Characterization of DitA3, the [Fe3S4] ferredoxin of an aromatic ring-hydroxylating dioxygenase from a diterpenoid-degrading microorganism

DitA3, a small soluble ferredoxin, is a component of a ring-hydroxylating dioxygenase involved in the microbial degradation of the diterpenoid, dehydroabietic acid. The anaerobic purification of a heterologously expressed his-tagged DitA3 yielded 20 mg of apparently homogeneous recombinant protein, rcDitA3, per liter of cell culture. Each mole of purified rcDitA3 contained 2.9 equivalents of iron and 4.2 equivalents of sulfur, indicating the presence of a single [Fe3S4] cluster. This conclusion was corroborated by UV-Visible absorption (ε412 = 13.4 mM− 1cm− 1) and EPR (gx,y = 2.00 and gz = 2.02) spectroscopies. The reduction potential of rcDitA3, determined using a highly oriented parallel graphite (HOPG) electrode, was − 177.0 ± 0.5 mV vs. the standard hydrogen electrode (SHE) (20 mM MOPS, 80 mM KCl, pH 7.0, 22 °C). This potential is similar to those of small, soluble Rieske-type ferredoxin components of aromatic-ring dihydroxylating dioxygenases. In contrast to these Rieske-type ferredoxins, DitA3 appears to exist as a dimer in solution. The dimeric ferredoxin may be more stable or may increase the catalytic efficiency of the dioxygenase by delivering the two reducing equivalents required for turnover of the oxygenase.