The 2-His-1-carboxylate facial triad an emerging structural motif in mononuclear non-heme iron(II)

otif for the iron active site, which we refer to as a 2-His-1-carboxylate facial triad, that contributes to its mechanistic versatility.
The 2-His-1-carboxylatefacial triad
The primary evidence for the 2-His-I -carboxylate facial triad derives t'n iiii the increasing number of crystal structures now available I i o m this class of enzymes. Five mononuclear nonheme iron(I1) enzymes have now been characterized by X-ray crystallography. From a comparison of the active sites of these enzymes (Fig. 2) [3-91, it is clear that the coordination spheres of the iron centers have some common features, despite the very different reactions they catalyze. All five of the enzymes have as endogenous ligands two histidines and one carboxylate that are arranged in an array encompassing one face of the octahedron (Fig. 2). This facial triad motif leaves three remaining sites for binding additional endogenous protein residues and/or exogenous ligands such as substrate(s) that can be used to tune the properties of the metal center. The 2-His-1-carboxylate facial triad is seen most clearly in the crystal structures of the extradiol-cleaving catechol dioxygenase 2,3-dihydroxybiphenyl 1,2-dioxygenase (BphC) and the pterin-dependent tyrosine hydroxylase (TyrH). The structure of BphC, solved independently by two laboratories, contains a square pyramidal iron active site with His145 occupying the axial position and His209 and Glu260 in the basal plane [3, 41. Two solvent molecules complete the metal-coordination sphere. The three endogenous amino acid residues are strictly conserved in the sequences of 35 different extradiol type dioxygenases which require Fe(I1) as well as that for a Mn-dependent dioxygenase [lo]. In the latter case, site-directed mutagenesis was used to demonstrate that these three amino acids are essential for dioxygenase activity [ 1I]. Crystallographic studies on the enzyme-substrate complex show that the substrate coordinates in a bidentate manner [4], as deduced from extended X-ray ab-
Eric L. HEGG and Lawrence QUE Jr Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, USA
(Received 4 July 1997) - EJB 97 094710
A 2-His-I-carboxylate facial triad is a common feature of the active sites in a number of mononuclear non-heme iron(I1) enzymes. This structural motif was established crystallographically for five different classes of enzymes and inferred from sequence similarity for two other classes. The 2-His-1-carboxylate facial triad anchors the iron in the active site and at the same time maintains three additional cis-oriented sites. These sites can be used to bind other endogenous ligands or exogenous ligands such as substrate giving the metal center great flexibility to use different mechanistic strategies to perform a and/or 02, variety of chemical transformations.
Keywords: 2-His-I -carboxylate facial triad; non-heme; oxygenase; iron ; mononuclear.
Mononuclear non-heme iron(I1) enzymes catalyze a diverse range of important reactions in the metabolism of amino acids, nucleic acids, and fatty acids, the biodegradation of aromatic compounds, and the biosynthesis of antibiotics such as penicillin and cephalosporin (Fig. 1) [ I , 21. This emerging class of enzymes now rivals heme enzymes in the diversity of oxidative transformations catalyzed. While many of the mechanistic strategies employed by these two classes of enzymes are similar, there are some significant differences. Heme enzymes feature an iron porphyrin cofactor that is ligated to an amino acid residue on the polypeptide chain; such a ligand environment leaves only one coordination site available for the binding of exogenous ligands, typically 0, for dioxygen-activating enzymes. As a consequence, NADH is the usual source of the two electrons needed for the reaction and a reductase is required to mediate electron transfer into the heme active site. In contrast, mononuclear nonheme iron enzymes employ a variety of electron sources, often metal-bound substrates or cofactors, to reduce 0, to the peroxide oxidation state. This mechanistic feature requires a more flexible metal-coordination environment to allow for the binding of a number of exogenous ligands. Understanding how the ligand environment around the metal center in mononuclear non-heme iron enzymes affects their chemistry and allows them to perform reactions typically associated with their heme counterparts is an area of intense current interest. Emerging from recent crystallographic studies on mononuclear non-heme enzymes is a comCorrespondence to L. Que Jr, Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, MN 55455, USA Fax: +1 612 624 7029. E-mail: que@ Abbreviations. BphC, 2,3-dhydroxybiphenyl 1,2-dioxygenase; TyrH, tyrosine hydroxylase; IPNS, isopenicillin N synthase; ACCO, l-aminocyclopropane-I-carboxylic acid oxidase; FeSOD, superoxide dismutase; SLO-1, soybean lipoxygenase. Note. This Minireview will be reprinted in EJB Reviews 1997 which will be available in April 1998.