Catalytic pathways of Euphorbia characias peroxidase reacting with hydrogen peroxide.
The reaction of Euphorbia characias latex peroxidase (ELP) with hydrogen peroxide as the sole substrate was studied by conventional and stopped-flow spectrophotometry. The reaction mechanism occurs via three distinct pathways. In the first (pathway I), ELP shows catalase-like activity: H2O2 oxidizes the native enzyme to compound I and subsequently acts as a reducing substrate, again converting compound I to the resting ferric enzyme. In the presence of an excess of hydrogen peroxide, compound I is still formed and further reacts in two other pathways. In pathway II, compound I initiates a series of cyclic reactions leading to the formation of compound II and compound III, and then returns to the native resting state. In pathway III, the enzyme is inactivated and compound I is converted into a bleached inactive species; this reaction proceeds faster in samples illuminated with bright white light, demonstrating that at least one of the intermediates is photosensitive. Calcium ions decrease the rate of pathway I and accelerate the rate of pathways II and III. Moreover, in the presence of calcium the inactive stable verdohemochrome P670 species accumulates. Thus, Ca2+ ions seem to be the key for all catalytic pathways of Euphorbia peroxidase.
Mura
Anna
A
Department of Applied Sciences in Biosystems, University of Cagliari, I-09042 Monserrato, Italy.
Pintus
Francesca
F
Lai
Paola
P
Padiglia
Alessandra
A
Bellelli
Andrea
A
Floris
Giovanni
G
Medda
Rosaria
R
D002118:Calcium / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D028481:Euphorbia / Q000201:enzymology* / Q000378:metabolism; D006418:Heme / Q000737:chemistry; D006735:Horseradish Peroxidase / Q000737:chemistry / Q000378:metabolism; D006861:Hydrogen Peroxide / Q000737:chemistry*; D007700:Kinetics; D008956:Models, Chemical; D010084:Oxidation-Reduction; D009195:Peroxidase / Q000737:chemistry* / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D013053:Spectrophotometry; D013997:Time Factors
D006418:Heme; D006861:Hydrogen Peroxide; D006735:Horseradish Peroxidase; D009195:Peroxidase; D002118:Calcium