Agriculture Reference
In-Depth Information
The six electron reduction of sulfite to sulfide is catalysed by SiR in plastids.
Plant SiR is a 65 kDa monomer and requires the presence of a siroheme and an FeS
cluster as cofactors, and ferredoxin as an electron donor (Nakayama et al.
2000
). In
contrast to other enzymes of the sulfur metabolism pathway, SiR is encoded by
single gene in
Arabidopsis
. The amino acid sequence and protein structure are very
similar to nitrite reductase (NiR) which catalyses a six electron reduction of nitrite
to ammonia in nitrate assimilation pathway. The 19 % amino acid sequence identity
suggests that these two enzymes have the same evolutionary origin.
Only recently a new enzyme in the sulfate assimilation pathway was identified
(Eilers et al.
2001
):
Arabidopsis
sulfite oxidase (SO; EC: 1.8.3.1) catalyses the
oxidation of sulfite to sulfate in a two electron reaction. It is localised in peroxi-
somes (Nowak et al.
2004
). Plant SO lacks the haem domain, which is known from
animal SO
s but it contains a molybdenum cofactor-binding domain. It was shown
that the terminal electron acceptor for plant SO is molecular oxygen, the reaction
converting O
2
into hydrogen peroxide. H¨nsch et al. (
2006
) also provided evidence
that hydrogen peroxide can oxidise sulfite non-enzymatically. It was shown that SO
can protect plants from an excess of sulfite which is toxic in high concentrations,
and SO
2
gas in the atmosphere (Brychkova et al.
2007
). However, the exact role of
SO in sulfur primary metabolism is still not clear.
'
Cysteine Biosynthesis
Cysteine is the key sulfur-containing compound in plants. It is synthesised by
incorporation of sulfide into the
-position of the serine carbon skeleton in the
terminal step of sulfur assimilation (Saito
2004
). Before the incorporation of
sulfide, serine needs to be activated to
O
-acetylserine (OAS). This process occurs
by acetyl transfer from acetyl coenzyme A, which is catalysed by serine
acetyltransferase (SAT; Serat; EC 2.1.3.30). Subsequently, OAS and sulfide are
the substrates for O-acetylserine (thiol) lyase (OAS-TL; EC: 2.5.1.47) which
catalyses the
ʲ
-replacement reaction (Hell and Wirtz
2011
; Takahashi
et al.
2011
). The enzymes involved in the cysteine biosynthesis process, SAT and
OAS-TL, are localised in plastids, mitochondria and cytosol (Saito
2000
; Fig.
3.3
).
Additionally,
ʲ
-cyanoalanine synthase was identified in mitochondria, which has a
similar catalytic activity to OAS-TL, and was previously identified as its mitochon-
drial form (Hatzfeld et al.
2000b
). This enzyme is important for detoxification of
cyanide produced e.g. during ethylene synthesis (Garcia et al.
2010
). The analysis
of whole plant protein extracts showed that entire SAT activity is always associated
with OAS-TL, and that an excess of free active OAS-TL is present (Hell and Wirtz
2011
). This and other results indicate that these two enzymes are associated and
form a hetero-oligomeric cysteine synthase complex (Hell and Wirtz
2008
,
2011
;
Saito
2004
; Takahashi et al.
2011
; Wirtz et al.
2001
; Wirtz and Hell
2006
). The
binding of OAS-TL to SAT stabilises SAT. In the complex, SAT is the only active
enzyme. The product OAS causes the release of OAS-TL from the complex which
ʲ
