Horseradish peroxidase isozyme C. A comparative study of native and recombinant enzyme produced by E. coli transformants

Mediatorless biosensor for H(2)O(2) based on recombinant forms of horseradish peroxidase directly adsorbed on polycrystalline gold

Four forms of horseradish peroxidase (HRP) have been used to prepare peroxidase-modified gold electrodes for mediatorless detection of peroxide: native HRP, wild type recombinant HRP, and two recombinant forms containing six-His tag at the C-terminus and at the N-terminus, respectively. The adsorption of the enzyme molecules on gold was studied by direct mass measurements with electrochemical quartz crystal microbalance. All the forms of HRP formed a monolayer coverage of the enzyme on the gold surface. However, only gold electrodes with adsorbed recombinant HRP forms exhibited high and stable current response to H(2)O(2) due to its bioelectrocatalytic reduction based on direct electron transfer between gold and HRP. The sensitivity of the gold electrodes modified with recombinant HRPs was in the range of 1.4-1.5 A M(-1) cm(-2) at -50 mV versus Agmid R:AgCl. The response to H(2)O(2) in the concentration range 0.1-40 microM was not dependent on the presence of a mediator (i.e. catechol) giving strong evidence that the electrode currents are diffusion limited. Lower detection limit for H(2)O(2) detection was 10 nM at the electrodes modified with recombinant HRPs.

A genetically engineered fusion protein with horseradish peroxidase as a marker enzyme for use in competitive immunoassays

Horseradish peroxidase is one of the most widely used marker enzymes in immunoassays. Several disadvantages are encountered upon chemical conjugation of peroxidase with antibodies or antigens, as are low reproducibility and undefined stoichiometry. We here describe for the first time the production of a recombinant fusion of a protein analyte with horseradish peroxidase in Escherichia coli, employing refolding of inclusion bodies and reconstitution with heme. The genetic fusion approach enables preparation of conjugates with 1:1 stoichiometry and defined structure. As a protein analyte, the human heart fatty acid binding protein (H-FABP) was chosen, which is a new and sensitive marker for acute myocardial infarction. The recombinant conjugate was fully active [650 U/mg with 2,2-azino-bis(3-ethyl-thiazoline-6-sulfonate) as substrate] and obtained in a yield of 12 mg/L of E. coli culture, which is better than that for recombinant peroxidase alone. The competitive immunoassay that was developed with the recombinant conjugate requires fewer incubation steps than the traditional sandwich ELISA format. It permitted the detection of H-FABP directly in plasma in the range of 10-1500 ng/mL which is the relevant range for clinical decision-making.

Tryptophanless recombinant horseradish peroxidase: stability and catalytic properties

The tryptophanless mutant of horseradish peroxidase, W117F, has been constructed and expressed in Escherichia coli. The mutation affects enzyme folding and stability. The optimum composition of the refolding medium requires the presence of ammonium sulfate. The yield of mutant is ca. 8000 U per liter of the optimized refolding medium with the specific activity of 1100-1500 U/mg (compared to 25, 000 U per liter and 2000 U/mg for the recombinant wild-type enzyme). The mutant is more stable in acid media, in the reaction course and toward irradiation. The effect of hydrogen peroxide pretreatment on radiation-induced inactivation of the wild-type and mutant enzyme indirectly indicates participation of Trp-117 in electron transfer pathways through the enzyme molecule. This is in agreement with the steady-state kinetic data interpreted in terms of Trp-117 participation in electron transfer within the Michaelis complex.

Wild-type and mutant forms of recombinant horseradish peroxidase C expressed in Escherichia coli. Substrate specificity and stability under irradiation

Two horseradish peroxidase C (HRPC) mutants with substitutions in the active center, i.e., Phe41-->His and Phe143-->Glu, were compared to the wild-type recombinant enzyme expressed in Escherichia coli in terms of the enzymatic activity and stability under irradiation. Both mutations caused a significant decrease in activity, but it was still possible to follow the effect of mutations on the key steps of the reaction mechanism. Phe41 can be considered a nonpolar barrier separating histidine residues in the active center and providing a firm noncovalent binding with the highly hydrophobic porphyrin ring. The replacement of Phe41 with the ionizable His residue destabilizes the enzyme. The Phe143-->Glu replacement creates a negative charge at the entrance of the heme-binding pocket, and protects the latter from both donor substrates and free radicals. The radiolytic inactivation of the wild-type and mutant forms of recombinant HRP suggested different binding sites for iodide, 2,2'-bis(3-ethylbenzothiasoline-6-sulfonate (ABTS), guaiacol, and O-phenylene diamine. The study of kinetics and inactivation is in agreement with the direct binding of iodide to the heme porphyrin ring. The results also suggest that the ABTS binding site is less accessible than that for O-phenylene diamine.

Effect of single-point mutations Phe41-->His and Phe143-->Glu on folding and catalytic properties of recombinant horseradish peroxidase expressed in E. coli

Wild-type recombinant and Phe41-->His and Phe143-->Glu mutant forms of horseradish peroxidase have been expressed in E. coli and reactivated from inclusion bodies with a yield of about 25%. The purified homogeneous preparations have been studied in the reaction of ABTS oxidation. The effect of mutations on heme entrapment and kinetics of ABTS oxidation demonstrates the essential role of the replaced residues in providing the hydrophobic crevice for the non-covalent heme binding.