Biology Reference
In-Depth Information
enzymes seem to be more widespread among microorganisms (as discussed in
the following section about genome analysis tools).
1. Lignin peroxidases
LiPs, first discovered in P. chrysosporium in the mid-1980s, are considered
as true ligninases as they directly catalyse lignin oxidation. These proteins
form a family of extracellular enzymes that are glycosylated (up to 20-30%),
monomeric and contain one ferric protoheme IX per molecule. Several iso-
forms and isoenzymes are present in fungal secretomes. LiPs have a molecu-
lar mass ranging between 35 and 48 kDa and an isoelectric point (pI) between
3.1 and 4.7.
The crystal structure of the LiP isozyme H
2
of P. chrysosporium was the
first to be solved (
Piontek et al., 1993; Poulos et al., 1993
), and showed an
organization in two domains essentially constituted of
a
-helices. The active
site of LiP is located at the centre of these two domains and is composed of a
heme-containing one iron atom. The iron atom is five-coordinated to the
four nitrogen atoms of the tetrapyrrolic structure and to the nitrogen of a
histidine residue. Two calcium ions (Ca
2
þ
) and four disulfide bonds stabilize
the three-dimensional structure. A post-translational modification resulting
in a hydroxylated residue (Trp171), always conserved in LiP sequences,
appears to be essential for the catalytic activity of these enzymes.
LiPs catalyse the monoelectronic and H
2
O
2
-dependent oxidation of a wide
variety of aromatic compounds through a multistep reaction. These reactions
induce the formation of aryl cationic radicals, which further undergo many
non-enzymatic reactions, generating a number of end products such as glyco-
late and oxalate. The enzyme interacts with its substrate by a 'ping-pong'
mechanism. Both the catalytic cycle (
Fig. 2
A) and the enzymatic intermedi-
aries are similar to those of the other peroxidases. Native LiPs are oxidized by
H
2
O
2
to give a first reaction intermediate, compound I, in which the iron is
present in an oxyferryl formO
Fe(IV), and a free radical cation which can be
located either on the heme or on a protein residue, depending on the isoenzyme
(O
¼¼
P
.
þ
). Compound I then oxidizes a first substrate molecule form-
ing compound II (O
Fe
4
þ
¼¼
Fe
4
þ
P) in which the iron is still present as Fe(IV),
and a substrate radical. Compound II oxidizes a second substrate molecule to
obtain the native form of LiP (Fe
3
þ
¼¼
P) and a second substrate radical. In
severe oxidative conditions (excess H
2
O
2
and absence of reducing substrate),
compound II reacts withH
2
O
2
to form a ferric-superoxo complex (Fe(III)O
2
.
),
a catalytically inactive form of the enzyme, named compound III.
The redox potential of LiPs, around 1.2 V at pH 3.0 (
Wong, 2008
), makes
these enzymes able to oxidize a broader range of substrates than other
peroxidases. Phenolic substrates are converted to phenoxy radicals, leading
