatically decreased to 1.3 mm in the presence of saturating concentration of pimeloyl-CoA, the second substrate. This result strongly suggests that the binding of pimeloyl-CoA induces a conformational change in the active site, and we propose that this new topology is complementary to d-alanine and to the putative reaction intermediate since they both have the same configuration. (+/-)-8-Amino-7-oxo-8-phosphonononaoic acid (1), the phosphonate derivative of the intermediate formed during the reaction, was our most potent inhibitor with a Ki of 7 microm. This compound behaved as a reversible slow-binding inhibitor, competitive with respect to l-alanine. Kinetic investigation showed that this slow process was best described by a one-step mechanism (mechanism A) with the following rate constants: k1 = 0.27 x 103 m-1.s-1, k2 = 1.8 s-1 and half-life for dissociation t1/2 = 6.3 min. The binding of compound 1 to the enzyme was also studied using ultraviolet/visible spectroscopy, and the data were consistent with the kinetic data (K = 4.2 microm). Among the other compounds tested, two potential transition state analogs, 4-carboxybutyl(1-amino-1-carboxyethyl)phosphonate (4) and 2-amino-3-hydroxy-2-methylnonadioic acid (5) were found to be competitive inhibitors with respect to l-alanine with Ki of 68 microm and 80 microm, respectively.

Biotin synthase mechanism: on the origin of sulphur

Biotin synthase catalyses the last step of the biosynthesis of biotin in microorganisms and plants. The active protein isolated from Bacillus sphaericus and Escherichia coli contains an iron-sulphur (FeS) cluster. The native enzymes were depleted of their iron and inorganic sulphide and the resulting apoenzymes were chemically reconstituted with FeCl3 and Na2[34S] to give labelled (Fe34S) enzymes. These enzymes were functional and when assayed in vitro produced labelled biotin containing about 65% of 34S. These data strongly support the hypothesis that the sulphur of biotin is derived from the (FeS) centre of the enzyme.

Biosynthesis of 3,6-dideoxyhexoses: in vivo and in vitro evidence for protein-protein interaction between CDP-6-deoxy-L-threo-D-glycero-4-hexulose 3-dehydrase (E1) and its reductase (E3)

CDP-6-deoxy-L-threo-D-glycero-4-hexulose 3-dehydrase (E1), together with its reductase (E3), catalyzes a novel deoxygenation reaction essential for the biosynthesis of 3,6-dideoxyhexoses. In an attempt to gain evidence substantiating the E1.E3 complex formation as a prerequisite for the C-3 deoxygenation activity, we have carried out experiments to study the interaction between these two proteins. The detection of a new species when a mixture of E1 and E3 was analyzed by size-exclusion chromatography was the initial indication supporting the proposed complex formation. Additional evidence for the expected complex formation was provided by the change of the CD spectrum of E1 upon its coupling with E3. The fact that the catalytic efficiency of this system is limited by the quantity of one enzyme, which becomes catalytically competent only after coupling with the second enzyme, further illustrated the importance of such a complex formation to the deoxygenation activity. By using the two-hybrid system which scores for interactions between two proteins coexpressed in yeast, the E1.E3 complex formation in vivo was also firmly established. These results, when considered with the incompatibility of other electron transfer proteins as replacements for E3 in this electron relay, nicely demonstrated the specificity of the E1-E3 recognition. The apparent dissociation constant of the E1.E3 complex formed in rapid equilibrium was estimated to be 288 +/- 22 nM from the correlation between the initial rate of the overall reaction and the concentration of one protein component, and the stoichiometry between E3 and E1 of this complex was deduced as 1.7. Interestingly, while the conformation of the E1.E3 complex was sensitive to the salt concentration in the buffer, the decrease in the catalytic activity at high ionic strength was most likely due to the retardation of the electron transfer mediated by E3. In conjunction with early mechanistic studies, the present data establish the significance of the E1.E3 complex formation for catalysis and, consequently, corroborate the mechanism proposed for the overall deoxygenation process.

Crystallization and preliminary X-ray study of the 8-amino-7-oxopelargonate synthase fromBacillus sphaericus

The 8-amino-7-oxopelargonate synthase (AOPS) cloned from Bacillus sphaericus, overproduced in Escherichia coli, has been crystallized in the pyridoxal 5'-phosphate (PLP)-bound form at pH 7.5, using polyethylene glycol as the precipitant. One crystal form corresponds to a tetragonal space group, with unit-cell dimensions a = b = 66, c = 181 A. These crystals do not diffract beyond 5 A, with conventional X-ray sources and cannot be used in the structure elucidation. A second crystal form is obtained when crystallization conditions are varied slightly by the addition of 0.2 M ammonium sulfate. The space group is P2(1)2(1)2(1), with unit-cell dimensions a = 68.9, b = 85.5, c = 125.9 A, indicating the presence of two molecules in the asymmetric unit (V(m) = 2.26 A(3) Da(-1); 46% water). These crystals diffract X-rays up to 3.2 A using in-house facilities and a preliminary data set has been collected. A second data set using the synchrotron radiation source W32 at LURE (Paris) has shown the crystals to diffract to at least 3 A, resolution, with good statistics. The structure determination of AOPS will provide a structural framework for the other alpha-amino ketone synthases for which no three-dimensional structure is yet available.

Mechanistic studies on the 8-amino-7-oxopelargonate synthase, a pyridoxal-5'-phosphate-dependent enzyme involved in biotin biosynthesis

The reaction mechanism of 8-amino-7-oxopelargonate (8-amino-7-oxononoate) synthase from Bacillus sphaericus, an enzyme dependent on pyridoxal 5'-phosphate (pyridoxal-P), which catalyzes the condensation of L-alanine with pimeloyl-CoA, the second step of biotin biosynthesis, has been studied. To facilitate mechanistic studies, an improved over-expression system in Escherichia coli, and a new continuous spectrophotometric assay for 8-amino-7-oxopelargonate synthase were designed. In order to discriminate between the two plausible basic mechanisms that can be put forth for this enzyme, that is: (a) formation of the pyridoxal-P-stabilized carbanion by abstraction of the C2-H proton of the alanine-pyridoxal-P aldimine, followed by acylation and decarboxylation, and (b) formation of the carbanion by decarboxylation followed by acylation, the fate of the C2-H proton of alanine during the course of the reaction has been examined using 1H NMR. Spectra of the 8-amino-7-oxopelargonate formed using either L-[2-2H]alanine in H2O or L-alanine in D2O, showed that the C2-H proton of alanine is lost during the reaction and that the C8-H proton of 8-amino-7-oxopelargonate is derived from the solvent, a result that is only consistent with mechanism (a). Furthermore 8-amino-7-oxopelargonate synthase catalyzes, in the absence of pimeloyl-CoA, the stereospecific exchange, with retention of configuration, of the C2-H proton of L-alanine with the solvent protons. Similarly, 8-amino-7-oxopelargonate synthase catalyzes the exchange of the C8-H proton of 8-amino-7-oxopelargonate. In addition to these exchange reactions, 8-amino-7-oxopelargonate synthase catalyzes an abortive transamination yielding an inactive pyridoxamine 5'-phosphate (pyridoxamine-P) form of 8-amino-7-oxopelargonate synthase and pyruvate. Kinetic analysis gave a rate constant of kexch. = 1.8 min-1 for the exchange reaction which is 10 times lower than the catalytic constant and a rate constant of ktrans. = 0.11 h-1 for the transamination. Finally deuterium kinetic isotope effects (KIE) were measured at position 2 of L-alanine (DV = 1.3) and in D2O (D2OV = 4.0). The magnitudes of the KIE are consistent with a partially rate-limiting abstraction of the C2-H proton of alanine and a partially rate-limiting reprotonation step. Taken together, all these results show that 8-amino-7-oxopelargonate synthase utilizes mechanism (a). 8-Amino-7-oxopelargonate synthase and 5-aminolevulinate synthase, which has also been shown to use mechanism (a), belong to a class of pyridoxal-P-dependent enzymes that catalyze the formation of alpha-oxoamines. Based on the fact that all these alpha-oxoamine synthases share strong sequence similarities, we postulate that they also share the same reaction mechanism.

Highly purified biotin synthase can transform dethiobiotin into biotin in the absence of any other protein, in the presence of photoreduced deazaflavin

Biotin synthase from Bacillus sphaericus has been purified to homogeneity from a recombinant strain. The UV-visible spectrum of the pure protein reveals the presence of a [2Fe-2S] cluster. The enzyme is active in the conversion of dethiobiotin to biotin in vitro, in the presence of NADPH, AdoMet and additional unidentified components from the crude extract of B. sphaericus wild type. We have also found that photoreduced deazaflavin can substitute for the crude extract and NADPH. In this system, biotin synthase is capable of transforming dethiobiotin into biotin in the absence of any other protein but at a substoichiometric level. When this assay was conducted in the presence of [35S]cysteine, no 35S was incorporated into biotin, contrary to what happens in the presence of the crude extract.

Mechanistic studies on CDP-6-deoxy-L-threo-D-glycero-4-hexulose 3-dehydrase identification of His-220 as the active-site base by chemical modification and site-directed mutagenesis

CDP-6-deoxy-L-threo-D-glycero-4-hexulose 3-dehydrase (E1) purified from Yersinia pseudotuberculosis is a pyridoxamine 5'-phosphate (PMP) dependent iron-sulfur-containing enzyme which catalyzes the C-O bond cleavage at C-3 of its substrate leading to the formation of 3,6-dideoxyhexose. This enzyme is rapidly inactivated by diethyl pyrocarbonate (DEP) at pH 6.0 and 25 degrees C. The inactivation of E1 by DEP, which is reversible upon treatment of hydroxylamine, appears to be attributable solely to the modification of histidine residues. The fact that coincubation of E1 with its substrate gave almost total protection against DEP inactivation and that only one less histidine residue was modified in the presence of substrate strongly suggested that inactivation is due to the modification of only one reactive histidine residue which resides in or near the active site of E1 and is critical for E1's activity. Sequence alignment between the translated ascC (E1) gene and several representative pyridoxal 5'-phosphate (PLP)/PMP dependent enzymes revealed that three of the four invariant residues, glycine, aspartate, and arginine found in all other aminotransferases, are conserved in the E1 sequence (G169, D191, and R403). However, the highly conserved lysine is replaced by a histidine residue (H220) in E1. In order to test whether H220 plays an essential role in E1 catalysis, H220N mutant was constructed and the encoding protein was found to exhibit nearly identical physical characteristics as the wild-type E1. Interestingly, the mutant protein had lost most of its catalytic activity, and one less histidine residue was modified upon treatment of H220N-mutated protein with DEP. Such a single-point mutation also impaired E1's capability of catalyzing the solvent hydrogen exchange at C-4' position of the PMP coenzyme. Our findings strongly suggested that H220 is most likely the active-site base which abstracts the C-4' proton from the PMP-substrate adduct and initiates the catalysis. Furthermore, E1's preservation of other invariant residues found in many PLP/PMP dependent enzymes allowed a speculation of their roles in E1 catalysis.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanistic studies on CDP-6-deoxy-delta 3,4-glucoseen reductase: the role of cysteine residues in catalysis as probed by chemical modification and site-directed mutagenesis

CDP-6-deoxy-delta 3,4-glucoseen reductase (E3), which catalyzes the reduction of the C-3 deoxygenation step during the formation of CDP-ascarylose, a 3,6-dideoxyhexose found in the lipopolysaccharide of Yersinia pseudotuberculosis, has been expressed at high level in Escherichia coli (670 times over the wild-type strain). This flavoenzyme, which also contains one plant ferredoxin type [2Fe-2S] cluster, was inactivated by 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) and N-ethylmaleimide. In both cases the inactivation followed a pseudo first order kinetics. The second order rate constant for the reaction of DTNB with E3 was 0.25 mM-1 min-1 at 20 degrees C, pH 8.0. Detailed characterization of the inactivated enzyme showed that neither the flavin nor the [2Fe-2S] cluster was altered during inactivation. Since this inactivation was reversible by treating the inactivated enzyme with 1 mM D,L-dithiothreitol (DTT), it was concluded that only cysteine residues were modified during inactivation. Analysis of the inactivation using the method developed by Tsou revealed that two cysteines react with DTNB at similar rates and modification of either one is enough to impair E3's activity. Tryptic digestion of E3 labeled with N-ethyl[2,3-14C]maleimide, followed by fractionation of the digest by high performance liquid chromatography, gave two labeled peptides, both of which were separately isolated as a pair of interconvertible diastereoisomers. Sequence analysis of these labeled peptides allowed the identification of Cys-75 and Cys-296 as the reactive cysteine residues. Interestingly, the C75S and C296S mutant proteins exhibit identical physical and comparable catalytic properties as the wild-type enzyme. Since Cys-296 is a conserved residue in the NAD(P) binding domain of enzymes belonging to the same class, this residue may be involved in stabilizing the charge-transfer complex between E3 and NADH, thus facilitating hydride transfer from the nicotinamide nucleotide to flavin. A chemically modified Cys-75 which is immediately adjacent to the [2Fe-2S] center in E3 may prevent the proper juxtaposition of the redox centers and thus impede electron transfer leading to enzyme inactivation. These results may be useful for placing constraints on the peptide folding comprising the active site of E3 for electron transfer between NADH, FAD, and the [2Fe-2S] center.

Studies of the biosynthesis of 3,6-dideoxyhexoses: molecular cloning and characterization of the asc (ascarylose) region from Yersinia pseudotuberculosis serogroup VA

The 3,6-dideoxyhexoses are found in the lipopolysaccharides of gram-negative bacteria, where they have been shown to be the dominant antigenic determinants. Of the five 3,6-dideoxyhexoses known to occur naturally, four have been found in various strains of Salmonella enterica (abequose, tyvelose, paratose, and colitose) and all five, including ascarylose, are present among the serotypes of Yersinia pseudotuberculosis. Although there exists one report of the cloning of the rfb region harboring the abequose biosynthetic genes from Y. pseudotuberculosis serogroup HA, the detailed genetic principles underlying a 3,6-dideoxyhexose polymorphism in Y. pseudotuberculosis have not been addressed. To extend the available information on the genes responsible for 3,6-dideoxyhexose formation in Yersinia spp. and facilitate a comparison with the established rfb (O antigen) cluster of Salmonella spp., we report the production of three overlapping clones containing the entire gene cluster required for CDP-ascarylose biosynthesis. On the basis of a detailed sequence analysis, the implications regarding 3,6-dideoxyhexose polymorphism among Salmonella and Yersinia spp. are discussed. In addition, the functional cloning of this region has allowed the expression of Ep (alpha-D-glucose cytidylyltransferase), Eod (CDP-D-glucose 4,6-dehydratase), E1 (CDP-6-deoxy-L-threo-D-glycero-4- hexulose-3-dehydrase), E3 (CDP-6-deoxy-delta 3,4-glucoseen reductase), Eep (CDP-3,6-dideoxy-D-glycero-D- glycero-4-hexulose-5-epimerase), and Ered (CDP-3,6-dideoxy-L-glycero-D-glycero-4-hexulose-4-reductase), facilitating future mechanistic studies of this intriguing biosynthetic pathway.

Cofactor characterization and mechanistic studies of CDP-6-deoxy-delta 3,4-glucoseen reductase: exploration into a novel enzymatic C-O bond cleavage event

The CDP-6-deoxy-delta 3,4-glucoseen reductase (E3) is a NADH-dependent enzyme which catalyzes the key reduction of the C-3 deoxygenation step during the formation of CDP-ascarylose, a 3,6-dideoxyhexose found in the lipopolysaccharide of Yersinia pseudotuberculosis. This highly purified enzyme is also a NADH oxidase capable of mediating the direct electron transfer from NADH to O2, forming H2O2. While previous work showed that E3 contains no common cofactor, one FAD and one plant ferredoxin type [2Fe-2S] center were found in this study to be associated with each molecule of E3. The iron-sulfur center is essential for E3 activity since bleaching of the [2Fe-2S] center leads to inactive enzyme. These results suggest that E3 employs a short electron-transport chain composed of both FAD and the iron-sulfur center to shuttle electrons from NADH to its acceptor. The order of electron flow, as indicated by EPR measurement with partially reduced E3, starts with hydride reduction of FAD by NADH. The iron-sulfur cluster, receiving electrons one at a time from the reduced flavin, relays the reducing equivalents via another iron-sulfur center in the active site of E1 to its final acceptor, the E1-bound PMP-glucoseen adduct. The participation of a one-electron-carrying iron-sulfur center in this reduction is advantageous since both electrons are dispatched from the same redox state of the prosthetic group, allowing electrons of equal energy to be delivered to the final acceptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Investigation of the first step of biotin biosynthesis in Bacillus sphaericus. Purification and characterization of the pimeloyl-CoA synthase, and uptake of pimelate

The pimeloyl-CoA synthase from Bacillus sphaericus has been purified to homogeneity from an overproducing strain of Escherichia coli. The purification yielded milligram quantities of the synthase with a specific activity of 1 unit/mg of protein. Analysis of the products showed that this enzyme catalysed the transformation of pimelate into pimeloyl-CoA with concomitant hydrolysis of ATP to AMP. Using a continuous spectrophotometric assay, we have examined the catalytic properties of the pure enzyme. The pH profile under Vmax. conditions showed a maximum around 8.5. Apparent Km values for pimelate, CoASH, ATP.Mg2- and Mg2+ were respectively 145 microM, 33 microM, 170 microM and 2.3 mM. The enzyme was inhibited by Mg2+ above 10 mM. This acid-CoA ligase exhibited a very sharp substrate specificity, e.g. neither GTP nor pimelate analogues (di- or mono-carboxylic acids) were processed. The bivalent metal ion requirement was also investigated: Mn2+ (73%) and Co2+ (32%) but not Ca2+ could replace Mg2+. The enzyme was inhibited by metal chelators such as 1,10-phenanthroline and EDTA. The synthase was a homodimer with a 28,000-M(r) subunit. N-Terminal sequencing definitely proved that this enzyme was encoded by the bioW gene. A careful study of pimelate uptake by B. sphaericus, E. coli and Pseudomonas dentrificans showed that this metabolite crossed the membrane of these microorganisms by passive diffusion, ruling out the involvement of the bioX gene product as pimelate carrier.

The 8-amino-7-oxopelargonate synthase from Bacillus sphaericus. Purification and preliminary characterization of the cloned enzyme overproduced in Escherichia coli

The 8-amino-7-oxopelargonate synthase [6-carboxyhexanoyl-CoA:L-alanine carboxyhexanoyltransferase (decarboxylating); EC 2.3.1.47] from Bacillus sphaericus involved in biotin biosynthesis was purified from an Escherichia coli overproducing strain. The purification afforded an electrophoretically homogeneous enzyme with a specific activity of 0.67 unit/mg. The purified enzyme is a monomer of 41 kDa. N-Terminal sequencing of the first 14 amino acid residues showed complete agreement with the predicted sequence from the bioF gene. The pure enzyme showed the characteristic absorption band (425 nm) of pyridoxal 5'-phosphate-dependent enzymes. Furthermore, the holoenzyme was resolved during an affinity step yielding the inactive apoenzyme, which recovered activity and the 425 nm-absorption band on dialysis against pyridoxal 5'-phosphate. Km values for L-alanine and pimeloyl-CoA were respectively 3 mM and 1 microM.

Analysis of tachykinin-binding site interactions using NMR and energy calculation data of potent cyclic analogues of substance P

The three-dimensional structures of [Cys3,6,Tyr8]-, [Gly2,Cys3,6,Tyr8]- and [DCys3,Cys6]substance P, designed as conformational analogues of substance P, have been studied by 1H-NMR (500 MHz) in different solvents and by energy calculations. As previously observed for substance P and physalaemin, two tachykinins acting via the NK-1 receptor, [Cys3,6,Tyr8]substance P presents an alpha-helical structure of the 4----8 sequence in methanol. This structure is stabilized by a beta-turn III via the formation of three hydrogen bonds involving the Cys-6, Phe-7 and Tyr-8 NH groups. In contrast to substance P, two of these hydrogen bonds are still present in dimethyl sulfoxide and in water the Cys-6 NH hydrogen bond is the only one remaining, such that a beta-turn structure inside the ring can be envisaged. In close agreement with the NMR data, the energy calculations lead to three types of folding for the core of [Cys3,6,Tyr8]substance P: a beta-turn III, a less stable beta-turn I (delta E = 3 kcal), and a beta-turn II (delta E = 4.6 kcal). The structure of Gly-Leu-Met-NH2 is strongly affected by changing the hydrophobicity of the medium. The most stable calculated conformation is the helix; however, numerous unrelated structures are destabilized by about 2-3 kcal/mol. These data are analyzed and discussed in connection with the high potency of [Cys3,6,Tyr8]substance P for both the NK-1 and NK-3 binding sites; that is the internal region of tachykinins (non-homologous amino acids) might present a similar three-dimensional structure when bound to the receptors (which may be at the origin of some lack of selectivity), whereas paradoxically the selectivity may be due to the common C-terminal sequence.

The NADPH-linked acetoacetyl-CoA reductase from Zoogloea ramigera. Characterization and mechanistic studies of the cloned enzyme over-produced in Escherichia coli

The NADPH-linked acetoacetyl-CoA reductase, (R)-3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.36), from the bacterium Zoogloea ramigera, involved in the formation of D-3-hydroxybutyryl-CoA for poly(D-3-hydroxybutyrate) biosynthesis, has been purified from an over-producing Escherichia coli strain. The purification was achieved in two steps, yielding an electrophoretically homogeneous enzyme of high specific activity (608 U/mg). The enzyme is an alpha 4 homotetramer of four 25-kDa subunits. It has a Km of 2 microM and a kcat/Km of 1.8 X 10(8) M-1 s-1 for acetoacetyl-CoA; it is inhibited by acetoacetyl-CoA above 10 microM. K is 10(-10) M for the dehydrogenation. Kinetic studies of the back reaction revealed a sequential mechanism involving a ternary complex. The stereospecificity of the hydride-equivalent transfer was demonstrated using NMR techniques to be 4S (B side). Using the fingerprint method proposed by Wierenga et al. [(1986) J. Mol. Biol. 187, 101-107], we identified a 28-residue stretch (residues 3-31) as a possible NADPH fold. Finally the specificity of the reductase was examined using 3-oxo-acyl-CoA analogs and analogs lacking the adenosine 3',5'-bisphosphate moiety of CoA. Only the straight-chain C5 analog (3-oxo-propionyl-CoA) was found to be an alternative substrate (40%) for the reductase.

Interaction of tachykinins with their receptors studied with cyclic analogues of substance P and neurokinin B

The activities of two groups of cyclic agonists of substance P (SP) have been studied. The disulfide bridge constraints have been designed on the basis of conformational studies on SP and physalaemin indicating an alpha-helical structure for the core of these two tachykinins (group I) and a folding of the C-terminal carboxamide towards the side chains of the glutamines 5 and 6 (group II). Only peptides simulating the alpha-helix present substantial potencies. [Cys3,6]SP is as active as SP in inhibiting 125I-labeled Bolton and Hunter SP-specific binding on rat brain synaptosomes and on dog carotid bioassay, two assays specific for the neurokinin 1 receptor. Moreover, [Cys3,6]SP is as potent as neurokinin B in inhibiting 125I-labeled Bolton and Hunter eledoisin-specific binding on rat cortical synaptosomes as well as in stimulating rat portal vein, two tests specific for the neurokinin 3 receptor. Interestingly, in contrast to neurokinin B, [Cys3,6]SP is a weak agonist of the neurokinin 2 receptor subtype, as evidenced by its binding potency in inhibiting 3H-labeled neurokinin A-specific binding on rat duodenum and in inducing the contractions of the rabbit pulmonary artery, a neurokinin 2-type bioassay. To increase the specificity of the cyclic analogue [Cys3,6]SP positions 8 and 9 were modified. [Cys3,6, Tyr8, Ala9]SP is slightly less selective than SP for the neurokinin 1 receptor subtype. [Cys2,5]neurokinin B constitutes a selective cyclic agonist for the neurokinin 3 receptor. The very weak potencies of the peptides from group II indicate that a certain degree of flexibility in the C-terminal moiety is required. Collectively, these results suggest that the neurokinin 1 and neurokinin 3 tachykinin receptors may recognize a similar three-dimensional structure of the core of the tachykinins. Different orientations of the common C-terminal tripeptide may be related to the selectivity for the different receptor subtypes.

Cyclization of peptides on a solid support. Application to cyclic analogs of substance P

The general conditions for cyclization of peptides on polymer matrix by disulfide bridge formation are reported. This procedure is based on attack of 3-nitro-2-pyridinesulfenyl group (Npys) by a thiol function. It has been used for synthesis of five cyclic analogs of Substance P.