Degradation of Diphenyl Ether in Sphingobium phenoxybenzoativorans SC_3 Is Initiated by a Novel Ring Cleavage Dioxygenase

Sphingobium phenoxybenzoativorans SC_3 degrades and utilizes diphenyl ether (DE) and 2-carboxy DE as its sole carbon and energy source. In this study, we report the degradation of DE and 2-carboxy DE initiated by a novel ring-cleavage angular dioxygenase (Dpe) in the strain. Dpe functions at the angular carbon and its adjacent carbon (C1a, C2) of a benzene ring in DE (or the 2-carboxy benzene ring in 2-carboxy DE) and cleaves the C1a-C2 bond (decarboxylation is simultaneously happened for 2-carboxy DE), yielding 2,4-hexadienal phenyl ester, which is subsequently hydrolyzed to muconic acid semialdehyde and phenol. Dpe is a type IV Rieske non-heme iron oxygenase (RHO) and consists of three components: a hetero-oligomer oxygenase, a [2Fe-2S]-type ferredoxin and a GR (glutathione reductase)-type reductase. Genetic analyses revealed that dpeA1A2 plays an essential role in degradation and utilization of DE and 2-carboxy DE in S. phenoxybenzoativorans SC_3. Enzymatic study showed that transformation of one molecule of DE needs two molecules of oxygen and two molecules of NADH, supporting the assumption that the cleavage of DE catalyzed by Dpe is a continuous two-step dioxygenation process: DE is dioxygenated at C1a, C2 to form an hemiacetal-like intermediates, which is further dioxygenated resulting the cleavage of the C1a-C2 bond to form one molecule of 2,4-hexadienal phenyl ester and two molecules of H 2 O. This study extends our knowledge of the mode and mechanism of ring-cleavage of aromatic compounds. IMPORTANCE Benzene ring-cleavage, catalyzed by dioxygenase, is the key and speed limiting step in the aerobic degradation of aromatic compounds. Previously reported ring-cleavage of DEs, the benzene ring needs to be firstly dihydroxylated at lateral position, and subsequently dehydrogenated and opened through extradiol cleavage. This process requires three enzymes (two dioxygenases and one dehydrogenase). In this study, we identified a novel angular dioxygenase (Dpe) in S. phenoxybenzoativorans SC_3. Under Dpe-mediated catalysis, the benzene ring of DE is dioxygenated at the angular position (C1a, C2) resulting in the C1a-C2 bond cleavage to generate a novel product 2,4-hexadienal phenyl ester. This process needs only one angular dioxygenase, Dpe. Thus the ring-cleavage catalyzed by Dpe represents a novel mechanism of benzene ring-cleavage.