Thermodynamic studies on anion binding to apotransferrin and to recombinant transferrin N-lobe half molecules.
Equilibrium constants for the binding of anions to apotransferrin, to the recombinant N-lobe half transferrin molecule (Tf/2N), and to a series of mutants of Tf/2N have been determined by difference UV titrations of samples in 0.1 M Hepes buffer at pH 7.4 and 25 degrees C. The anions included in this study are phosphate, sulfate, bicarbonate, pyrophosphate, methylenediphosphonic acid, and ethylenediphosphonic acid. There are no significant differences between anion binding to Tf/2N and anion binding to the N-lobe of apotransferrin. The binding of simple anions like phosphate appears to be essentially equivalent for the two apotransferrin binding sites. The binding of pyrophosphate and the diphosphonates is inequivalent, and the studies on the recombinant Tf/2N show that the stronger binding is associated with the N-terminal site. Anion binding constants for phosphate, pyrophosphate, and the diphosphonates with the N-lobe mutants K206A, K296A, and R124A have been determined. Anion binding tends to be weakest for the K296A mutant, but the variation in log K values among the three mutants is surprisingly small. It appears that the side chains of K206, K296, and R124 all make comparable contributions to anion binding. There are significant variations in the intensities of the peaks in the difference UV spectra that are generated by the titrations of the mutant apoproteins with these anions. These differences appear to be related more to variations in the molar extinction coefficients of the anion-protein complexes rather than to differences in binding constants. (+info)
Point mutations in the guanine phosphoribosyltransferase from Giardia lamblia modulate pyrophosphate binding and enzyme catalysis.
Guanine phosphoribosyltransferase (GPRTase) from Giardia lamblia, an enzyme required for guanine salvage and necessary for the survival of this parasitic protozoan, has been kinetically characterized. Phosphoribosyltransfer proceeds through an ordered sequential mechanism common to many related purine phosphoribosyltransferases (PRTases) with alpha-D-5-phosphoribosyl-1-pyrophosphate (PRPP) binding to the enzyme first and guanosine monophosphate (GMP) dissociating last. The enzyme is a highly unique purine PRTase, recognizing only guanine as its purine substrate (K(m) = 16.4 microM) but not hypoxanthine (K(m) > 200 microM) nor xanthine (no reaction). It also catalyzes both the forward (kcat = 76.7 s-1) and reverse (kcat = 5.8.s-1) reactions at significantly higher rates than all the other purine PRTases described to date. However, the relative catalytic efficiencies favor the forward reaction, which can be attributed to an unusually high K(m) for pyrophosphate (PPi) (323.9 microM) in the reverse reaction, comparable only with the high K(m) for PPi (165.5 microM) in Tritrichomonas foetus HGXPRTase-catalyzed reverse reaction. As the latter case was due to the substitution of threonine for a highly conserved lysine residue in the PPi-binding loop [Munagala et al. (1998) Biochemistry 37, 4045-4051], we identified a corresponding threonine residue in G. lamblia GPRTase at position 70 by sequence alignment, and then generated a T70K mutant of the enzyme. The mutant displays a 6.7-fold lower K(m) for PPi with a twofold increase in the K(m) for PRPP. Further attempts to improve PPi binding led to the construction of a T70K/A72G double mutant, which displays an even lower K(m) of 7.9 microM for PPi. However, mutations of the nearby Gly71 to Glu, Arg, or Ala completely inactivate the GPRTase, suggesting the requirement of flexibility in the putative PPi-binding loop for enzyme catalysis, which is apparently maintained by the glycine residue. We have thus tentatively identified the PPi-binding loop in G. lamblia GPRTase, and attributed the relatively higher catalytic efficiency in the forward reaction to the unusual loop structure for poor PPi binding in the reverse reaction. (+info)
Dialysate iron therapy: infusion of soluble ferric pyrophosphate via the dialysate during hemodialysis.
BACKGROUND: Soluble iron salts are toxic for parenteral administration because free iron catalyzes free radical generation. Pyrophosphate strongly complexes iron and enhances iron transport between transferrin, ferritin, and tissues. Hemodialysis patients need iron to replenish ongoing losses. We evaluated the short-term safety and efficacy of infusing soluble ferric pyrophosphate by dialysate. METHODS: Maintenance hemodialysis patients receiving erythropoietin were stabilized on regular doses of intravenous (i.v.) iron dextran after oral iron supplements were discontinued. During the treatment phase, 10 patients received ferric pyrophosphate via hemodialysis as monthly dialysate iron concentrations were progressively increased from 2, 4, 8, to 12 micrograms/dl and were then sustained for two additional months at 12 micrograms/dl (dialysate iron group); 11 control patients were continued on i.v. iron dextran (i.v. iron group). RESULTS: Hemoglobin, serum iron parameters, and the erythropoietin dose did not change significantly from month 0 to month 6, both within and between the two groups. The weekly dose of i.v. iron (mean +/- SD) needed to maintain iron balance during month 6 was 56 +/- 37 mg in the i.v. iron group compared with 10 +/- 23 mg in the dialysate iron group (P = 0.001). Intravenous iron was required by all 11 patients in the i.v. iron group compared with only 2 of the 10 patients receiving 12 micrograms/dl dialysate iron. The incidence of adverse effects was similar in both groups. CONCLUSIONS: Slow infusion of soluble iron pyrophosphate by hemodialysis may be a safe and effective alternative to the i.v. administration of colloidal iron dextran in maintenance hemodialysis patients. (+info)
Inhibition of an ecto-ATP-diphosphohydrolase by azide.
Cell surface ATPases (ecto-ATPases or E-ATPases) hydrolyze extracellular ATP and other nucleotides. Regulation of extracellular nucleotide concentration is one of their major proposed functions. Based on enzymatic characterization, the E-ATPases have been divided into two subfamilies, ecto-ATPases and ecto-ATP-diphosphohydrolases (ecto-ATPDases). In the presence of either Mg2+ or Ca2+, ecto-ATPDases, including proteins closely related to CD39, hydrolyze nucleoside diphosphates in addition to nucleoside triphosphates and are inhibited by millimolar concentrations of azide, whereas ecto-ATPases appear to lack these two properties. This report presents the first systematic kinetic study of a purified ecto-ATPDase, the chicken oviduct ecto-ATPDase (Strobel, R.S., Nagy, A.K., Knowles, A.F., Buegel, J. & Rosenberg, M.O. (1996) J. Biol. Chem. 271, 16323-16331), with respect to ATP and ADP, and azide inhibition. Km values for ATP obtained at pH 6.4 and 7.4 are 10-30 times lower than for ADP and the catalytic efficiency is greater with ATP as the substrate. The enzyme also exhibits complicated behavior toward azide. Variable inhibition by azide is observed depending on nucleotide substrate, divalent ion, and pH. Nearly complete inhibition by 5 mm azide is obtained when MgADP is the substrate and when assays are conducted at pH 6-6.4. Azide inhibition diminishes when ATP is the substrate, Ca2+ as the activating ion, and at higher pH. The greater efficacy of azide in inhibiting ADP hydrolysis compared to ATP hydrolysis may be related to the different modes of inhibition with the two nucleotide substrates. While azide decreases both Vmax and Km for ADP, it does not alter the Km for ATP. These results suggest that the apparent affinity of azide for the E.ADP complex is significantly greater than that for the free enzyme or E.ATP. The response of the enzyme to three other inhibitors, fluoride, vanadate, and pyrophosphate, is also dependent on substrate and pH. Taken together, these results are indicative of a discrimination between ADP and ATP by the enzyme. A mechanism of azide inhibition is proposed. (+info)
Presence of a vacuolar H+-pyrophosphatase in promastigotes of Leishmania donovani and its localization to a different compartment from the vacuolar H+-ATPase.
Inorganic pyrophosphate promoted the acidification of an intracellular compartment in permeabilized promastigotes of Leishmania donovani, as measured by Acridine Orange uptake. The proton gradient generated by pyrophosphate was collapsed by addition of nigericin or NH4Cl. Pyrophosphate-driven proton translocation was stimulated by potassium ions, and inhibited by NaF, the pyrophosphate analogues imidodiphosphate and aminomethylenediphosphonate (AMDP), dicyclohexylcarbodiimide, and the thiol reagents p-hydroxymercuribenzoate and N-ethylmaleimide, all at concentrations similar to those that inhibit the plant vacuolar proton-pumping pyrophosphatase (H+-PPase). The proton translocation activity had a pH optimum in the range 7.0-7.5, and was unaffected by bafilomycin A1 (40 nM), concanamycin A (5 nM), sodium o-vanadate (500 microM) and KNO3 (200 mM). AMDP-sensitive pyrophosphate hydrolysis was also detected in promastigotes, and potassium ions also stimulated this activity. Sodium ions disrupted pH gradients established in the presence of ATP but not in the presence of pyrophosphate, and sequential addition of ATP and pyrophosphate resulted in partially additive Acridine Orange accumulation, suggesting that the vacuolar H+-PPase is in a different intracellular compartment from the vacuolar H+-ATPase and Na+/H+ exchanger of L. donovani promastigotes. Separation of promastigote extracts on Percoll gradients yielded a dense fraction that contained H+-PPase activity but lacked ATPase activity and markers for mitochondria, glycosomes and lysosomes. The organelles in this fraction appeared by electron microscopy to consist of electron-dense vacuoles. In summary, these results indicate that, in contrast to plant vacuoles, vacuolar H+-PPase and vacuolar ATPase activities are present in different compartments in L. donovani promastigotes. (+info)
Novel enzymatic oxidation of Mn2+ to Mn3+ catalyzed by a fungal laccase.
Fungal laccases are extracellular multinuclear copper-containing oxidases that have been proposed to be involved in ligninolysis and degradation of xenobiotics. Here, we show that an electrophoretically homogenous laccase preparation from the white rot fungus Trametes versicolor oxidized Mn2+ to Mn3+ in the presence of Na-pyrophosphate, with a Km value of 186 microM and a Vmax value of 0.11 micromol/min/mg protein at the optimal pH (5.0) and a Na-pyrophosphate concentration of 100 mM. The oxidation of Mn2+ involved concomitant reduction of the laccase type 1 copper site as usual for laccase reactions, thus providing the first evidence that laccase may directly utilize Mn2+ as a substrate. (+info)
The specificity-conferring code of adenylation domains in nonribosomal peptide synthetases.
BACKGROUND: Many pharmacologically important peptides are synthesized nonribosomally by multimodular peptide synthetases (NRPSs). These enzyme templates consist of iterated modules that, in their number and organization, determine the primary structure of the corresponding peptide products. At the core of each module is an adenylation domain that recognizes the cognate substrate and activates it as its aminoacyl adenylate. Recently, the crystal structure of the phenylalanine-activating adenylation domain PheA was solved with phenylalanine and AMP, illustrating the structural basis for substrate recognition. RESULTS: By comparing the residues that line the phenylalanine-binding pocket in PheA with the corresponding moieties in other adenylation domains, general rules for deducing substrate specificity were developed. We tested these in silico 'rules' by mutating specificity-conferring residues within PheA. The substrate specificity of most mutants was altered or relaxed. Generalization of the selectivity determinants also allowed the targeted specificity switch of an aspartate-activating adenylation domain, the crystal structure of which has not yet been solved, by introducing a single mutation. CONCLUSIONS: In silico studies and structure-function mutagenesis have defined general rules for the structural basis of substrate recognition in adenylation domains of NRPSs. These rules can be used to rationally alter the specificity of adenylation domains and to predict from the primary sequence the specificity of biochemically uncharacterized adenylation domains. Such efforts could enhance the structural diversity of peptide antibiotics such as penicillins, cyclosporins and vancomycins by allowing synthesis of 'unnatural' natural products. (+info)
In vivo gammadelta T cell priming to mycobacterial antigens by primary Mycobacterium tuberculosis infection and exposure to nonpeptidic ligands.
BACKGROUND: The recognition of phosphorylated nonpeptidic microbial metabolites by Vgamma9Vdelta2 T cells does not appear to require the presence of MHC molecules or antigen processing, permitting rapid responses against microbial pathogens. These may constitute an important area of natural anti-infectious immunity. To provide evidence of their involvement in immune reactivities against mycobacteria, we measured the responsiveness of peripheral blood Vgamma9Vdelta2 T cells in children with primary Mycobacterium tuberculosis (MTB) infections. MATERIALS AND METHODS: Peripheral blood mononuclear cells from 22 children with MTB infections and 16 positivity of tuberculin (PPD)-negative healthy children were exposed to nonpeptidic antigens in vitro and the reactivity of the Vgamma9Vdelta2 T cell subset with these antigens was determined using proliferation and cytokine assays. Also, responses of gammadelta T cells from rhesus monkeys stimulated with phosphoantigens in vivo were measured. RESULTS: The Vgamma9Vdelta2 T cell responses were highly increased in infected children in comparison with age-matched controls. This augmented Vgamma9Vdelta2 T cell reactivity subsided after successful antibiotic chemotherapy, suggesting that persistent exposure to mycobacterial antigens is required for the maintenance of gammadelta T cell activation in vivo. The in vivo reactivity of Vgamma9Vdelta2 T cells to phosphoantigens was also analyzed in a rhesus monkey model system. Intravenous injections of phosphoantigens induced an activated state of simian Vgamma9Vdelta2 T cells which decreased after 2 months, i.e., with a time course similar to that seen in MTB-infected children. CONCLUSIONS: The increased reactivity of Vgamma9Vdelta2 T cells to phosphoantigens appears to be dependent on constant antigenic exposure. Consequently, the assessment of Vgamma9Vdelta2 responses may be useful for monitoring the efficacy of antimycobacterial therapies. (+info)