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PubMed1987

Chorismate aminations: partial purification of Escherichia coli PABA synthase and mechanistic comparison with anthranilate synthase.

Chorismate is converted by regiospecific amination/aromatization sequences to o-aminobenzoate and p-aminobenzoate (PABA) by anthranilate synthase (AS) and PABA synthase (PABS), respectively. We report here the first partial purification of the large subunit of Escherichia coli PABA synthase, previously reported to be quantitatively inactivated in purification attempts. The subunit encoded by the pabB gene was overexpressed from a T7 promoter and purified 9-fold to 25-30% homogeneity. The pabB subunit appears unusually sensitive to inactivation by glycerol so this cosolvent is contraindicated. The Km for chorismate is 42 microM in the ammonia-dependent conversion to PABA, and we estimate a turnover number of 2.6 min-1. A variety of chorismate analogues have been prepared and examined. Of these compounds, cycloheptadienyl analogue 11 has been found to be the most potent inhibitor of Serratia marcescens anthranilate synthase (Ki = 30 microM for an RS mixture) and of the E. coli pabB subunit of PABA synthase (Ki = 226 microM). Modifications in the substituents at C-3 [enolpyruyl ether, (R)- or (S)-lactyl ether, glycolyl ether] or C-4 (O-methyl) of chorismate lead to alternate substrates. The Vmax values for (R)- and (S)-lactyl ethers are down 10-20-fold for each enzyme, and V/K analyses show the (S)-lactyl chorismate analogue to be preferred by 12/1 over (R)-lactyl for anthranilate synthase while a 3/1 preference was observed for (R)-/(S)-lactyl analogues by PABA synthase. The glycolyl ether analogue of chorismate shows 15% Vmax vs. chorismate for anthranilate synthase but is actually a faster substrate (140%) than chorismate with PABA synthase, suggesting the elimination/aromatization step from an aminocyclohexadienyl species may be rate limiting with AS but not with PABS. Indeed, studies with (R)-lactyl analogue 14 and anthranilate synthase led to accumulation of an intermediate, isolable by high-performance liquid chromatography and characterized by NMR and UV-visible spectroscopy as 6-amino-5-[(1-carboxyethyl)oxy]-1,3-cyclohexadiene-1-carboxylic acid (17). This is the anticipated intermediate predicted by our previous work with conversion of synthetic trans-6-amino-5-[(1-carboxyethenyl)oxy]-1,3-cyclohexadiene-1-carbo xylic acid (2) to anthranilate by the enzyme. Compound 17 is quantitatively converted to anthranilate on reincubation with enzyme, but at a 1.3-10-fold lower Vmax than starting lactyl substrate 14 under the conditions investigated; the basis for this kinetic variation is not yet determined.

Anthranilate Synthase↗
PubMed1990

Monofunctional chorismate mutase from Bacillus subtilis: kinetic and 13C NMR studies on the interactions of the enzyme with its ligands.

The interaction of the monofunctional chorismate mutase from Bacillus subtilis with chorismate and prephenate has been studied kinetically and by NMR spectroscopy with 13C specifically labeled substrates. Prephenate dominates the population of enzyme-bound species, and the "off" rate constant (approximately 60 s-1) obtained from line-broadening experiments is close to the value of kcat for chorismate (50 s-1) determined kinetically. The calculated "on" rate constant for prephenate (8 x 10(5) M-1 s-1) is similar to the value of kcat/Km for chorismate (5 x 10(5) M-1 s-1). The kinetic parameters of the Bacillus mutase are remarkably insensitive to pH over a wide range and display no solvent isotope effect. These results suggest that the enzyme-catalyzed reaction may be encounter controlled (slowed from the diffusion limit by some feature of the enzyme's active site) and that kcat for chorismate is determined by the product off rate. There is now no evidence to suggest that the skeletal rearrangement on the enzyme surface occurs by a pathway other than a pericyclic process.

Bacillus subtilis↗
PubMed1977

Rearrangement of chorismate to prephenate. Use of chorismate mutase inhibitors to define the transition state structure.

The enzymically catalyzed conversion of chorismate to prephenate may proceed through either a chair-like or a boat-like transition state. To distinguish between these alternatives, we have prepared a series of structural analogues of the two possible transition state structures and tested them as inhibitors of chorismate mutase-prephenate dehydrogenase from Escherichia coli K12. The results indicate that the enzymically catalyzed reaction passes through a chair-like intermediate. None of the compounds studied is an ideal transition state analogue; it seems likely that the partial bond structure of the transition state precludes the corresponding orientation of the side chain in stable molecules. Nevertheless, the new inhibitors are stronger than any previously available, and the degree of inhibition is consistent with bacteriostatic activity recently observed in some of the compounds.

Alkanes↗
PubMed1989

Aurintricarboxylic acid is a potent inhibitor of phosphofructokinase.

Aurintricarboxylic acid (ATA) was found to be a very potent inhibitor of purified rabbit liver phosphofructokinase (PFK), giving 50% inhibition at 0.2 microM. The inhibition was in a manner consistent with interaction at the citrate-inhibitory site of the enzyme. The data suggest that inhibition of PFK by ATA was not due to denaturation of the enzyme or the irreversible binding of inhibitor, since the inhibition could be reversed by addition of allosteric activators of PFK, i.e. fructose 2,6-bisphosphate or AMP. Two other tricarboxylic acids, agaric acid and (-)-hydroxycitrate, were found to inhibit PFK. ATA at much higher concentrations (500 microM) was shown to inhibit fatty acid synthesis from endogenous glycogen in rat hepatocytes; however, protein synthesis was not altered.

Animals↗
PubMed1988

Catalysis of concerted reactions by antibodies: the Claisen rearrangement.

Monoclonal antibodies were prepared against a transition state analog inhibitor of chorismate mutase (EC 5.4.99.5). One of the antibodies catalyzes the rearrangement of chorismate to prephenate with rate accelerations of more than 2 orders of magnitude compared to the uncatalyzed reaction. Saturation kinetics were observed, and at 25 degrees C the values of kcat and Km were 1.2 X 10(-3) s-1 and 5.1 X 10(-5) M respectively. The transition state analog was shown to be a competitive inhibitor of the reaction with Ki equal to 0.6 microM. These results demonstrate the feasibility of using rationally designed immunogens to generate antibodies that catalyze concerted reactions.

Animals↗
PubMed1990

Serotonin uptake, storage, and synthesis in an immortalized committed cell line derived from mouse teratocarcinoma.

We report the isolation and characterization of a serotoninergic cell line, 1C11, derived from a mouse teratocarcinoma. The clone 1C11 was immortalized through the expression of the simian virus 40 oncogenes. 1C11 presents two states: an immature epithelial-like state (1C11 precursor) and a more differentiated state (1C11). After induction by dibutyryl cyclic AMP and cyclohexanecarboxylic acid, almost 100% of 1C11 cells continue to divide and have acquired a neural-like phenotype. 1C11* cells coexpress several neural markers, such as synaptophysin (the membrane constituent of synaptic vesicles), the neuropeptide [Met5]enkephalin, and the neurotransmitter serotonin. 1C11* cells store endogenous serotonin and are able to synthesize serotonin from L-tryptophan and to catabolize it by monoamine oxidase B. Moreover, the cells take up serotonin by a carrier-mediated mechanism very similar to that of serotoninergic neurons. The expression of the simian virus 40 oncogenes, which promoted immortalization, does not therefore prevent further differentiation. This inducible cell line constitutes a valuable model for cellular and molecular studies concerning the physiology and the pharmacological modulation of the serotoninergic phenotype.

Animals↗
PubMed1976

Inhibition of RNA-directed DNA polymerase by aurintricarboxylic acid.

Commercial-grade aurintricarboxylic acid (ATA) inhibits poly(A), poly(C) and viral RNA-directed DNA synthesis by detergent-disrupted virions of Moloney murine leukemia virus. Paper chromatography of crude ATA yields two active components, which appear to behave identically, and at least two inactive components. The concentration of ATA needed to inhibit polymerase activity is proportional to the concentration of viral protein. The inhibition is neither attributable to contaminating heavy metal ions in the ATA preparation nor to chelation by ATA of Mn2+ or Zn2+, the necessary co-factors. Inhibition of the polymerase reaction by ATA greatly increases the Km for the primer [oligo(T)/oligo(dG)], while it only slightly lowers the Vmax and does not affect the Km's for the template [poly(A)/poly(C)] or the substrate (TTP/dGTP). Thus, ATA seems to reduce specifically the affinity of the polymerase for the DNA primer molecule.

Aurintricarboxylic Acid↗
PubMed1985

Purification and properties of NADH/NADPH-dependent p-hydroxybenzoate hydroxylase from Corynebacterium cyclohexanicum.

Crude soluble extracts of Corynebacterium cyclohexanicum, grown on cyclohexanecarboxylic acid, were found to contain 4-hydroxybenzoate 3-hydroxylase which functions with NADH as well as NADPH. The purified enzyme preparation was electrophoretically homogeneous and contained FAD as prosthetic group. The relative molecular mass of the enzyme was estimated to be about 47000 by native and denaturated acrylamide gel electrophoresis, indicating that it is monomeric. The enzyme was stable at 60 degrees C for 10 min. The enzyme was highly specific for p-hydroxybenzoate. The activity was inhibited by several aromatic analogues of p-hydroxybenzoate such as p-aminobenzoate, p-fluorobenzoate, o-hydroxybenzoate, m-hydroxybenzoate, 2,4-dihydroxygenzoate, and 2,5-dihydroxybenzoate. The Km value for NADH was fairly constant, about 45 microM, in the pH range 7.0-8.4, whereas the Km value for NADPH increased from 63 microM to 170 microM as the pH rose from 7.0 to 8.4. V values in the same pH range, however, were approximately constant in both cases; about 30 mumol min-1 mg-1 for NADH, and 26 mumol min-1 mg-1 for NADPH. Mg2+ was required for full activity of the enzyme in low concentrations of phosphate buffer. The enzyme was inhibited by C1- which was non-competitive with respect to NADH, NADPH and p-hydroxybenzoate.

4-Hydroxybenzoate-3-Monooxygenase↗
PubMed1988

Purification and properties of 4-hydroxycyclohexanecarboxylate dehydrogenase from Corynebacterium cyclohexanicum.

4-Hydroxycyclohexanecarboxylate dehydrogenase, which requires NAD as a cofactor, was detected in crude soluble extracts of Corynebacterium cyclohexanicum grown on cyclohexanecarboxylic acid as the sole carbon source. The dehydrogenase was purified from extracts to an electrophoretically homogenous state by ammonium sulfate precipitation and chromatography on DEAE-650s, agarose-NAD and hydroxyapatite. The enzyme consisted of two identical subunits and had a native relative molecular mass of 53,600. There were two residues each of cysteine and tryptophan in the enzyme molecule. Oxo acid rather than hydroxy acid was routinely used as substrate for assay of the enzyme. The enzyme is highly specific for 4-oxocyclohexanecarboxylic acid: the carboxyl group is essential and the position of carbonyl group is important; neither the 2-oxo nor the 3-oxo homologue was used as substrate. A methyl substitution on the ring of 4-oxocyclohexanecarboxylate resulted in an almost complete loss of its activity. The reduction product was identified as trans-4-hydroxycyclohexanecarboxylic acid by gas-liquid chromatography and mass spectrometry. It was used as a substrate for the reverse reaction in the presence of NAD but not its cis-isomer. The enzyme was specific for the B-side (pro-S) hydrogen of NADH in the hydrogen transfer from NADH to 4-oxocyclohexanecarboxylate. The Km values for 4-oxocyclohexanecarboxylate and NADH in the reduction reaction at pH 6.8 were 0.50 mM and 0.28 mM, respectively, whereas those for trans-4-hydroxycyclohexanecarboxylate and NAD in the oxidation reaction at pH 8.8 were 0.51 mM and 0.23 mM, respectively. The equilibrium constant of the reaction was 1.79 x 10(-10) M. The enzyme was strongly inhibited by N-bromosuccinimide.

Alcohol Oxidoreductases↗
PubMed1990

Osmotic stress and abscisic acid induce expression of the wheat Em genes.

The early-methionine-labelled (Em) polypeptide is the single most abundant cytosolic protein of dry wheat embryos. It is encoded by messenger RNA which accumulates during the later (maturation) stages of embryogenesis. The accumulation of Em mRNA can be induced in isolated developing embryos, in culture, by the application of the plant growth regulator, abscisic acid, which prevents precocious germination. Precocious germination is also inhibited by the culture of embryos under conditions of osmotic stress when accumulation of Em mRNA is induced. This induction occurs in the absence of any significant increase in the endogenous levels of embryonic abscisic acid although there is a requirement for the continued presence of the growth regulator. Additionally, expression of Em genes can be repeated during early germination, if imbibing embryos are subjected to osmotic stress. Induction of Em-gene expression by osmotic stress is consistent with the proposed role of the Em polypeptide in mediating the remarkable tolerance of cereal embryos to the programmed desiccation undergone during their maturation.

Abscisic Acid↗
PubMed1977

The effect of conformationally restricted amino acid. Analogues on the frog spinal cord in vitro.

1 The isolated spinal cord of the frog (Rana pipiens) was used to examine the structural requirement for the activity of neutral amino acids. The potencies of the aliphatic amino acids, gamma-aminobutyric acid (GABA), beta-alanine and glycine were compared with the potencies of conformationally restricted cyclopentane and cyclohexane amino acid analogues. Both motoneurone hyperpolarizing and primary afferent depolarizing activity were examined in this study. 2 On motoneurones beta-alanine was the most potent aliphatic amino acid and glycine the least potent. Of the substituted aminocyclopentane carboxylic acids, that compound with a separation of amino and carboxylic acid groups closest to that of the extended GABA molecule (4.74 A) had a potency similar to GABA. As the separation decreased the hyperpolarizing activity fell off rapidly. The substituted aminocyclohexane carboxylic acids were generally inactive even at a concentration of 10 mM. 3 Strychnine blocked the motoneurone hyperpolarizing responses to all compounds with a distance between the amino and carboxylic acid groups of 3.66 A or less, but did not block the response of compounds with a distance of 4.08 A or greater. Picrotoxin and bicuculline antagonized all the responses to varying degrees and therefore were of little value in characterizing the responses. 4 On the primary afferents GABA was the most potent aliphatic amino acid and glycine the least potent. The substituted aminocyclohexane carboxylic acids were generally inactive on primary afferents. The response of the substituted aminocyclopentane carboxylic acid whose separation of amino and carboxylic acid groups was closest to that of the extended GABA molecule was most similar to the GABA response. However, (+/-)-cis-3-aminocyclopentane-carboxylic acid (separation=4.08 A),which mimicked the action of GABA on motoneurones, closely mimicked the depolarizing action of beta-alanine on primary afferents. 5 The findings suggest that the hyperpolarizing GABA receptor on motoneurones will accept a molecule whose amino and carboxylic acid groups are separated by a distance of 4.08 A or greater while the glycine receptor will accept a compound with a distance of 3.66 A or less. The depolarizing GABA receptors on primary afferents appear to be more selective since they are not activated by (+/-)-cis-3-aminocyclopentane carboxylic acid (separation = 4.08 A), while the motoneurone receptors are.

Action Potentials↗
PubMed1989

Nucleotide sequence of Escherichia coli isochorismate synthetase gene entC and evolutionary relationship of isochorismate synthetase and other chorismate-utilizing enzymes.

Biochemical analysis of the enzymatic activity catalyzing the conversion of chorismate to isochorismate in the enterobactin biosynthetic pathway attributed the reaction to the isochorismate synthetase enzyme, designated EntC. However, the lack of mutations defining this activity has hampered the precise identification of the entC structural gene. In this study, we engineered a stable insertion mutation into the chromosomal region between the enterobactin genes fepB and entE. This mutation disrupted the structural gene for a previously identified 44-kilodalton protein and eliminated production of 2,3-dihydroxybenzoic acid, the catechol precursor of enterobactin. The complete nucleotide sequence of this gene was determined and compared with the sequences of other genes encoding chorismate-utilizing proteins. The similarities observed in these comparisons not only indicated that the locus is entC but also supported the premise that these enzymes constitute a family of related proteins sharing a common evolutionary origin. In addition, in this and the accompanying paper (M. S. Nahlik, T. J. Brickman, B. A. Ozenberger, and M. A. McIntosh, J. Bacteriol. 171:784-790, 1989), evidence is presented indicating that the entA product is potentially a secondary factor in the chorismate-to-isochorismate conversion and that the prototypic entC lesion (entC401) resides in the structural gene for the EntA protein. Finally, polarity effects from the insertion mutation in entC on downstream biosynthetic genes indicated that this locus is the promoter-proximal cistron in an ent operon comprising at least five genes. Appropriate regulatory signals upstream of entC suggest that this operon is regulated by iron through interaction with the Fur repressor protein.

Amino Acid Sequence↗
PubMed1976

Biosynthesis of chloramphenicol in Streptomyces sp. 3022a. Identification of p-amino-L-phenylalanine as a product from the action of arylamine synthetase on chorismic acid.

Products obtained from the action of arylamine synthetase on [G-14C]chorismic acid were fractionated by gel filtration and ion exchange column chromatography to yield a partially purified radioactive component with an arylamine function. From its ultraviolet absorption spectrum and thin-layer chromatographic behaviour the product was considered to be p-aminophenylalanine and the identification was confirmed by co-crystallization with an authentic specimen. Specific deamination of the product with L-amino-acid oxidase indicated that it was the L-epimer. These results strengthen previous evidence that arylamine synthetase is at a branch point in the shikimic acid pathway, specifically diverting intermediates to the synthesis of chloramphenicol.

Amino Acid Oxidoreductases↗
PubMed1985

Abscissic acid localization by light microscopic immunohistochemistry in Chenopodium polyspermum L. Effect of water stress.

An indirect immunohistochemical technique was developed using a rabbit anti-abscissic acid (ABA) serum and the soluble peroxidase-antiperoxidase (PAP) complex for the localization of endogenous ABA in the aerial parts of Chenopodium. Terminal bud, axillary bud bearing nodes, and adult leaves were prefixed by a soluble carbodiimide to obtain the coupling of ABA on cellular proteins and postfixed by a conventional mixture of aldehydes. They were then embedded in paraffin or in plastic. Numerous controls were carried out on sections and on a model system to test the validity of the technique. Based on the staining patterns observed along the plant, an apico-basal gradient of ABA was revealed. In the older buds, ABA was mainly concentrated in the quiescent meristematic cells of the apex. Phloem cells of the main axis and chloroplasts of the leaves were specifically labeled. No reaction product was visualized in the parenchyma cells or in the cambial zone. Water stress, which is known to increase ABA content, induced an increase of immunoreactivity within the same compartments. This physiological test validates the stain.

Abscisic Acid↗