Purification and Physical-Chemical Characterization of the Three Hydroperoxidases from the Symbiotic Bacterium Sinorhizobium meliloti

Three genes encoding heme hydroperoxidases (katA, katB, and katC) have been identified in the soil bacterium Sinorhizobium meliloti. The recombinant proteins were overexpressed in Escherichia coli and purified in order to achieve a spectral and kinetic characterization. The three proteins contain heme b with high-spin Fe(III). KatB is an acidic bifunctional homodimeric catalase-peroxidase exhibiting both catalase (k cat = 2400 s-1) and peroxidase activity and having a high affinity for hydrogen peroxide (apparent K M = 1.6 mM). KatA and KatC are acidic monofunctional homotetrameric catalases. Although different in size (KatA is a small subunit catalase while KatC is a large subunit catalase) both enzymes exhibit the same heme type and a similar affinity for H2O2 (apparent K M values of 160 and 150 mM). However, the turnover rate of KatA (k cat = 279000 s-1) exceeds that of KatC (k cat = 3100 s-1) significantly. The kinetic parameters are in good agreement with the physiological role of these heme proteins. KatB is the housekeeping hydroperoxidase exhibiting the highest affinity for hydrogen peroxide, while KatA has the lowest H2O2 affinity but the highest k cat/K M value (1.75 × 106 M-1 s-1), in agreement with the hydrogen peroxide inducibility of the encoding gene. Moreover, the lower catalytic efficiency of KatC (2.1 × 104 M-1 s-1) appears to be enough for growing in the stationary phase and/or under heat or salt stress (conditions that are known to favor katC expression).