Binding Interaction of a Ring-hydroxylating Dioxygenase with Fluoranthene in Pseudomonas aeruginosa DN1

Abstract

Pseudomonas aeruginosa DN1 can efficiently utilize fluoranthene as its sole carbon source, and the first step in the biodegradation process is catalyzed by a ring-hydroxylating dioxygenase (RHD). To better understand the binding interaction of RHD with fluoranthene in the strain DN1, the genes encoding alpha subunit (RS30940) and beta subunit (RS05115) of the RHD were functionally characterized using gene knockout approach and homology modeling combined with molecular docking. The results showed that the mutants lacking the characteristic alpha subunit and/or beta subunit failed to degrade fluoranthene effectively. Based on the translated protein sequence and Ramachandran plot, 96.5 % of the primary amino-acid sequences of the alpha subunit in the modeled structure of the RHD were in the permitted region, 2.3 % in the allowed region, but 1.2 % in the disallowed area. The active center of the alpha subunit constituted a triangle structure of the mononuclear iron atom and the two oxygen atoms coupled with a catalytic ternary of His217-His222-Asp372 for the dihydroxylation reaction with fluoranthene. Amino acid residues adjacent to fluoranthene were nonpolar groups, and the C7-C8 positions on the fluoranthene ring were estimated to be the best oxidation sites. The distance of C7-O and C8-O was 3.77 Å and 3.04Å respectively, and both of them were parallel. The results demonstrated that the dihydroxylation reaction was initiated at C7-C8 positions of the fluoranthene ring by RHD in P. aeruginosa DN1, indicating that the binding interaction may be useful for predicting substrate conversion of RHDs.