myo-Inositol oxygenase: a radical new pathway for O₂and C–H activation at a nonheme diiron cluster

Abstract

The enzyme myo-inositol oxygenase (MIOX) catalyzes conversion of myo-inositol (cyclohexan-1,2,3,5/4,6-hexa-ol or MI) to D-glucuronate (DG), initiating the only known pathway in humans for catabolism of the carbon skeleton of cell-signaling inositol (poly)phosphates and phosphoinositides. Recent kinetic, spectroscopic, and crystallographic studies have shown that the enzyme activates its substrates, MI and O 2 , at a carboxylate-bridged nonheme diiron(II/III) cluster, making it the first of many known nonheme diiron oxygenases to employ the mixed-valent form of its cofactor. Evidence suggests that (1) the Fe(III) site coordinates MI via its C1 and C6 hydroxyl groups, (2) the Fe(II) site reversibly coordinates O 2 to produce a superoxo-diiron(III/III) intermediate, and (3) the pendant oxygen atom of the superoxide ligand abstracts hydrogen from C1 to initiate the unique C-C-bond-cleaving, four-electron oxidation reaction. This review recounts the studies leading to the recognition of the novel cofactor requirement and catalytic mechanism of MIOX and forecasts how remaining gaps in our understanding might be filled by additional experiments.Bacterial multi-component monooxygenases [BMMs; e.g., soluble methane monooxygenase (sMMO), toluene/o-xylene monooxygenase (ToMO), and phenol hydroxylase], plant fatty acyl desaturases (e.g., stearoyl acyl carrier protein Δ 9 -desaturase) and the R2 subunits of conventional class I ribonucleotide reductases (RNR-R2s) all use carboxylate-bridged dinuclear iron clusters to activate O 2 for cleavage of strong C-H or O-H bonds. 1-5 Each of these reaction begins with the reduction of O 2 to the peroxide oxidation state by the diiron(II/ II) form of the cofactor. In several cases, peroxide-bridged diiron(III/III) intermediates have been directly characterized. 6-11 Several of the peroxide complexes are known or believed to undergo O-O-bond cleavage to generate high-valent iron complexes that cleave the target C/ O-H bonds. 1-5 For example, the diiron(III/IV) complex, X, in the RNR-R2 reaction oxidizes a tyrosine residue by one electron, cleaving the phenolic O-H bond and activating the protein for participation in nucleotide reduction with its partner subunit, RNR-R1. [12][13][14][15][16][17][18][19][20][21] Similarly, the diiron(IV/IV) complex, Q, in the sMMO reaction cleaves a C-H bond of methane to initiate its hydroxylation. 1,3,7,[22][23][24] In each of these reactions, a diiron(III/III) form of the cluster is generated at the end of the oxidation sequence. For the reactions that are catalytic, a complete "turnover" therefore requires reduction of the cluster back to the O 2 -reactive diiron(II/II) state by additional proteins, with electrons provided ultimately by NAD(P)H. 1,3,4 Although this Please send correspondence to: J.

myo-Inositol Oxygenase Overexpression Accentuates Generation of Reactive Oxygen Species and Exacerbates Cellular Injury following High Glucose Ambience

“…MIOX is a non-heme di-iron enzyme that oxidizes myo-inositol to glucuronic acid via a G-X pathway (48), which also oxidizes the less abundant chiro isomer of inositol, and its metabolites enter the pentose pathway (24). MIOX is predominantly expressed in kidneys of pigs and humans (26,49); however, it remains to be investigated whether this enzyme is responsible for causing oxidant stress, particularly in the renal proximal tubules.…”

Crystal structure of PhnZ in complex with substrate reveals a di-iron oxygenase mechanism for catabolism of organophosphonates

“…MIOX catalyzes the oxidative cleavage of the C1-C6 carbon-carbon bond in myoinositol to form D-glucuronic acid, using a mixed oxidation state Fe(II)/Fe(III) di-iron active site to activate molecular oxygen and bind the substrate (reviewed in ref. 24). Despite low overall sequence conservation between MIOX and PhnZ (15% identity, SI Appendix, Fig.…”

Pathways for Arene Oxidation in Non-Heme Diiron Enzymes: Lessons from Computational Studies on Benzoyl Coenzyme A Epoxidase

“…28 In contrast, soluble Δ 9 desaturase, 29 inserting a double bond into an alkyl chain by double hydrogen abstraction, seems to operate via a 1,1-μ-hydroperoxodiiron(III) intermediate, 30 whereas for myo-inositol oxygenase, 31 a superoxo-diiron(III) species is held responsible for the initial C-H cleavage. 31,32 Methane monooxygenase can oxidize π-systems of olefins and aromatics, 33 and so can several synthetic model non-heme diiron systems. 34 On the other hand, non-heme diiron enzymes natively hydroxylating aromatic substrates also exist; long known examples include phenol hydroxylase, 35 toluene 4-monooxygenase (T4MO) 36 and toluene/o-xylene monooxygenase (ToMO).…”