Crystallization of 5-enolpyruvylshikimate 3-phosphate synthase from Escherichia coli

User-friendly one-step disposable signal-on bioassay for glyphosate detection in water samples

The onsite detection of glyphosate requires an easy-to-handle, low-cost and disposable assay for untrained users as requested by the ASSURED guidelines. A new strategy based on the expression of fusion proteins is proposed here. A glyphosate oxidase derived from Bacillus subtilis and the 6E10 variant of the dye peroxidase from Pseudomonas putida, both fused with the carbohydrate binding module (CBM) 3a from Clostridium thermocellum, were designed and expressed, leading to GlyphOx-CBM and 6E10-CBM. Cell lysates were used to immobilise both enzymes on cotton buds' heads without any purification. The cotton buds exhibit glyphosate oxidase activity when dipped into a glyphosate-contaminated water sample containing the 6E10-CBM chromogenic substrates. The chromophore could be quantified both in the solution and on the cotton buds' heads. Photography followed by image analysis allows to detect glyphosate with a linear range of 0.25-2.5 mM and a limit of detection (LoD) of 0.12 mM. When the chromogenic substrates are replaced by luminol, the chemiluminescence reaction allows the detection of glyphosate with a linear range of 2-500 μM and a LoD of 0.45 μM. No interference was observed using glyphosate analogues (glycine, sarcosine, aminomethylphosphonic acid) or other herbicides used in a mixture. Only cysteine was found to inhibit 6E10-CBM. Two river waters spiked with glyphosate lead to recoveries of 64-131%. This work describes a very easy-to-handle and inexpensive signal-on bioassay for glyphosate detection in real surface water samples.

Mechanism of the EPSP synthase catalyzed reaction: evidence for the lack of a covalent carboxyvinyl intermediate in catalysis

In order to detect covalent reaction intermediates in the 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase reaction, we have investigated the interaction of EPSP synthase with the reaction product EPSP. An exchange of EPSP-methylene protons could be demonstrated by incubating EPSPS with EPSP in D2O. Since trace amounts of contaminating Pi would lead to reversal of EPSPS reaction and hence methylene proton exchange, we added pyruvate kinase, ADP, Mg++ and K+. Under these conditions, any contaminating Pi that is converted to PEP is trapped as ATP. No exchange of EPSP protons with those of the solvent could be detected in the presence of this trap system, suggesting that enzyme-bound EPSP is unable to form a covalent tetrahedral complex. Incorporation of [14C] from [14C]-S3P and [14C]-PEP into EPSP could be detected, but only in the absence of a PEP (or Pi) trap system. This indicates that for the exchange reaction, Pi is required, and also indicates the absence of a covalent intermediate, unless the carboxyvinyl-enzyme-bound S3P is completely restricted from exchange.

Reaction of 5-enol-pyruvoylshikimate-3-phosphate synthase with diethyl pyrocarbonate: evidence for an essential histidine residue

5-enol-Pyruvoylshikimate-3-phosphate synthase catalyzes the reversible condensation of phosphoenolpyruvate and shikimate 3-phosphate to yield 5-enol-pyruvoylshikimate 3-phosphate and inorganic phosphate. The enzyme is a target for the nonselective herbicide glyphosate (N-phosphonomethylglycine). Diethyl pyrocarbonate inactivated this enzyme with a second-order rate constant of 220 M-1 min-1 at pH 7.0 and 0 degrees C. The rate of inactivation is pH dependent and the pH inactivation rate data show the involvement of a group with a pKa of 6.8. Almost all of the original activity was recovered by treatment of the inactivated enzyme with hydroxylamine. The difference spectrum of the inactivated and native enzyme reveals a single peak at 242 nm but no trough at around 278 nm is observed. Complete inactivation required the modification of four histidine residues per molecule of the enzyme. However, statistical analysis of the residual activity and the extent of modification shows that among the four modifiable residues, only one is critical for activity. Furthermore, this inactivation is prevented by the substrates of the enzyme. The above results indicated that one histidine is located within or very close to the active site and may play an important role in catalysis.