Metallopeptide-Based Mimics Suggest Substrate Orientation Directs Sulphur Oxidation vs Hydrogen Atom Abstraction Reactions in Mononuclear Non-Haem Iron Dioxygenases

Abstract

Active metallopeptide based mimics of substratebound cysteine dioxygenase (CDO) and isopenicillin-N-synthase (IPNS) suggest that differential reactivity between the two metalloenzymes results from the orientation of a chemically active S(3pπ) dominated MO relative to the Fe(III)-O2 -moiety.Mononuclear non-haem iron (mnhFe) dioxygenases represent a large class of enzymes that facilitate a diverse array of reactions including: H-atom abstraction, hydroxylation, oxygen atom transfer and aromatic ring opening reactions. [1][2][3][4][5][6] Despite the rich chemistries these enzymes promote, mnhFe dioxygenases have similar active-site structures. In their active forms these enzymes possess a reduced high-spin six-coordinate Fe(II) cofactor. The Fe(II) centre is ligated to the enzyme by a facial triad of N/O containing residues and the remaining coordination sites are occupied by water ligands. These water ligands are labile and will exchange with O2, and in some cases, substrate.A subclass of mnhFe enzymes include those that can oxidise thiol containing substrates. For example, isopenicillin-Nsynthase (IPNS) 7-12 converts δ-(L-α-aminoadipoyl)-L-cysteinyl-Dvaline into isopenicillin-N in the biosynthesis of penicillins and cephalosporins while cysteine dioxygenase (CDO) [13][14][15][16][17][18] converts cysteine into cysteine sulphinic acid (Fig. 1). Both CDO and IPNS appear to modify substrate following coordination of the Satom to the Fe(II)-centre followed by the formation of an Fe(III)-O2 -intermediate upon O2 binding. Following Fe(III)-O2 - formation the oxidation mechanisms of CDO and IPNS branch from one another (Fig. 1). In CDO it has been proposed that the superoxo ligand attacks the coordinated S-atom. O-O bond cleavage then leads to the formation of an Fe(IV)-oxo intermediate with a coordinated sulphenate. In IPNS it has been proposed that the initial step following Fe(III)-O2 -generation is the abstraction of a hydrogen atom from the methylene α to the coordinated sulphur atom.Significant efforts have been devoted to understanding the origins of the differential reactivities observed by different mnhFe dioxygenases. 2,3,6,[19][20][21] Considering the similarity in electronic structures of both the Fe(II) forms and the initial Fe(III)-O2 -intermediates substrate orientation and outersphere interactions are the likely means of dictating differential chemical reactivity in thiol-modifying mnhFe dioxygenases.