A simple two step procedure for purification of the catalytic domain of chicken tryptophan hydroxylase 1 in a form suitable for crystallization

Tryptophan hydroxylase (TPH) [EC 1.14.16.4] catalyzes the conversion of tryptophan to 5-hydroxytryptophan, which is the first and rate-determining step in the biosynthesis of the neurotransmitter serotonin. We have expressed the catalytic domain of chicken (Gallus gallus) TPH isoform 1 in Escherichia coli in high yield. The enzyme was highly purified using only one anion exchange and one gel filtration, with a yield of 11 mg/L culture and a specific activity of 0.60 micromol/min/mg. The K(m) values were determined to K(m, tryptophan)=7.7+/-0.7 microM, K(m, BH4)=324+/-10 microM and K(m, O2)=39+/-2 microM. Substrate inhibition by tryptophan was observed at concentrations above 15 microM. Furthermore, the purified enzyme has been crystallized without 7,8-dihydro-L-biopterin and a data set to 3A resolution has been collected.     

Identification of the catalytic subunit of acetohydroxyacid synthase in Haemophilus influenzae and its potent inhibitors

Acetohydroxyacid synthase (AHAS; EC 2.2.1.6) is a thiamin diphosphate- (ThDP)- and FAD-dependent enzyme that catalyzes the first common step in the biosynthetic pathway of the branched-amino acids (BCAAs) leucine, isoleucine, and valine. The gene from Haemophilus influenzae that encodes the AHAS catalytic subunit was cloned, overexpressed in Escherichia coli BL21(DE3), and purified to homogeneity. The purified H. influenzae AHAS catalytic subunit (Hin-AHAS) appeared as a single band on SDS-PAGE gel, with a molecular mass of approximately 63 kDa. The enzyme catalyzes the condensation of two molecules of pyruvate to form acetolactate, with a K(m) of 9.2mM and the specific activity of 1.5 micromol/min/mg. The cofactor activation constant (K(c)=13.5 microM) and the dissociation constant (K(d)=3.3 microM) of ThDP were also determined by enzymatic assay and tryptophan fluorescence quenching studies, respectively. We screened a chemical library to discover new inhibitors of the Hin AHAS catalytic subunit. Through which, AVS-2087 (IC(50)=0.53 microM), KSW30191 (IC(50)=1.42 microM), and KHG20612 (IC(50)=4.91 microM) displayed potent inhibition as compare to sulfometuron methyl (IC(50)=276.31 microM).     

Anthracycline binding to synthetic and natural membranes. A study using resonance energy transfer

The binding of adriamycin and its two analogues 4'-epidoxorubicin and 4'-deoxydoxorubicin to synthetic and mitochondrial membranes was investigated by using resonance energy transfer between these drugs and two fluorescent probes, diphenylhexatriene (DPH) and tryptophan. The fluorescence of the lipid probe DPH in both types of membranes and tryptophan in mitochondria was quenched by the anthracyclines in a dose-dependent manner. In sonicated, fluid-phase dimyristoyl-L-alpha-phosphatidylcholine (DMPC) vesicles, the half-quenching concentration (K50) of adriamycin was 17 +/- 1 microM, whereas in bilayers containing a 1:1 molar ratio of DMPC to cardiolipin (CL), the value was 8 +/- 1 microM. In liver and heart mitochondria, the K50 values were 8 +/- 2 and 11 +/- 3 microM, respectively. Similar results were obtained for the other two drugs. Replacing a nonionic with an ionic medium or decreasing the pH from pH 7.7 to pH 6.9 increased the K50 value of adriamycin for DPH in DMPC/CL (1:1 molar) liposomes and in mitochondria. Higher concentrations of anthracycline were needed to quench the fluorescence of tryptophan. The results suggest that these drugs interact with both phospholipids and proteins and that the cardiotoxicity of adriamycin is unlikely to be related to the amount of drug bound to heart mitochondria.     

Evidence for tryptophan residues in the cation transport path of the Na(+),K(+)-ATPase

A family of aryl isothiouronium derivatives was designed as probes for cation binding sites of Na(+),K(+)-ATPase. Previous work showed that 1-bromo-2,4,6-tris(methylisothiouronium)benzene (Br-TITU) acts as a competitive blocker of Na(+) or K(+) occlusion. In addition to a high-affinity cytoplasmic site (K(D) < 1 microM), a low-affinity site (K(D) approximately 10 microM) was detected, presumably extracellular. Here we describe properties of Br-TITU as a blocker at the extracellular surface. In human red blood cells Br-TITU inhibits ouabain-sensitive Na(+) transport (K(D) approximately 30 microM) in a manner antagonistic with respect to extracellular Na(+). In addition, Br-TITU impairs K(+)-stimulated dephosphorylation and Rb(+) occlusion from phosphorylated enzyme of renal Na(+),K(+)-ATPase, consistent with binding to an extracellular site. Incubation of renal Na(+),K(+)-ATPase with Br-TITU at pH 9 irreversibly inactivates Na(+),K(+)-ATPase activity and Rb(+) occlusion. Rb(+) or Na(+) ions protect. Preincubation of Br-TITU with red cells in a K(+)-free medium at pH 9 irreversibly inactivates ouabain-sensitive (22)Na(+) efflux, showing that inactivation occurs at an extracellular site. K(+), Cs(+), and Li(+) ions protect against this effect, but the apparent affinity for K(+), Cs(+), or Li(+) is similar (K(D) approximately 5 mM) despite their different affinities for external activation of the Na(+) pump. Br-TITU quenches tryptophan fluorescence of renal Na(+),K(+)-ATPase or of digested "19 kDa membranes". After incubation at pH 9 irreversible loss of tryptophan fluorescence is observed and Rb(+) or Na(+) ions protect. The Br-TITU appears to interact strongly with tryptophan residue(s) within the lipid or at the extracellular membrane-water interface and interfere with cation occlusion and Na(+),K(+)-ATPase activity.     

Purification and characterization of a Ca(2+)-dependent protein kinase from sandalwood (Santalum album L.): evidence for Ca(2+)-induced conformational changes

An early development-specific soluble 55 kDa Ca(2+)-dependent protein kinase has been purified to homogeneity from sandalwood somatic embryos and biochemically characterized. The purified enzyme, swCDPK, resolved into a single band on 10% polyacrylamide gels, both under denaturing and non-denaturing conditions. swCDPK activity was strictly dependent on Ca(2+), K(0.5) (apparent binding constant) for Ca(2+)-activation of substrate phosphorylation activity being 0.7 microM and for autophosphorylation activity approximately 50 nM. Ca(2+)-dependence for activation, CaM-independence, inhibition by CaM-antagonist (IC(50) for W7=6 microM, for W5=46 microM) and cross-reaction with polyclonal antibodies directed against the CaM-like domain of soybean CDPK, confirmed the presence of an endogenous CaM-like domain in the purified enzyme. Kinetic studies revealed a K(m) value of 1.3 mg/ml for histone III-S and a V(max) value of 0.1 nmol min(-1) mg(-1). The enzyme exhibited high specificity for ATP with a K(m) value of 10 nM. Titration with calcium resulted in the enhancement of intrinsic emission fluorescence of swCDPK and a shift in the lambda(max) emission from tryptophan residues. A reduction in the efficiency of non-radiative energy transfer from tyrosine to tryptophan residues was also observed. These are taken as evidence for the occurrence of Ca(2+)-induced conformational change in swCDPK. The emission spectral properties of swCDPK in conjunction with Ca(2+) levels required for autophosphorylation and substrate phosphorylation help understand mode of Ca(2+) activation of this enzyme.     

Alkaline phosphatase from the hyperthermophilic bacterium T. maritima requires cobalt for activity

The hyperthermophilic bacterium Thermotoga maritima encodes a gene sharing sequence similarities with several known genes for alkaline phosphatase (AP). The putative gene was isolated and the corresponding protein expressed in Escherichia coli, with and without a predicted signal sequence. The recombinant protein showed phosphatase activity toward the substrate p-nitrophenyl-phosphate with a k(cat) of 16 s(-1) and a K(m) of 175 microM at a pH optimum of 8.0 when assayed at 25 degrees C. T. maritima phosphatase activity increased at high temperatures, reaching a maximum k(cat) of 100 s(-1), with a K(m) of 93 microM at 65 degrees C. Activity was stable at 65 degrees C for >24 h and at 90 degrees C for 5 h. Phosphatase activity was dependent on divalent metal ions, specifically Co(II) and Mg(II). Circular dichroism spectra showed that the enzyme gains secondary structure on addition of these metals. Zinc, the most common divalent metal ion required for activity in known APs, was shown to inhibit the T. maritima phosphatase enzyme at concentrations above 0.3 moles Zn: 1 mole monomer. All activity was abolished in the presence of 0.1 mM EDTA. The T. maritima AP primary sequence is 28% identical when compared with E. coli AP. Based on a structural model, the active sites are superimposable except for two residues near the E. coli AP Mg binding site, D153 and K328 (E. coli numbering) corresponding to histidine and tryptophan in T. maritima AP, respectively. Sucrose-density gradient sedimentation experiments showed that the protein exists in several quaternary forms predominated by an octamer.     

WrbA from Escherichia coli and Archaeoglobus fulgidus is an NAD(P)H:quinone oxidoreductase

WrbA (tryptophan [W] repressor-binding protein) was discovered in Escherichia coli, where it was proposed to play a role in regulation of the tryptophan operon; however, this has been put in question, leaving the function unknown. Here we report a phylogenetic analysis of 30 sequences which indicated that WrbA is the prototype of a distinct family of flavoproteins which exists in a diversity of cell types across all three domains of life and includes documented NAD(P)H:quinone oxidoreductases (NQOs) from the Fungi and Viridiplantae kingdoms. Biochemical characterization of the prototypic WrbA protein from E. coli and WrbA from Archaeoglobus fulgidus, a hyperthermophilic species from the Archaea domain, shows that these enzymes have NQO activity, suggesting that this activity is a defining characteristic of the WrbA family that we designate a new type of NQO (type IV). For E. coli WrbA, the K(m)(NADH) was 14 +/- 0.43 microM and the K(m)(benzoquinone) was 5.8 +/- 0.12 microM. For A. fulgidus WrbA, the K(m)(NADH) was 19 +/- 1.7 microM and the K(m)(benzoquinone) was 37 +/- 3.6 microM. Both enzymes were found to be homodimeric by gel filtration chromatography and homotetrameric by dynamic light scattering and to contain one flavin mononucleotide molecule per monomer. The NQO activity of each enzyme is retained over a broad pH range, and apparent initial velocities indicate that maximal activities are comparable to the optimum growth temperature for the respective organisms. The results are discussed and implicate WrbA in the two-electron reduction of quinones, protecting against oxidative stress.     

Astaxanthin binding and structural stability of the apple snail carotenoprotein ovorubin

Ovorubin (OR) is the major perivitellin of the eggs of Pomacea canaliculata. The astaxanthin (ASX) binding and structural stability of OR were investigated by fluorescence spectroscopy and circular dichroism (CD). The apo-OR (without astaxanthin) shows a single, high affinity binding site for ASX (K(D)=0.5 microM). The quenching of tryptophan fluorescence by ASX indicates that about 22% are near the carotenoid-binding site in a non-polar environment, as indicated by tryptophan resonance energy transfer to the ligand. Secondary structure (alpha+beta) was virtually not affected by cofactor removal. Holo-OR shows unusually high thermal stability. The removal of ASX does not affect the thermal or chemical stability of the quaternary structure. In conclusion, although subtle changes were observed, ASX is not essential for OR stability, unlike most invertebrate carotenoproteins. This supports the idea that OR plays an important physiological role in the storage, transport and protection of carotenoids during snail embryogenesis.     

Tryptophan Availability Modulates Serotonin Release from Rat Hypothalamic Slices

Application of a novel in vitro experimental system has allowed us to describe the relationship between tryptophan availability and serotonin release from rat hypothalamic slices. Superfusing hypothalamic slices with a physiologic medium containing l-tryptophan (1, 2, 5, or 10 microM) caused dose-dependent elevations in tissue tryptophan levels; the magnitude of the elevations produced by supplementing the medium with less than 5 microM tryptophan was within the physiologic range for rat brain tryptophan levels. Slice serotonin levels rose biphasically as the tryptophan concentration in the medium was increased. Superfusing the slices with medium supplemented with a low tryptophan concentration (1 or 2 microM) caused proportionally greater incremental changes in serotonin levels than the increases caused by further elevating the tryptophan concentration (5 or 10 microM). The spontaneous release of serotonin from the slices exhibited a dose-dependent relationship with the tryptophan concentration of the superfusion medium. Electrically evoked serotonin release, which was calcium-dependent and tetrodotoxin-sensitive, also increased in proportion to the medium tryptophan concentration. These data suggest that the rate at which serotonin is released from hypothalamic nerve terminals is coupled to brain tryptophan levels. Accelerations in hypothalamic serotonin synthesis, caused by elevating brain tryptophan levels, result in proportionate increases in the rates of serotonin release during rest and with membrane depolarization.     

Tryptophan of facultative methylotrophic bacteria Pseudomonas sp. M. II. Regulation of tryptophan biosynthesis

Regulation of tryptophan biosynthesis of facultative methylotrophic Pseudomonas sp. M was studied. Repression of the trpE, trpD and trpC genes by tryptophan was demonstrated. It was also shown that the trpE and trpDC genes are derepressed noncoordinately. No regulation of the trpF gene product could be demonstrated, indicating that its synthesis is constitutive. The trpA and trpB genes are inducible by indol-3-glycerophosphate. Anthranilate synthase and tryptophan synthase were sensitive to the feedback inhibition. The tryptophan concentrations giving 50% inhibition were estimated to be 9 microM and 1 microM, respectively. Experimental evidence for activation of the N-5-phosphoribosyl anthranilate isomerase and for inhibition of the indol-3-glycerophosphate synthase by some tryptophan intermediates was obtained.     

Crystallographic and mass spectrometric characterisation of eIF4E with N7-alkylated cap derivatives

Structural complexes of the eukaryotic translation initiation factor 4E (eIF4E) with a series of N(7)-alkylated guanosine derivative mRNA cap analogue structures have been characterised. Mass spectrometry was used to determine apparent gas-phase equilibrium dissociation constants (K(d)) values of 0.15 microM, 13.6 microM, and 55.7 microM for eIF4E with 7-methyl-GTP (m(7)GTP), GTP, and GMP, respectively. For tight and specific binding to the eIF4E mononucleotide binding site, there seems to be a clear requirement for guanosine derivatives to possess both the delocalised positive charge of the N(7)-methylated guanine system and at least one phosphate group. We show that the N(7)-benzylated monophosphates 7-benzyl-GMP (Bn(7)GMP) and 7-(p-fluorobenzyl)-GMP (FBn(7)GMP) bind eIF4E substantially more tightly than non-N(7)-alkylated guanosine derivatives (K(d) values of 7.0 microM and 2.0 microM, respectively). The eIF4E complex crystal structures with Bn(7)GMP and FBn(7)GMP show that additional favourable contacts of the benzyl groups with eIF4E contribute binding energy that compensates for loss of the beta and gamma-phosphates. The N(7)-benzyl groups pack into a hydrophobic pocket behind the two tryptophan side-chains that are involved in the cation-pi stacking interaction between the cap and the eIF4E mononucleotide binding site. This pocket is formed by an induced fit in which one of the tryptophan residues involved in cap binding flips through 180 degrees relative to structures with N(7)-methylated cap derivatives. This and other observations made here will be useful in the design of new families of eIF4E inhibitors, which may have potential therapeutic applications in cancer.     

Oxidation of tryptophan by redox intermediates of myeloperoxidase and inhibition of hypochlorous acid production

The neutrophil enzyme myeloperoxidase catalyzes the oxidation of tyrosine to tyrosyl radicals, which cross-link to proteins and initiate lipid peroxidation. Tryptophan is present in plasma at about the same concentration as tyrosine and has a similar one-electron reduction potential. In this investigation, we have determined the ability of myeloperoxidase to catalyze the oxidation of tryptophan to assess whether or not this reaction may contribute to oxidative stress at sites of inflammation. We show that tryptophan is a poor substrate for myeloperoxidase because, even though it reacts rapidly with compound I (kI 2.1 x 10(6) M(-1)s(-1)), it reacts sluggishly with compound II (kII 7 M(-1)s(-1)). Tryptophan reversibly inhibited production of hypochlorous acid by purified myeloperoxidase by converting the enzyme to a mixture of compound II and compound III. It gave 50% inhibition (I50) at a concentration of 2 microM. In contrast, it was an ineffective inhibitor of hypochlorous acid production by human neutrophils (I50 80 microM) unless superoxide dismutase was present (I50 5 microM). We propose that compound I of myeloperoxidase will oxidize tryptophan at sites of inflammation. Enzyme turnover will result from the reaction of superoxide or tyrosine with compound II. Thus, tryptophan radicals are potential candidates for exacerbating oxidative stress during inflammation.     

Characterization of tescalcin, a novel EF-hand protein with a single Ca2+-binding site: metal-binding properties, localization in tissues and cells, and effect on calcineurin

The tescalcin gene is preferentially expressed during mouse testis differentiation. Here, we demonstrate that this gene encodes a 24 kDa Ca(2+)- and Mg(2+)-binding protein with one consensus EF-hand and three additional domains with EF-hand homology. Equilibrium dialysis with (45)Ca(2+) revealed that recombinant tescalcin binds approximately one Ca(2+) ion at physiological concentrations (pCa 4.5). The intrinsic tryptophan fluorescence of tescalcin was significantly reduced by Ca(2+), indicative of a conformational change. The apparent K(d) for Ca(2+) was 0.8 microM. A point mutation in the consensus EF-hand (D123A) abolished (45)Ca(2+) binding and prevented the fluorescence quenching, demonstrating that the consensus EF-hand alone mediates the Ca(2+)-induced conformational change. Tescalcin also binds Mg(2+) (K(d) 73 microM), resulting in a much smaller fluorescence decrease. In the presence of 1 mM Mg(2+), tescalcin's Ca(2+) affinity is shifted to 3.5 microM. These results illustrate that tescalcin should bind Mg(2+) constitutively in a quiescent cell, replacing it with Ca(2+) during stimulation. We also show that tescalcin is most abundant in adult mouse heart, brain, and stomach, as well as in HeLa and HL-60 cells. Immunofluorescence microscopy revealed that tescalcin is present in the cytoplasm and nucleus, with concentration in membrane ruffles and lamellipodia in the presence of serum, where it colocalizes with the small guanosine triphosphatase Rac-1. Tescalcin shares sequence and functional homology with calcineurin-B homologous protein (CHP), and we found that tescalcin, like CHP, can inhibit the phosphatase activity of calcineurin A. Hence, tescalcin is a novel calcineurin B-like protein that binds a single Ca(2+) ion.     

Different degrees of cooperativity of the Ca2+-induced changes in fluorescence intensity of solubilized sarcoplasmic reticulum ATPase

The tryptophan fluorescence emission of sarcoplasmic reticulum Ca2+-ATPase was studied both in purified ATPase vesicles and in ATPase solubilized with the nonionic detergent dodecyloctaethyleneglycolmonoether (C12E8). Fluorescence intensity changes in purified ATPase were titrated as a function of free Ca2+ in the medium. It exhibited a cooperative pattern, with a Hill number of 2.21 +/- 0.02 and K0.5 = 0.51 microM Ca2+. Upon solubilization of the ATPase, the cooperative pattern of fluorescence change was lost; the Hill number was 0.96 and K0.5 = 1.4 microM Ca2+. When solubilization was carried out in the presence of 0.5 or 1.0 mM CaCl2, followed by the titrations of fluorescence change in the micromolar Ca2+ range, the cooperative pattern was preserved under the same concentrations of C12E8 which would otherwise promote the loss in cooperativity. For the ATPase solubilized in millimolar Ca2+, the Hill number was 1.98 with a K0.5 = 1.5 microM Ca2+. The maximal amount of Ca2+ bound to the high affinity sites corresponded to approximately 1 mol of calcium/mol of polypeptide chains, both in purified ATPase vesicles and in the soluble ATPase. A model is suggested, which involves a minimum of 4 interacting Ca2+ sites (tetramers). Cooperativity is accounted for in the model by the predominance in the absence of Ca2+ of low affinity state (E') of the Ca2+ site (K'D = 5.7 x 10(-4) M), which would be congruent to 90 times more concentrated than (E), the high affinity state (KD = 1.9 x 10(-7) M). Simulations derived from this model fit the experimental data.     

Susceptibility of cerebellar granule neurons from GM2/GD2 synthase-null mice to apoptosis induced by glutamate excitotoxicity and elevated KCl: Rescue by GM1 and LIGA20

Our previous study showed an impaired regulation of Ca(2+) homeostasis in cultured cerebellar granule neurons (CGN) from neonatal mice lacking GM2, GD2 and all gangliotetraose gangliosides, due to disruption of the GM2/GD2 synthase (GalNAc-T) gene. In the presence of depolarizing concentration (55 mM) K(+), these cells showed persistent elevation of intracellular Ca(2+) ([Ca(2+)]( i )) leading to apoptosis and cell destruction. This was in contrast to CGN from normal littermates whose survival was enhanced by high K(+). In this study we demonstrate that glutamate has the same effect as K(+) on CGN from these ganglioside-deficient knockout (KO) mice and that apoptosis in both cases is averted by exogenous GM1. Even more effective rescue was obtained with LIGA20, a semi-synthetic derivative of GM1. LC(50) of glutamate in the KO cells was 3.1 microM, compared to 46 microM in normal CGN. [Ca(2+)]( i ) measurement with fura-2 revealed no difference in glutamate-stimulated Ca(2+) influx between the 2 cell types. However, reduction of [Ca(2+)]( i ) following application of Mg(2+) was significantly impaired in the mutant CGN. The rescuing effects of exogenous GM1 and LIGA20 corresponded to their ability to restore Ca(2+) homeostasis. The greater potency of LIGA20 is attributed to its greater membrane permeability with resultant ability to insert into both plasma and nuclear membranes at low concentration (相似文献   

(6R)-Tetrahydrobiopterin Increases the Activity of Tryptophan Hydroxylase in Rat Raphe Slices

The effects of (6R)- and (6S)-tetrahydrobiopterin (BPH4), tetrahydroneopterin, and 6-methyltetrahydropterin on the activity of tryptophan hydroxylase were investigated in rat raphe slices. The activity of tryptophan hydroxylase was estimated by measurement of 5-hydroxytryptophan (5-HTP) formation under inhibition of aromatic L-amino acid decarboxylase with use of HPLC-fluorometric detection. (6R)-BPH4 (the naturally occurring form) at 42 microM, tetrahydroneopterin at 50 microM, and 6-methyltetrahydropterin at 100 microM increased tryptophan hydroxylase activity to 350, 145, and 146% of control values, respectively. (6S)-BPH4, however, had no significant effects on tryptophan hydroxylase activity. These results suggest that tryptophan hydroxylase is subsaturating in vivo for the naturally occurring cofactor, (6R)-BPH4, and that the concentration of (6R)-BPH4 may play an important role for the regulation of tryptophan hydroxylase activity in vivo.     

Unique metal dependency of cytosolic alpha-mannosidase from Thermotoga maritima,a hyperthermophilic bacterium

A putative cytosolic alpha-mannosidase gene from a hyperthermophilic marine bacterium Thermotoga maritima was cloned and expressed in Escherichia coli. The purified recombinant enzyme appeared to be a homodimer of a 110-kDa subunit. The enzyme showed metal-dependent ability to hydrolyze p-nitrophenyl-alpha-D-mannopyranoside. In the absence of a metal, the enzyme was inactive. Cobalt and cadmium supported high activity (60 U/mg at 70 degrees C), while the activity with zinc and chromium was poor. Cobalt (0.8 mol) bound to 1 mol monomer with a K(d) of 70 microM. The optimum pH and temperature were 6.0 and 80 degrees C, respectively. The activity was inhibited by swainsonine, but not by 1-deoxymannojirimycin, which is in agreement with the features of cytosolic alpha-mannosidase.     

Examination of substrate-induced conformational changes in the Na+/glucose cotransporter

Conformations of the Na+/glucose cotransporter were examined using tryptophan fluorescence and substrates to induce cotransporter conformational changes. Addition of Na+ but not K+ or TMA+ resulted in a saturable quenching of tryptophan fluorescence with a K0.5 for Na+ of 28 mM. In the presence of saturating Na+ concentrations, d-glucose but not l-glucose, fructose, or phlorizin resulted in a partial return of tryptophan fluorescence to approximately 70% of the substrate-free levels. This return of tryptophan fluorescence was a saturable function of d-glucose concentration with a K0.5 of 43 microM. The three conformations were compared with respect to their sensitivity to tryptophan quench reagents. Acrylamide quenching was unaffected by substrates. In contrast, I- quenching decreased 40% in the presence of Na+, while Cs+ quenching increased 64%. Addition of saturating d-glucose concentrations resulted in the return of I- quenching to 90% of the substrate-free values and reduced Cs+ quenching to substrate-free levels. In contrast, phlorizin did not mimic the effect of d-glucose on tryptophan fluorescence. These results are interpreted in terms of a second substrate-induced cotransporter conformational change which based on similar substrate specificities appears directly related to cotransporter-mediated Na+ and d-glucose transport.     

Kinetic and physical evidence that the diheme enzyme MauG tightly binds to a biosynthetic precursor of methylamine dehydrogenase with incompletely formed tryptophan tryptophylquinone

Methylamine dehydrogenase (MADH) contains the protein-derived cofactor tryptophan tryptophylquinone (TTQ) which is generated by the posttranslational modification of two endogenous tryptophan residues. The modifications are incorporation of two oxygens into one tryptophan side chain and the covalent cross-linking of that side chain to a second tryptophan residue. This process requires at least one accessory gene, mauG. Inactivation of mauG in vivo results in production of an inactive 119 kDa tetrameric alpha 2beta 2 protein precursor of MADH with incompletely synthesized TTQ. This precursor can be converted to active MADH with mature TTQ in vitro by reaction with MauG, a 42 kDa diheme enzyme. Steady-state kinetic analysis of the MauG-dependent conversion of the precursor to mature MADH with completely synthesized TTQ yielded values of k cat of 0.20 +/- 0.01 s (-1) and K m of 6.6 +/- 0.6 microM for the biosynthetic precursor protein in an in vitro assay. In the absence of an electron donor to initiate the reaction it was possible to isolate the MauG-biosynthetic precursor (enzyme-substrate) complex in solution using high-resolution size-exclusion chromatography. This stable complex is noncovalent and could be separated into its component proteins by anion-exchange chromatography. In contrast to the enzyme-substrate complex, a mixture of MauG and its reaction product, mature MADH, did not elute as a complex during size-exclusion chromatography. The differential binding of MauG to its protein substrate and protein product of the reaction indicates that significant conformational changes in one or both of the proteins occur during catalysis which significantly affects the protein-protein interactions.     

Purification and characterization of an inducible aromatic amino acid-sensitive form of chorismate mutase from Solanum tuberosum L. tubers

An amino acid-sensitive form of chorismate mutase (CM) has been purified over 1000-fold from disks excised from tubers of Solanum tuberosum L. cv White Rose. Purification was accomplished by chromatography on Matrix Blue A followed by affinity chromatography with tryptophan as ligand. CM assays performed in the absence of tryptophan yielded pH-dependent sigmoidal kinetics. At pH 8.0, sigmoidal kinetics were observed with a Hill coefficient of 1.66 (S0.5 = 188 microM). However, a shift from sigmoidal to hyperbolic kinetics was observed when assays were performed at pH 8.5. Addition of 9 microM tryptophan to the assay resulted in maximum activation of the enzyme with a Ka of 1.2 microM. When assayed in the presence of tryptophan, hyperbolic kinetics were observed over the pH range 6.0-8.0. Addition of tryptophan also decreased the Km for chorismate from 185 to 45 microM. Tryptophan (0.1 mM) completely protected CM from inhibition by phenylalanine (1.8 mM) and tyrosine (1.8 mM). However, in the absence of the activator, phenylalanine and tyrosine exhibited 50% inhibition at 0.80 and 0.68 mM concentrations, respectively. Both phenylalanine and tyrosine competitively inhibited CM activity with Ki values of 550 and 440 mM, respectively. Arogenate (1.0 mM) had no effect on CM activity in either the presence or absence of tryptophan. Analytical isoelectric focusing yielded an isoelectric point of 4.73.