The 2‐His‐1‐Carboxylate Facial Triad — An Emerging Structural Motif in Mononuclear Non‐Heme Iron(II) Enzymes

Abstract

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 and/or 02, giving the metal center great flexibility to use different mechanistic strategies to perform a 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 com-

SyrB2 in syringomycin E biosynthesis is a nonheme Fe II α-ketoglutarate- and O 2 -dependent halogenase

“…2A). The holo form (phosphopantetheinylated) of the T domain was generated posttranslationally in situ through action of Sfp (20) and aspartate as putative ligands for Fe II (26)(27). The formation of the Fe IV AO ferryl species is likely to be a common intermediate in both the oxygenase and halogenase mechanism.…”

“…In this regard, nature usually turns to high-valent oxoiron, both in heme and nonheme contexts (24,26,32), to hydroxylate unactivated methyl and methylene groups. This type of reaction may be the chemical niche for nonheme Fe II halogenases.…”

Nitric Oxide Impairs Normoxic Degradation of HIF-1α by Inhibition of Prolyl Hydroxylases

“…This may be exemplified by spectroscopic studies when NO directly binds to the ferrous ion in protocatechuate 4,5-dioxygenase and catechol 2,3-dioxygenase (Arciero et al, 1985) or to isopenicillin N synthase (Roach et al, 1995). These enzymes coordinate Fe 2ϩ in their catalytic domain in a 2-histidine-1-carboxylate facial triad that is the defining structural motif of mononuclear nonheme iron(II) enzymes (Hegg and Que, 1997). Interestingly, HIF-1␣ prolyl hydroxylases belong to a nonheme Fe 2ϩ -containing family of enzymes.…”

Finding Intermediates in the O2 Activation Pathways of Non-Heme Iron Oxygenases

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