There are a number of ways of classifying enzymes. One method of enzyme classification, as mentioned earlier, is by grouping enzymes according to the reactions they catalyze. In this type of classification scheme, peroxidases are grouped under the oxidoreductase class - those which catalyze oxidation-reduction reactions - and are given the EC number 1.11.1.7.
Another method of classification is by studying the structure of proteins and coming up with a classification hierarchy according to structure similarities. However, since enzyme structure is a bit complex, there are many ways of grouping enzymes by structure. Several databases on classification by structure are available on the internet. The most well known of these include SCOP, CATH, and FSSP.
Here are some general observations that are common to most of the attempts at structural classification of peroxidases:
Primary structure
The number of amino acids differs from one peroxidase chain to another. The
Horseradish Peroxidase (HRP) monomer on
the left
consists of 306 amino acids and one heme
group
.
Secondary structure
Most peroxidase monomers consist mostly of alpha-helices
(magenta) . A typical chain would consist
of 10 alpha helical segements,
( yellow). In plant peroxidases, three
of these are the most conserved
(green). For HRP, there are 3
additional helices present
(red).
Tertiary Structure
The HRP monomer shown on the right actually contains two domains
, typical of most peroxidases. Two anti-parallel helices from the two
domains form a crevice in which the heme group is inserted
(domain 1 - red; domain 2 - blue).
Quaternary Structure
The number of subunits or chains depends on the peroxidase. The HRP isozyme
on the right consists of 6 identical chains or monomers
. Each single unit contains a heme group
.