Peroxiredoxins (Prx) and glutaredoxins
(Grx) are key players in redox metabolism. While Prx reduce hydroperoxides and can also act as chaperones and sensors, Grx catalyze
dithiol-disulfide exchange reactions and exert different functions in iron metabolism (see Deponte
2013 and Deponte
2017 for review). For example, enzymatically active Grx utilize reduced glutathione (GSH) for the reduction of inter- or intramolecular disulfide
bonds (Fig. 1).
1: Traditional model of the Grx-dependent reduction of inter-
(left side) or intramolecular (right side) disulfides by
GSH. Grx have either one or two reactive cysteine residues
at the active site and can therefore be classified as monothiol
and dithiol glutaredoxins, respectively. Only one cysteine
residue is required for the mechanism on the left side. See Eckers
et al. 2009
Prx and Grx are known for a long time, more recent studies revealed that these proteins form mechanistically
and funtionally highly heterogeneous groups. Thus, Prx and Grx are great examples of proteins that underwent
subtle structural changes throughout evolution resulting in completely different properties and functions.
The following questions are addressed:
How do the mono- and dithiol isoforms of Prx or Grx differ?
What makes Prx and Grx active/inactive in standard enzymatic assays?
How do some Prx act as chaperones and Grx bind iron-sulfur clusters?
Which features determine the quaternary structure of Prx and Grx?
How are Prx- and Grx-dependent reactions exactly catalyzed?
Regarding the diversity of Prx, we recently established the Prx5-type isoform from Plasmodium falciparum (PfAOP) as a model enzyme for
Grx/GSH-dependent peroxiredoxins (Djuika
et al. 2013). A mutational follow-up analysis of the enzyme revealed a novel mechanism for balancing substrate turnover and Prx inactivation (Staudacher
et al. 2015). This mechanistic model was further supported and strenghtened by stopped-flow kinetic measurements in collaboration with Madia Trujillo and Rafael Radi as well intracellular ratiometric redox measurements with roGFP2-tagged PfAOP in collaboration with Bruce Morgan (Staudacher
et al. 2017). In addition, results from a collaboration with Peer Bork's group pointed towards an unusual prokaryotic ancestry and gene fusion event resulting in a dual
localization of PfAOP in malaria parasites (Djuika
et al. 2015).
Regarding the diversity of Grx, we previously characterized and compared three novel Grx-isoforms from yeast in collaboration with Johannes Herrmann's group (Mesecke
et al. 2008a, Mesecke
et al. 2008b,
et al. 2009): Grx6 and Grx7 both have a significant
enzymatic activity with low molecular weight model substrates
in contrast to most other monothiol glutaredoxins. However,
the apparently similar proteins have quite different properties
considering catalytic turnover, Fe-S
cluster binding and subunit oligomerization
2: Oligomerization of yeast Grx6 and Grx7 and glutathione-dependent
binding of a Fe-S cluster in Grx6. See Mesecke
et al. 2008a
Yeast Grx8 is an
unusual dithiol glutaredoxin because it requires both cysteine
residues for catalysis and has a rather low activity with
model substrates (Eckers
et al. 2009). Based on our studies on Grx6-8, we proposed a refined model of Grx catalysis and postulated the existence of two distinct glutathione interaction sites
(Fig. 3), which we later confirmed experimentally (Begas et al. 2017). The model is suited to explain the observed phenomena for
mono- and dithiol Grx and can be used for the prediction of mechanistic features and
physiological functions of Grx-isoforms (Deponte
2013 and Deponte
2017). For example, based on a comparative study on yeast Grx7 and the dithiol Grx from P. falciparum,
we showed that Grx can also efficiently convert low-molecular weight disulfide substrates using an alternative mechanism
(Begas et al. 2015).
3: Refined mechanistic model for Grx-dependent catalysis.
Click on picture to enlarge. See Eckers
et al. 2009