Journal of Student Research 2017

Journal Student Research

130

Figure 1.

N

N

Fe

N

N

N

N

Figure 1. Native HRP heme group with distal histidine sidechain

depicted.

Catalytic Cycle

HRP’s catalytic cycle is a three step mechanism consisting of the native HRP (Fe (III)), compound I and compound II (Figure 2) 4 . The first step in the catalytic cycle is the formation of compound I by the cleavage of H ² O ² and the release of water. This reaction increases the oxidation state of the enzyme by two equivalents. Compound I contains an iron-oxo group (Fe+4=O) and a porphyrin -cation radical 4 . This highly oxidized species oxidizes organic substrates, such as OIV, by a 1 electron transfer producing an organic radical intermediate and the second enzyme intermediate, compound II 4 . Compound II contains an oxoferryl group4 and is one oxidizing equivalent above the resting enzyme. A second organic substrate molecule is subsequently oxidized by a second 1 electron transfer reaction, thus reducing the enzyme back to its resting Fe(III)-heme form 4,8 and releasing a second water molecule. In the enzyme catalytic cycle, reduction of compound II is the slow step due its lower redox potential 6,7 . Figure 2

O

Native HRP (ground state)

N +

N

N

N

O H

H O O H

2

Fe

Fe

N

N

N

N

N

N

Compound I

N

N

AH

+

3 +

O H

A ·

O

A ·

N

N

Fe

AH

N

N

H +

N

Compound II

N