Biology Reference
In-Depth Information
horseradish peroxidase (HRP) but the dissociation constants of ferrous dioxygen are higher in case
of Tyr249Phe (129µM) when compared to wild-type KatG (84 µM) and HRP (1.9 µM). The conversion
of compound II to compound III is very much faster in case of variant Tyr249Phe than in case of
HRP and other haem peroxidases.
In monofunctional catalases H
2
O
2
oxidation and formation of O
2
occurs in two steps. In the fi rst
step, one molecule of H
2
O
2
is oxidized by the catalase enzyme to form an intermediate compound I
[oxoiron (IV) Porphyrin π-cation radical intermediate] which oxidizes another molecule of H
2
O
2
to
O
2
in the second step. The redox intermediates during catalase cycle of KatGs from different sources
(
Synechocystis
sp. PCC 6803,
Burkholderia pseudomallei
and
Mycobacterium tuberculosis
) have been
determined by the oxidation of of peroxoacetic acid and H
2
O
2
. The intermediates with different spectral
features differed among the three KatGs (Jakopitsch
et al
., 2007). Employing gas chromatography-mass
spectrometry, O
2
evolved from a 50:50 mixture of H
2
18
O
2
and H
2
16
O
2
by KatGs from
Synechocystis
sp.
strain PCC 6803 and
M
.
tuberculosis
was shown by two electron oxidation without breaking the O-O
bond. The formation of
18
O
2
and
16
O
2
is indicative of two electron oxidation because O
2
evolution as
16
O
18
O has not been detected. This has been found to be the case in case of KatG variants (D152S,
H123E, W122F, Y249F and R439A) that exhibited a very low catalase activity (Vlasits
et al
., 2007).
These results thus support the conclusions drawn earlier on monofunctional catalases (Jarnagin and
Wang, 1958; Kato
et al
., 2004).
The kinetics of H
2
O
2
oxidation by wild-type KatG and its variants
(Trp122Phe, Asp152Ser and Glu253Gln) of
Synechocystis
sp. strain PCC 6803 have been probed by
sequential-mixing stopped-fl ow spectroscopy (Vlasits
et al
., 2010).
A molecular peculiarity of catalase-peroxidases is the presence of three large insertions in their
catalytic domain, two of which are typical for KatGs. A gene duplication event in the ancestral
hydroperoxidase gene may be responsible for the presence of an extra C-terminal copy. Both the
N-terminal and C-terminal copies orient themselves and facilitate the functioning of both the haem
group and essential amino acid residues at the active site (Zámocký
et al
., 2001). Phylogenetic
relationships in class I superfamily of bacterial, fungal and plant peroxidases (60 known sequences)
possessing prosthetic haem group, peroxidase motif and peroxidatic activity, revealed the existence of
four clearly separated clades, i.e. C-terminal and N-terminal domains of catalases-peroxidases, Apxs
and CcPs. Evidences are presented in favour of the gene duplication event in catalse-peroxidases
having occurred in the later phase of evolution after the formation of individual peroxidase families
(Zámocký, 2004). An overview of the evolution of catalases from bacteria to humans covering
their distribution, phylogeny structure and function of these enzymes has been presented. Their
physiological response to oxidative stress and diseases resulting due to defi ciency of catalases in
human beings has been highlighted (Zámocký
et al
., 2008). Evolution of structure and function of
class I peroxidases has been studied by analyzing over 940 sequences in all available genomes of
prokaryotes and eukaryotes. Phylogenetic analysis of 193 class I peroxidases showed the presence
of two new well separated clades of hybrid peroxidases that might represent evolutionary bridges
between catalase-peroxidases and CcPs (typeA) as well as between APxs (Zámocký
et al
., 2010).
H
2
O
2
as a signalling molecule
:
H
2
O
2
has emerged as a signalling molecule that infl uences the
transcription by changing the expression of hundreds of genes. Besides affecting the genes involved
in ROS detoxifi cation, the range of cellular processes infl uenced by H
2
O
2
signalling encompass signal
transduction, transcriptional regulation and the metabolism of proteins, carbohydrates and lipids
(Stone and Yang, 2006; Vandenbroucke
et al
., 2008). In
E
.
coli
, transcription factors SoxR and OxyR
respond to O
2
•-
and H
2
O
2
stress, respectively. The reaction of OxyR with H
2
O
2
causes the formation
of an intramolecular disulfi de bond that brings about a conformational change and makes it active
