C-terminal catalytic domain of the large/alpha subunit (ahdA1c) of a ring-hydroxylating dioxygenase from Sphingomonas sp. strain P2 and related proteins
C-terminal catalytic domain of the large subunit (ahdA1c) of the AhdA3A4A2cA1c salicylate 1-hydroxylase complex from Sphingomonas sp. strain P2, and related Rieske-type non-heme iron aromatic ring-hydroxylating oxygenases (RHOs, also known as aromatic ring hydroxylating dioxygenases). AhdA3A4A2cA1c is one of three known isofunctional salicylate 1-hydroxylase complexes in strain P2, involved in phenanthrene degradation, which catalyze the monooxygenation of salicylate, the metabolite of phenanthene degradation, to produce catechol. This complex prefers salicylate over other substituted salicylates; the other two salicylate 1-hydroxylases have different substrate preferences. RHOs utilize non-heme Fe(II) to catalyze the addition of hydroxyl groups to the aromatic ring, an initial step in the oxidative degradation of aromatic compounds. RHOs are composed of either two or three protein components, and are comprised of an electron transport chain (ETC) and an oxygenase. The ETC transfers reducing equivalents from the electron donor to the oxygenase component, which in turn transfers electrons to the oxygen molecules. The oxygenase components are oligomers, either (alpha)n or (alpha)n(beta)n. The alpha subunits are the catalytic components and have an N-terminal domain, which binds a Rieske-like 2Fe-2S cluster, and a C-terminal domain which binds the non-heme Fe(II). The Fe(II) is co-ordinated by conserved His and Asp residues. Other oxygenases belonging to this subgroup include the alpha subunits of anthranilate 1,2-dioxygenase from Burkholderia cepacia DBO1, a polycyclic aromatic hydrocarbon dioxygenase from Cycloclasticus sp. strain A5 (PhnA dioxygenase), salicylate-5-hydroxylase from Ralstonia sp. U2, ortho-halobenzoate 1,2-dioxygenase from Pseudomonas aeruginosa strain JB2, and the terephthalate 1,2-dioxygenase system from Delftia tsuruhatensis strain T7. This subfamily belongs to the SRPBCC (START/RHO_alpha_C/PITP/Bet_v1/CoxG/CalC) domain superfamily of proteins that bind hydrophobic ligands. SRPBCC domains have a deep hydrophobic ligand-binding pocket.