General Features of the Enzymes

The resting state of HRP C is high-spin Fe(III) (d ) with a magnetic susceptibility of 5.23 Bohr magnetons (96). However, it cannot be described adequately by the spin quantum number S =, which would correspond to a high-spin species with five unpaired d electrons and a 

The coordination- and spin state of HRP C are dependent on pH, although little change occurs between pH 4 and pH 10. At pH 3.1, the resting state enz5mie is a 5-c HS form in which His 170 has been replaced as ligand by a water molecule (104). The scission of the Fe-His bond occurs from a 6-c intermediate with a water molecule boimd at the distal site. At high pH values, a transition to an alkaline form occurs 

The majority of reactions catalyzed by horseradish peroxidase can be summarized by Eq. (1), in which AH2 represents a reducing substrate. 

Expression of peroxidases that catalyze crosslinking reactions can he induced by wounding or infection. Well-documented examples include the production of acidic peroxidase isoenzymes in woimded tissue of tobacco (132), potato (133), and tomato (134), and also in wheat plants infected with the powdery mildew fungus, Erysiphe graminis (135). It is known that peroxidase activity can be induced on wounding of horseradish leaves (56), but other mechanisms of defense in horseradish remain to be demonstrated. 

The ability of HRP to degrade the plant hormone indole-3-acetic acid (lAA) in the absence of hydrogen peroxide was noted as early as 1955 (136). Plant peroxidases are now known to be of major importance in the metabolism of lAA (137) (note that they are often referred to as indole acetic acid oxidases in the older literature). The mechanism of lAA oxidation by HRP C is complex and has been studied experimentally in great detail by several groups (23, 137). Reaction products include indole-3-methanol, indole-3-aldehyde, and 3-methylene oxin-dole, which is probably a nonenzymatic conversion product of indole-3-methylhydroperoxide. The most important developments in this area have been reviewed (23). 

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