Agriculture Reference
In-Depth Information
step APR transfers two electrons to APS to produce sulfite; the electrons are derived
from GSH (Bick et al.
1998
). Subsequently, sulfite is reduced to sulfide by
ferredoxin-dependent sulfite reductase (SiR; EC: 1.8.7.1); this reaction requires a
transfer of six electrons from ferredoxin to sulfite. Sulfide is then incorporated into
the amino acid skeleton of
O
-acetylserine (OAS) to form cysteine. This reaction is
catalysed by OAS thiol-lyase (OAS-TL; EC: 2.5.1.47) (Kopriva
2006
; Leustek
et al.
2000
; Takahashi et al.
2011
).
APR is a key enzyme of the sulfur assimilation pathway. It is regulated by
various environmental factors and signalling molecules (Kopriva
2006
; Koprivova
et al.
2008
). Similarly to ATPS, APR is encoded by a small multigene family and
three isoforms are known in
Arabidopsis
. It was shown in various studies that APR1
and APR3 are co-regulated and share the highest sequence similarity. However,
APR2 responds differently to hormone treatments (Koprivova et al.
2008
) which
indicates specific functions of particular APR isoforms. The amino acid sequence of
APR suggests a multi-domain structure. Its precursor is synthesised with an
N-terminal plastid transit peptide. In the mature protein the N-terminal domain is
similar in predicted amino acid sequence to PAPS reductase (PAPR) from bacteria,
and the C-terminal domain with thioredoxin (Trx). Due to the lack of the C-terminal
domain, PAPR requires thioredoxin or glutaredoxin as a cofactor (Kopriva
et al.
2007
). This may suggest the role of C-terminal domain as redox cofactor
(Leustek et al.
2000
).
Arabidopsis
APR is a dimer of 45 kDa subunits (Kopriva and
Koprivova
2004
) binding a [Fe
4
S
4
] iron-sulfur cluster (Kopriva et al.
2001
). PAPR
consist of two 28 kDa subunits without any prostetic groups. A single conserved
cysteine residue is responsible for its activity and dimerization.
APK catalyses the transfer of phosphate from ATP to APS to form PAPS which
is a sulfate donor for sulfotransferases. Four genes coding APK are found in the
Arabidopsis
genome, all located on different chromosomes and with a high level of
similarity. Three of the isoforms contain chloroplast transit peptides at the
N-termini and these have been confirmed to be localised in plastids (Mugford
et al.
2009
). The APK3 isoform does not contain the N-terminal extension and it
is likely to be responsible for cytosolic activity. Little is known about the biochem-
istry and the functions of the individual plant APKs. However, they have a
significant effect on sulfur metabolism. It was shown that
apk1apk2
double mutant
has a dramatically low glucosinolate level and also substantially higher levels of
cysteine and GHS than wild type plants (Mugford et al.
2009
). This suggests a
compensation of low glucosinolate level by increases in cysteine and GSH. This
subsequently may indicate that primary sulfate metabolism is up-regulated in this
mutant, implying an important role of APK in controlling sulfur distribution in
plants. Plants with APK1 as the only active APK isoform showed the wild type
phenotype suggesting the major contribution of this isoform to total enzyme
activity (Mugford et al.
2010
). The analysis of mutants lacking various APR
isoforms and differences in tissue-specific expression between the isoforms indicate
specific roles of particular isoforms in plant sulfur metabolism (Kopriva et al.
2012
;
Mugford et al.
2009
).
