Subunit A of the Class III Extradiol dioxygenase, Protocatechuate 4,5-dioxygenase, and similar enzymes
This subfamily includes the A subunit of protocatechuate (PCA) 4,5-dioxygenase (LigAB) and two subfamilies of unknown function. The A subunit is the smaller, non-catalytic subunit of LigAB. PCA 4,5-dioxygenase catalyzes the oxidization and subsequent ring-opening of PCA (or 3,4-dihydroxybenzoic acid), which is an intermediate in the breakdown of lignin and other compounds. PCA 4,5-dioxygenase is one of the aromatic ring opening dioxygenases which play key roles in the degradation of aromatic compounds. As members of the Class III extradiol dioxygenase family, the enzymes use a non-heme Fe(II) to cleave aromatic rings between a hydroxylated carbon and an adjacent non-hydroxylated carbon. LigAB-like class III enzymes are usually composed of two subunits, designated A and B, which form a tetramer composed of two copies of each subunit.