A retrospective look at the cationic peanut peroxidase structure

Zo-peroxidase: Crystal structure and sequence of a highly-glycosylated peroxidase resistant to high concentrations of H2O2 from Japanese radish

Understanding Peroxidase (PRXs) enzymatic diversity and functional significance from a three-dimensional point of view is a key point for structural and mechanistic studies. In this context, Zo-peroxidase (ZoPrx) a member of the class III peroxidases and secreted by plants, differs from all previously described PRXs because of its remarkable catalytic stability in the presence of hydrogen peroxide. In this work, we present the crystallographic structure of ZoPrx isolated from Japanese radish, at 2.05 Å resolution. The mature enzyme consists of a single monomer of 308 residues exhibiting the same fold as all previously described members of the plant PRXs superfamily. Furthermore, the enzyme contains a heme b group as the prosthetic group and two Ca²⁺ binding sites. Moreover, seven N-glycosylation sites were found in the structure, and 49 glycans bound to the two ZoPrx molecules found in the asymmetric unit are clearly visible in the electron density map. The comparison of ZoPrx coordinates with homologous enzymes revealed minor structural changes, in which the residue 177 appears to be responsible for enlarging the access to the heme cavity, the only structural finding which may be related to the H₂O₂ tolerance of ZoPrx and detected by X-ray crystallography. Because of its characteristics, ZoPrx has a broad range of potential applications from chemical synthesis to environmental biocatalysis, thus its aminoacidic sequence, partially completed using the electron density, and the three-dimensional structure itself, become a possible starting point to engineering heme-peroxidases to enhance oxidative stability.

A role of glycosyl moieties in the stabilization of bitter gourd (Momordica charantia) peroxidase

The possible role of carbohydrate moieties in the stabilization of proteins has been investigated by using bitter gourd peroxidase as a model system. A comparative study of glycosylated and non-glycosylated isoenzymes of bitter gourd peroxidase was performed at various temperatures, pH, water-miscible organic solvents, detergents and chaotropic agent like urea. The pH-optima and temperature-optima of both glycosylated and non-glycosylated isoforms of bitter gourd peroxidase remained unchanged. The probes employed were changes in the enzyme activity and fluorescence. The glycosylated form of peroxidase retained greater fraction of enzyme activity against the exposure caused by various physical and chemical denaturants. The unfolding of both forms of enzyme in the presence of high urea concentrations, studied by fluorescence, indicated greater perturbations in the conformation of non-glycosylated preparation. The different properties examined thus indicated that glycosylation plays an important role in the stabilization of native conformation of proteins against the inactivation caused by various types of denaturants.

Purification and partial characterization of broccoli (Brassica oleracea Var. Italica) peroxidases

Three peroxidase (POD) isoenzymes were purified from a soluble extract of broccoli stems. The acidic and neutral PODs were purified to homogeneity by using ion exchange and hydrophobic chromatography. The basic POD was purified by cation exchange and gel filtration chromatography. The neutral and basic PODs had molecular masses of approximately 43 kDa, and the acidic POD had a molecular mass of 48 kDa by SDS-PAGE. pI was approximately 4, 5, and 8 for acidic, neutral, and basic PODs, respectively. Optimum activity using guaiacol as the H donor was obtained at pH approximately 6 for both neutral and basic PODs and at pH approximately 4 for acidic POD. All three of the purified isoenzymes are glycosylated. Reaction rates with various substrates including guaiacol, guaiacol/MBTH, DMAB/MBTH, and ferulic acid/MBTH were different among the isoenzymes. K(m) and amino acid composition were also determined.

Heat inactivation and reactivation of broccoli peroxidase

Heat inactivation characteristics differed for acidic (A), neutral (N), and basic (B) broccoli peroxidase. At 65 degrees C, A was the most heat stable followed by N and B. The activation energies for denaturation were 388, 189, and 269 kJ/mol for A, N, and B, respectively. Reactivation of N occurred rapidly, within 10 min after the heated enzyme was cooled and incubated at room temperature. The extent of reactivation varied from 0 to 50% depending on the isoenzyme and heating conditions (temperature and time). The denaturation temperature allowing the maximum reactivation was 90 degrees C for A and horseradish peroxidase (HRP) and 70 and 80 degrees C for B and N, respectively. In all cases, heat treatment at low temperatures for long times prevented reactivation of the heated enzymes. Calcium (5 mM) increased the thermal stability of N and B but had no effect on reactivation. The presence of 0.05% bovine serum albumin decreased thermal stability but increased the extent of reactivation of A..

Investigation of cationic peanut peroxidase glycans by electrospray ionization mass spectrometry

Cationic peanut peroxidase (CP) was isolated from peanut (Arachis hypogaea) cell suspension culture medium. CP is a glycoprotein with three N-linked glycan sites at Asn60, Asn144, and Asn185. ESI-MS of the intact purified protein reveals the microheterogeneity of the glycans. Tryptic digestion of CP gave a near complete sequence coverage by ESI-MS. The glycopeptides from the tryptic digestion were separated by RP HPLC identified by ESI-MS and the structure of the glycan chains determined by ESI-MS/MS. The glycans are large structures of up to 16 sugars, but most of their non-reducing ends have been modified giving a mixture of shorter chains at each site. Good agreement was found with the one glycan previously analyzed by (1)H NMR. This work is the basis for the future studies on the role of the glycans on stability and folding of CP and is another example of a detailed structural characterization of complex glycoproteins by mass spectrometry.