posted on 2021-09-22, 18:07authored byChristelle Caux, Bruno Guigliarelli, Corinne Vivès, Frédéric Biaso, Marius Horeau, Hawra Hassoune, Isabelle Petit-Hartlein, Céline Juillan-Binard, Stephane Torelli, Franck Fieschi, Vincent Nivière
MsrPQ is a new type of methionine
sulfoxide reductase (Msr) system
found in bacteria. It is specifically involved in the repair of periplasmic
methionine residues that are oxidized by hypochlorous acid. MsrP is
a periplasmic molybdoenzyme that carries out the Msr activity, whereas
MsrQ, an integral membrane-bound hemoprotein, acts as the physiological
partner of MsrP to provide electrons for catalysis. Although MsrQ
(YedZ) was associated since long with a protein superfamily named
FRD (ferric reductase domain), including the eukaryotic NADPH oxidases
and STEAP proteins, its biochemical properties are still sparsely
documented. Here, we have investigated the cofactor content of the E. coli MsrQ and its mechanism of reduction by the
flavin reductase Fre. We showed by electron paramagnetic resonance
(EPR) spectroscopy that MsrQ contains a single highly anisotropic
low-spin (HALS) b-type heme located on the periplasmic side of the
membrane. We further demonstrated that MsrQ holds a flavin mononucleotide
(FMN) cofactor that occupies the site where a second heme binds in
other members of the FDR superfamily on the cytosolic side of the
membrane. EPR spectroscopy indicates that the FMN cofactor can accommodate
a radical semiquinone species. The cytosolic flavin reductase Fre
was previously shown to reduce the MsrQ heme. Here, we demonstrated
that Fre uses the FMN MsrQ cofactor as a substrate to catalyze the
electron transfer from cytosolic NADH to the heme. Formation of a
specific complex between MsrQ and Fre could favor this unprecedented
mechanism, which most likely involves transfer of the reduced FMN
cofactor from the Fre active site to MsrQ.