STRESS RESPONSES - Cyanobacteria - page 695

PCC 73102 is exceptional because it is absent in most of the nitrogen-fi xing species examined
[ Cyanothece CCY 0110, T . erythraeum IMS101, A . variabilis ATCC 29413, No . spumigena CCY 9414
and Anabaena sp. strain PCC 7120] as also in M . aeruginosa NIES-843, T . elongatus BP-1, A . marina
MBIC11017, strains of Cyanothece sp. (ATCC 51142, PCC 7424) and Lyngbya sp. PCC 8106 (Bernroitner
et al ., 2009).
The Prxs rely on the sulphur atom of a conserved cysteine residue termed as the peroxidative Cys
(Cys P ) that helps in the cleavage of the peroxyl -O-OH bond. The second cysteine (if present) is known
as the resolving cysteine (Cys R ) residue. It is on this basis that the Prxs were originally grouped into
two subfamilies: 2-Cys Prx and 1-Cys Prx. On the basis of the catalytic mechanism, the 2-Cys Prx are
divided into 'typical' which possess the Cys P and Cys R residues and form homodimers through the
intersubunit disulfi de bond and 'atypical' which possess one Cys P and form intramolecular cysteine
in the same polypeptide. The basic catalytic mechanism in all Prxs is suggested to take place in two
steps. In the fi rst step, the activated cysteine (Cys P -SH) is oxidized by a peroxide substrate to sulphenic
acid (Cys P -SOH) with concurrent reduction of the peroxide (ROH). In the second step, the sulphenic
acid then reacts with the sulphydryl group of Cys R forming an intersubunit disulphide bond and
simultaneously releasing water. The regeneration of oxidized catalytic thiol depends on glutathione
(R'SH) or thioredoxin, glutaredoxin, cyclophilin or tryperodoxin (Wood et al ., 2003; Stork et al ., 2005).
Prxs are effi cient scavengers of very low concentrations of H 2 O 2 . Substrates include H 2 O 2 , alkyl
hydroperoxides and peroxynitrite. Prxs are important components of antioxidant defence system
and also play a signifi cant role in redox-mediated signal transduction at least in some eukaryotes
(Dietz et al ., 2003; Veal et al ., 2004; Dietz, 2011). These reactions have been best explained in case of
2-Cys Prxs which carry out a cycle of three distinct redox transformations within a single active site.
Thus Cys P goes through three different stages in the peroxidatic cycle: sulphydryl (-2) - sulphenic
acid (O) - disulphide(-1) - sulphydryl (-2) (Aran et al ., 2009).
In some non-photosynthetic bacteria AhpC, the 2-Cys Prx gets reducing power from a
fl avoprotein AhpF, an NADPH disulphide oxidoreductase (Poole, 2005). In plants, the chloroplast
2-Cys Prx is reduced by electrons received from CDSP32, a protein containing two Trx domains
in tandem (Tel-Or et al ., 1985). The homologues of AhpF and CDSP32 are absent in cyanobacterial
genomes. The regeneration of oxidized form of 2-Cys Prx in eukaryotes is known to occur through
an electron transfer from NADPH via NADPH: thioredoxin oxidoreductase (NTR) and a Trx together
in a single protein (Pérez-Ruiz et al ., 2006). Only in case of Anabaena sp. strain PCC 7120 genome
an NTR sequence has been reported (Florencio et al ., 2006). Cyanobacterial 2-Cys Prxs are also
known as thioredoxin peroxidases because they use Trx as electron donor to reduce peroxides. The
regeneration of oxidized form of 2-Cys Prx is suggested to be through the reduced ferredoxin and
the enzyme ferredoxin: thioredoxin oxidoreductase plays a key role in supplying electrons to Trx
(Meyer et al ., 2005; Buchanan and Balmer, 2005; Florencio et al ., 2006; Schürmann and Buchanan, 2008).
Subsequently, the reduced Trx, catalyzes the reduction of a variety of target enzymes, including Prx,
by disulphide/dithiol exchange (Buchanan and Balmer, 2005). In the thermophilic cyanobacterium
T . elongatus BP-1, NADPH thioredoxin reductase C functions as an electron donor to 2-Cys Prx. An
NAD(P)H oxidase complex induced by oxidative stress was shown to consist of NADPH thioredoxin
reductase C and 2-Cys Prx. This provided evidence for their interaction in vivo and further in vitro
NADPH-dependent H 2 O 2 reduction assays required the presence of NADPH thioredoxin reductase
C and 2-Cys Prx suggesting that the reductase supplies reducing power from NADPH to 2-Cys Prx.
The NADPH thioredoxin reductase C is a unique enzyme that consists of a NADPH thioredoxin
reductase domain and a thioredoxin-like domain with conserved -CXYC- motif in each of them.
A point mutation in the conserved motif of the reductase domain resulted in the loss of NADPH