|Gene:||wrbA||Accession Numbers: EG11540 (EcoCyc), b1004, ECK0995|
Subunit composition of
NAD(P)H:quinone oxidoreductase = [WrbA]4
NAD(P)H:quinone oxidoreductase = WrbA
The purified WrbA protein has NAD(P)H:quinone oxidoreductase activity [Patridge06]. WrbA is related to the flavodoxin family of proteins [Grandori94]. Unlike the flavodoxins, WrbA does not have a stabilized semiquinone state. It rapidly takes up two electrons, generating the fully reduced form [Noll06]. Purified WrbA protein binds one FMN per monomer with a binding constant of 2 µM at room temperature, which is weaker than that of typical flavodoxins [Grandori98, Patridge06]. Binding of FMN appears to be pH-dependent [Patridge06], and it increases the thermal stability and promotes tetramerization of WrbA [Natalello07].
WrbA is a multimer in solution, existing in an equilibrium between the dimeric and tetrameric form [Grandori98, Patridge06]. Crystal structures show WrbA to be a dimer of dimers. Structural comparisons to flavodoxin and the mammalian NAD(P)H:quinone oxidoreductase Nqo1 allow interpretation of the differences in the cofactor requirements and the catalytic functions of these proteins [Wolfova07, Carey07]. Additional crystal structures of WrbA in complex with benzoquinone or NADH suggest that binding of quinones and NADH to the FMN cofactor is mutually exclusive [Andrade07]. Steady-state kinetic analysis suggests a ping-pong reaction mechanism and show two-plateau Michaelis-Menten plots that are dependent on the temperature at which the enzyme had been held. This result implies allosteric regulation of the enzyme [Kishko12].
Comparison of the crystal structures of the apo- and holoenzyme forms of WrbA led to improved understanding of the functional similarities and differences of WrbA compared to the flavodoxins [Wolfova09].
It was initially reported that WrbA copurifies with the Trp repressor protein TrpR and enhances the formation or stability of TrpR binding to its operator target sites [Yang93a]. However, a later report showed that WrbA does not specifically affect the DNA binding affinity or mode of binding of TrpR [Grandori98]. WrbA alone does not bind to the trp operator DNA [Yang93a]. The association between WrbA and TrpR observed by [Yang93a] may therefore be due to structural rather than functional reasons [Grandori98].
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