Introduction

Toxic molecules such as superoxide and hydroxide radicals can be found in cells due to the presence of oxygen. These are byproducts of aerobic respiration. They are eliminated by a number of enzymes present inside the cell. Superoxide, for example, is destroyed by superoxide dismutase. The degradation, however, produces more hydrogen peroxide (H2O2), which is, in turn, destroyed by peroxidase. Peroxidases reduce H2O2 to water while oxidizing a variety of substrates. Thus, peroxidases are oxidoreductases which use H2O2 as electron acceptor for catalyzing different oxidative reactions. The overall reaction is as follows:

Some enzymes require the presence of certain molecules, which are not part of their primary structure or amino acid sequence, for their enzymatic activity. These molecules are called cofactors. In peroxidase, the bound cofactor necessary for its activity is heme. Heme is a complex between an iron ionand the molecule protoporphyrin IX.

Different types or species of peroxidases have been found and studied. Structure and sequence data of these can be found and downloaded at the RCSB Protein Data Bank. Experiments have shown that although peroxidases share a similar general structure, there are also a number of marked differences. The enzyme displayed on the right is one of the seven peroxidase isozymes extracted from Horseradish roots (Amoracia Rusticana). Its PDB ID code is 1ATJ. An introduction to the Protein Data Bank can be viewed by clicking here.