Biology Reference
In-Depth Information
iodide antioxidant system because of their inefficient catalysis toward iodide (see
below) as well as their various locations in the cell (Colin et al.
2003
). Numerous
studies did demonstrate that iodide-oxidizing enzymes, iodoperoxidases (vIPOs),
play the central role in the iodine metabolism rather than vBrPOs. These
homodimeric enzymes being encoded by a multigenic family are responsible for
both iodide uptake and ROS scavenging. They have a strict specificity to iodide
because of their special halide binding site topology and the fine-tuning of the
vanadate cofactor electronegativity (Colin et al.
2005
; Pacios and G´lvez
2010
).
Iodoperoxidases have a lower affinity to iodide (Km,I
-
¼
2.5 mM) and, more
important, a higher catalytic turnover rate of iodide (kcat,I
-
¼
462 s
1
) than
vBrPOs (Km,I
-
38 s
1
). Moreover, a seven times higher
specific activity toward iodide of the purified iodoperoxidases compared to vBrPOs
(cf. 1,200 vs. 180 U mg
1
) represents a high efficiency of iodide incorporation and
the crucial role in iodine metabolism (Colin et al.
2003
).
When
L. digitata
is unstressed and submerged during high tide, iodide is taken
up from seawater (0.3
18.1 mM, kcat,I
-
¼
¼
M) probably by facilitated diffusion down its concentration
gradient to the apoplast (K
m
upper et al.
1998
). There, the iodoperoxidases mediates
the oxidation of iodide. However, this process requires a steady flow of H
2
O
2
into
the apoplast, which was estimated to as small as approx. 5
€
upper et al.
1998
).
So far, the source of H
2
O
2
is not absolutely clear yet, but it is supposed to be
produced by cell wall oxidases or membrane-bound enzymes with an extracellular
domain. Intracellular sources of H
2
O
2
might also be responsible for the functioning
of the iodoperoxidase. However, it lacks detailed studies, which investigate the role
of vIPOs under, for example, high light conditions when H
2
O
2
is produced by APX
or the photorespiratory pathway in response to excessively absorbed light energy.
Depending on the age of the sporophyte and the regions of the thallus, iodide
may accumulate between 13,000 and even 150,000-fold relative to natural seawater
in the apoplast of
L. digitata
(K
M(K
m
€
upper et al.
1998
). In contrast to these vast pools,
iodide contents in related
Ectocarpus
and
Fucus
species are only a fraction of that
one found in Laminariales (Saenko et al.
1978
; Cock et al.
2010
). This substantial
difference can be ascribed to the lack of iodoperoxidases in species of these genera.
Only bromoperoxidases being characterized by their substrate preference to bro-
mide over iodide fix comparable small amounts of iodide. This is supported by the
recent analysis of the genome sequence of
Ectocarpus siliculosus
, which revealed
that only one haloperoxidase, a vBrPO, is encoded (Cock et al.
2010
).
In the apoplast, iodide is non-covalently associated with biomolecules such as
phenolic compounds, polysaccharides, and proteins (Verhaeghe et al.
2008
;K
€
upper
et al.
2008
). This unique type of accumulation ensures that iodide can be readily and
rapidly mobilized for chemical defence and under oxidative stress conditions during
high tide (Verhaeghe et al.
2008
). It was demonstrated by extracellular addition of
elicitors (e.g., oligoguluronates) mimicking pathogen attack and oxidants (e.g.,
H
2
O
2
)to
L. digitata
that iodide is released into the seawater within seconds after
addition. This response represents a very fast mechanism to acclimate to an external
abiotic and biotic stressor. However, because these high iodide concentrations
are not directly toxic to both pro- and eukaryotes (K
€
€
upper et al.
2008
), iodocarbons
