In plants, it has been proposed that hexacoordinate (class 1) non-symbiotic Hbs (nsHb-1) function in vivo as peroxidases. However, little is known about peroxidase activity of nsHb-1. We evaluated the peroxidase activity of rice recombinant Hb1 (a nsHb-1) by using the guaiacol/H2O2 system at pH 6.0 and compared it to that from horseradish peroxidase (HRP). Results showed that the affinity of rice Hb1 for H2O2 was 86-times lower than that of HRP (K(m)=23.3 and 0.27 mM, respectively) and that the catalytic efficiency of rice Hb1 for the oxidation of guaiacol using H2O2 as electron donor was 2838-times lower than that of HRP (k(cat)/K(m)=15.8 and 44,833 mM(-1) min(-1), respectively). Also, results from this work showed that rice Hb1 is not chemically modified and binds CO after incubation with high H2O2 concentration, and that it poorly protects recombinant Escherichia coli from H2O2 stress. These observations indicate that rice Hb1 inefficiently scavenges H2O2 as compared to a typical plant peroxidase, thus indicating that non-symbiotic Hbs are unlikely to function as peroxidases in planta.
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