Pulsed Multifrequency Electron Paramagnetic Resonance Spectroscopy Reveals Key Branch Points for One- vs Two-Electron Reactivity in Mn/Fe Proteins

Selective isotope labeling probes the chemical capacity and reaction mechanism of a heterobimetallic Mn/Fe protein

The R2-like ligand binding oxidase (R2lox) forms a novel tyrosine-valine crosslink upon O2 activation, reflecting an overall two-electron oxidation reaction. However, the mechanism through which the crosslink is formed is under debate, as the reaction can be initiated through either tyrosine OH or valine CH bond activation. Here, we utilized selective isotopic labeling and several spectroscopic techniques to probe the mechanism of this oxidation process. The results suggest that R2lox is capable of performing CH bond activation, with both solvent and CH kinetic isotope effects of approximately 2. Signatures of a high-valent MnIV/FeIV intermediate were observed through rapid-freeze-quench EPR that resemble an analogous intermediate found in the heterobimetallic radical-initiating enzyme, class Ic ribonucleotide reductase. This study provides a lower bound on the free energies of CH bonds that can be cleaved by Mn/Fe cofactors and suggests the potential for such enzymes to functionally replace Fe/Fe cofactors in a range of reactions.

Structural Elucidation of the Reduced Mn(III)/Fe(III) Intermediate of the Radical-Initiating Metallocofactor in Chlamydia trachomatis Ribonucleotide Reductase

Ribonucleotide reductases (RNRs) are the sole de novo source of deoxyribonucleotides for DNA synthesis and repair across all organisms and carry out their reaction via a radical mechanism. RNR from Chlamydia trachomatis generates its turnover-initiating cysteinyl radical by long-range reduction of a Mn(IV)/Fe(III) cofactor, producing a Mn(III)/Fe(III) intermediate. Herein, we characterize the protonation states of the inorganic ligands in this reduced state using advanced pulse electron paramagnetic resonance (EPR) spectroscopy and 2H-isotope labeling. A strongly coupled deuteron is observed by hyperfine sublevel correlation (HYSCORE) spectroscopy experiments and indicates the presence of a bridging hydroxo ligand. Isotope-dependent EPR line broadening analysis and the magnitude of the estimated Mn-Fe exchange coupling constant together suggest a μ-oxo/μ-hydroxo core. Two distinct signals detected in electron-nuclear double resonance (ENDOR) spectra are attributable to less strongly coupled hydrons of a terminal water ligand to Mn(III). Together, these experiments imply that the reduced cofactor has a mixed μ-oxo/μ-hydroxo core with a terminal water ligand on Mn(III). This structural assignment sheds light generally on the reactivity of Mn/Fe heterobimetallic sites and, more specifically, on the proton-coupling in the electron transfer that initiates ribonucleotide reduction in this subclass of RNRs.

Accessing a synthetic FeIIIMnIV core to model biological heterobimetallic active sites

Metalloproteins with dinuclear cores are known to bind and activate dioxygen, with a subclass of these proteins having active sites containing FeMn cofactors and activities ranging from long-range proton-coupled electron transfer (PCET) to post-translational peptide modification. While mechanistic studies propose that these metallocofactors access FeIIIMnIV intermediates, there is a dearth of related synthetic analogs. Herein, the first well-characterized synthetic FeIII-(μ-O)-MnIV complex is reported; this complex shows similar spectroscopic features as the catalytically competent FeIIIMnIV intermediate X found in Class Ic ribonucleotide reductase and demonstrates PCET function towards phenolic substrates. This complex is prepared from the oxidation of the isolable FeIII-(μ-O)-MnIII species, whose stepwise assembly is facilitated by a tripodal ligand containing phosphinic amido groups. Structural and spectroscopic studies found proton movement involving the FeIIIMnIII core, whereby the initial bridging hydroxido ligand is converted to an oxido ligand with concomitant protonation of one phosphinic amido group. This series of FeMn complexes allowed us to address factors that may dictate the preference of an active site for a heterobimetallic cofactor over one that is homobimetallic: comparisons of the redox properties of our FeMn complexes with those of the di-Fe analogs suggested that the relative thermodynamic ease of accessing an FeIIIMnIV core can play an important role in determining the metal ion composition when the key catalytic steps do not require an overly potent oxidant. Moreover, these complexes allowed us to demonstrate the effect of the hyperfine interaction from non-Fe nuclei on 57Fe Mössbauer spectra which is relevant to MnFe intermediates in proteins.

Bioinformatic Discovery of a Cambialistic Monooxygenase

Dinuclear monooxygenases mediate challenging C-H bond oxidation reactions throughout nature. Many of these enzymes are presumed to exclusively utilize diiron cofactors. Herein we report the bioinformatic discovery of an orphan dinuclear monooxygenase that preferentially utilizes a heterobimetallic manganese-iron (Mn/Fe) cofactor to mediate an O2-dependent C-H bond hydroxylation reaction. Unlike the structurally similar Mn/Fe-dependent monooxygenase AibH2, the diiron form of this enzyme (SfbO) exhibits a nascent enzymatic activity. This behavior raises the possibility that many other dinuclear monooxygenases may be endowed with the capacity to harness cofactors with a variable metal content.

Enzymatic Hydroxylation of Aliphatic C-H Bonds by a Mn/Fe Cofactor

The aerobic oxidation of carbon-hydrogen (C-H) bonds in biology is currently known to be accomplished by a limited set of cofactors that typically include heme, nonheme iron, and copper. While manganese cofactors perform difficult oxidation reactions, including water oxidation within Photosystem II, they are generally not known to be used for C-H bond activation, and those that do catalyze this important reaction display limited intrinsic reactivity. Here we report that the 2-aminoisobutyric acid hydroxylase from Rhodococcus wratislaviensis, AibH1H2, requires manganese to functionalize a strong, aliphatic C-H bond (BDE = 100 kcal/mol). Structural and spectroscopic studies of this enzyme reveal a redox-active, heterobimetallic manganese-iron active site at the locus of O2 activation and substrate coordination. This result expands the known reactivity of biological manganese-iron cofactors, which was previously restricted to single-electron transfer or stoichiometric protein oxidation. Furthermore, the AibH1H2 cofactor is supported by a protein fold distinct from typical bimetallic oxygenases, and bioinformatic analyses identify related proteins abundant in microorganisms. This suggests that many uncharacterized monooxygenases may similarly require manganese to perform oxidative biochemical tasks.

Enzymatic Hydroxylation of Aliphatic C-H Bonds by a Mn/Fe Cofactor

Manganese cofactors activate strong chemical bonds in many essential enzymes. Yet very few manganese-dependent enzymes are known to functionalize ubiquitous carbon-hydrogen (C-H) bonds, and those that catalyze this important reaction display limited intrinsic reactivity. Herein, we report that the 2-aminoisobutyric acid hydroxylase from Rhodococcus wratislaviensis requires manganese to functionalize a C-H bond possessing a bond dissociation enthalpy (BDE) exceeding 100 kcal/mol. Structural and spectroscopic studies of this enzyme reveal a redox-active, heterobimetallic manganese-iron active site that utilizes a manganese ion at the locus for O ₂ activation and substrate coordination. Accordingly, this enzyme represents the first documented Mn-dependent monooxygenase in biology. Related proteins are widespread in microorganisms suggesting that many uncharacterized monooxygenases may utilize manganese-containing cofactors to accomplish diverse biological tasks.

Site-Differentiated MnIIFeII Complex Reproducing the Selective Assembly of Biological Heterobimetallic Mn/Fe Cofactors

Class Ic ribonucleotide reductases (RNRIc) and R2-like ligand-binding oxidases (R2lox) are known to contain heterobimetallic Mn^IIFe^II cofactors. How these enzymes assemble Mn^IIFe^II cofactors has been a long-standing puzzle due to the weaker binding affinity of Mn^II versus Fe^II. In addition, the heterobimetallic selectivity of RNRIc and R2lox has yet to be reproduced with coordination complexes, leading to the hypothesis that RNRIc and R2lox overcome the thermodynamic preference for coordination of Fe^II over Mn^II with their carefully constructed three-dimensional protein structures. Herein, we report the selective formation of a heterobimetallic Mn^IIFe^II complex accomplished in the absence of a protein scaffold. Treatment of the ligand Py₄DMcT (L) with equimolar amounts of Fe^II and Mn^II along with two equivalents of acetate (OAc) affords [LMn^IIFe^II (OAc)₂(OTf)]⁺ (Mn^IIFe^II) in 80% yield, while the diiron complex [LFe^IIFe^II(OAc)₂(OTf)]⁺ (Fe^IIFe^II) is produced in only 8% yield. The formation of Mn^IIFe^II is favored regardless of the order of addition of Fe^II and Mn^II sources. X-ray diffraction (XRD) of single crystals of Mn^IIFe^II reveals an unsymmetrically coordinated carboxylate ligand─a primary coordination sphere feature shared by both RNRIc and R2lox that differentiates the two metal binding sites. Anomalous XRD studies confirm that Mn^IIFe^II exhibits the same site selectivity as R2lox and RNRIc, with the Fe^II (d⁶) center preferentially occupying the distorted octahedral site. We conclude that the successful assembly of Mn^IIFe^II originates from (1) Fe-deficient conditions, (2) site differentiation, and (3) the inability of ligand L to house a dimanganese complex.