STRUCTURE OF AN AROMATIC-RING-HYDROXYLATING DIOXYGENASE-NAPHTHALENE 1,2-DIOXYGENASE

Background: Pseudomonas sp. NCIB 9816-4 utilizes a multicomponent enzy me system to oxidize naphthalene to (+)-cis-(1R,PS)-dihydroxy-1,2-dihy dronaphthalene. The enzyme component catalyzing this reaction, naphtha lene 1,2-dioxygenase (NDO), belongs to a family of aromatic-ring-hydro xylating dioxygenases that oxidize aromatic hydrocarbons and related c ompounds to cis-arene diols. These enzymes utilize a mononuclear non-h eme iron center to catalyze the addition of dioxygen to their respecti ve substrates. The present study was conducted to provide essential st ructural information necessary for elucidating the mechanism of action of NDO. Results: The three-dimensional structure of NDO has been dete rmined at 2.25 Angstrom resolution. The molecule is an alpha(3) beta(3 ) hexamer. The alpha subunit has a beta-sheet domain that contains a R ieske [2Fe-2S] center and a catalytic domain that has a novel fold dom inated by an antiparallel nine-stranded beta-pleated sheet against whi ch helices pack. The active site contains a non-heme ferrous ion coord inated by His208, His213, Asp362 (bidentate) and a water molecule. Asn 201 is positioned further away, 3.75 Angstrom, at the missing axial po sition of an octahedron, In the Rieske [2Fe-2S] center, one iron is co ordinated by Cys81 and Cys101 and the other by His83 and His104. Concl usions: The domain structure and iron coordination of the Rieske domai n is very similar to that of the cytochrome bc(1) domain. The active-s ite iron center of one of the alpha subunits is directly connected by hydrogen bonds through a single amino acid, Asp205, to the Rieske [2Fe -2S] center in a neighboring ex subunit. This is likely to be the main route for electron transfer.