Characterization of Water Coordination to Ferrous Nitrosyl Complexes with fac-N2O, cis-N2O2, and N2O3 Donor Ligands

Improved resolution of 3-mercaptopropionate dioxygenase active site provided by ENDOR spectroscopy offers insight into catalytic mechanism

3-mercaptopropionate (3MPA) dioxygenase (MDO) is a mononuclear nonheme iron enzyme that catalyzes the O2-dependent oxidation of thiol-bearing substrates to yield the corresponding sulfinic acid. MDO is a member of the cysteine dioxygenase family of small molecule thiol dioxygenases and thus shares a conserved sequence of active site residues (Serine-155, Histidine-157, and Tyrosine-159), collectively referred to as the SHY-motif. It has been demonstrated that these amino acids directly interact with the mononuclear Fe-site, influencing steady-state catalysis, catalytic efficiency, O2-binding, and substrate coordination. However, the underlying mechanism by which this is accomplished is poorly understood. Here, pulsed electron paramagnetic resonance spectroscopy [1H Mims electron nuclear double resonance spectroscopy] is applied to validate density functional theory computational models for the MDO Fe-site simultaneously coordinated by substrate and nitric oxide (NO), (3MPA/NO)-MDO. The enhanced resolution provided by electron nuclear double resonance spectroscopy allows for direct observation of Fe-bound substrate conformations and H-bond donation from Tyr159 to the Fe-bound NO ligand. Further inclusion of SHY-motif residues within the validated model reveals a distinct channel restricting movement of the Fe-bound NO-ligand. It has been argued that the iron-nitrosyl emulates the structure of potential Fe(III)-superoxide intermediates within the MDO catalytic cycle. While the merit of this assumption remains unconfirmed, the model reported here offers a framework to evaluate oxygen binding at the substrate-bound Fe-site and possible reaction mechanisms. It also underscores the significance of hydrogen bonding interactions within the enzymatic active site.

Synthesis, Structure and Reactivity of a Mononuclear N,N,O-Bound Fe(II) α-Keto-Acid Complex

A bulky, tridentate phenolate ligand (ImPh2 NNOtBu ) was used to synthesise the first example of a mononuclear, facial, N,N,O-bound iron(II) benzoylformate complex, [Fe(ImPh2 NNOtBu )(BF)] (2). The X-ray crystal structure of 2 reveals that the iron centre is pentacoordinate (τ=0.5), with a vacant site located cis to the bidentate BF ligand. The Mössbauer parameters of 2 are consistent with high-spin iron(II), and are very close to those reported for α-ketoglutarate-bound non-heme iron enzyme active sites. According to NMR and UV-vis spectroscopies, the structural integrity of 2 is retained in both coordinating and non-coordinating solvents. Cyclic voltammetry studies show that the iron centre has a very low oxidation potential and is more prone to electrochemical oxidation than the redox-active phenolate ligand. Complex 2 reacts with NO to form a S=3 /2 {FeNO}7 adduct in which NO binds directly to the iron centre, according to EPR, UV-vis, IR spectroscopies and DFT analysis. Upon O2 exposure, 2 undergoes oxidative decarboxylation to form a diiron(III) benzoate complex, [Fe2 (ImPh2 NNOtBu )2 (μ2 -OBz)(μ2 -OH)2 ]+ (3). A small amount of hydroxylated ligand was also observed by ESI-MS, hinting at the formation of a high-valent iron(IV)-oxo intermediate. Initial reactivity studies show that 2 is capable of oxygen atom transfer reactivity with O2 , converting methyl(p-tolyl)sulfide to sulfoxide.

1H-HYSCORE Reveals Structural Details at the Fe(II) Active Site of Taurine:2-Oxoglutarate Dioxygenase

Proton Hyperfine Sublevel Correlation (¹H-HYSCORE) experiments have been used to probe the ligation structure of the Fe(II) active site of taurine:2-oxoglutarate dioxygenase (TauD), a non-heme Fe(II) hydroxylase. To facilitate Electron Paramagnetic Resonance (EPR) experiments, Fe(II) derivatives of the enzyme were studied using nitric oxide as a substitute for molecular oxygen. The addition of NO to the enzyme yields an S = 3/2 {FeNO}⁷ paramagnetic center characterized by nearly axial EPR spectra with g_⊥ = 4 and g_|| = 2. Using results from (i) an X-ray crystallographic study of TauD crystallized under anaerobic conditions in the presence of both cosubstrate 2-oxoglutarate and substrate taurine, (ii) a published theoretical description of the {FeNO}⁷ derivative of this form of the enzyme, and (iii) previous ²H-Electron Spin Echo Envelope Modulation (ESEEM) studies, we were able to assign the proton cross peaks detected in orientation-selected ¹H-HYSCORE spectra. Discrete contributions from the protons of two coordinated histidine ligands were resolved. If substrate taurine is absent from the complex, orientation-selective HYSCORE spectra show cross peaks that are less resolved and when combined with information obtained from continuous wave EPR, support an alternate binding scheme for 2-oxoglutarate. HYSCORE studies of TauD in the absence of 2-oxoglutarate show additional ¹H cross peaks that can be assigned to two distinct bound water molecules. In addition, ¹H and ¹⁴N cross peaks that arise from the coordinated histidine side chains show a change in NO coordination for this species. For all of the TauD species, ¹H hyperfine couplings and their orientations are sensitive to the detailed electronic structure of the {FeNO}⁷ center.

Multielectron Chemistry within a Model Nickel Metalloprotein: Mechanistic Implications for Acetyl-CoA Synthase

The acetyl coenzyme A synthase (ACS) enzyme plays a central role in the metabolism of anaerobic bacteria and archaea, catalyzing the reversible synthesis of acetyl-CoA from CO and a methyl group through a series of nickel-based organometallic intermediates. Owing to the extreme complexity of the native enzyme systems, the mechanism by which this catalysis occurs remains poorly understood. In this work, we have developed a protein-based model for the Ni_P center of acetyl coenzyme A synthase using a nickel-substituted azurin protein (NiAz). NiAz is the first model nickel protein system capable of accessing three (Ni^I/Ni^II/Ni^III) distinct oxidation states within a physiological potential range in aqueous solution, a critical feature for achieving organometallic ACS activity, and binds CO and -CH₃ groups with biologically relevant affinity. Characterization of the Ni^I-CO species through spectroscopic and computational techniques reveals fundamentally similar features between the model NiAz system and the native ACS enzyme, highlighting the potential for related reactivity in this model protein. This work provides insight into the enzymatic process, with implications toward engineering biological catalysts for organometallic processes.

Spectroscopic analyses of 2-oxoglutarate-dependent oxygenases: TauD as a case study

A wide range of spectroscopic approaches have been used to interrogate the mononuclear iron metallocenter in 2-oxoglutarate (2OG)-dependent oxygenases. The results from these spectroscopic studies have provided valuable insights into the structural changes at the active site during substrate binding and catalysis, thus providing critical information that complements investigations of these enzymes by X-ray crystallography, biochemical, and computational approaches. This mini-review highlights taurine hydroxylase (taurine:2OG dioxygenase, TauD) as a case study to illustrate the wealth of knowledge that can be generated by applying a diverse array of spectroscopic investigations to a single enzyme. In particular, electronic absorption, circular dichroism, magnetic circular dichroism, conventional and pulse electron paramagnetic, Mössbauer, X-ray absorption, and resonance Raman methods have been exploited to uncover the properties of the metal site in TauD.