Non‐Coordinative Binding of O₂ at the Active Center of a Copper‐Dependent Enzyme

Abstract

Molecular oxygen (O2) is a sustainable oxidation reagent. O2 is strongly oxidizing but kinetically stable and its final reaction product is water. For these reasons learning how to activate O2 and how to steer its reactivity along desired reaction pathways is a longstanding challenge in chemical research.[1] Activation of ground‐state diradical O2 can occur either via conversion to singlet oxygen or by one‐electron reduction to superoxide. Many enzymes facilitate activation of O2 by direct fomation of a metal‐oxygen coordination complex concomitant with inner sphere electron transfer. The formylglycine generating enzyme (FGE) is an unusual mononuclear copper enzyme that appears to follow a different strategy. Atomic‐resolution crystal structures of the precatalytic complex of FGE demonstrate that this enzyme binds O2 juxtaposed, but not coordinated to the catalytic CuI. Isostructural complexes that contain AgI instead of CuI or nitric oxide instead of O2 confirm that formation of the initial oxygenated complex of FGE does not depend on redox activity. A stepwise mechanism that decouples binding and activation of O2 is unprecedented for metal‐dependent oxidases, but is reminiscent of flavin‐dependent enzymes.

Mechanistic Insight into Peptidyl‐Cysteine Oxidation by the Copper‐Dependent Formylglycine‐Generating Enzyme

“…After geometry optimization, an O 2 ‐unbound species ( 3 RC ) was obtained with dioxygen in its triplet ground state. This is consistent with the experimental finding of entrapment of the non‐coordinative complex with dioxygen by freeze‐quenching [14] . 3 RC can readily transform to the O 2 ‐bound state ( 3 IM1 ) via the transition state, 3 TS1 , with a free energy barrier of 2.9 kcal mol −1 .…”

“…observed a transient chromophore in the reaction of enzyme‐substrate complex with dioxygen [10c] . A more recent crystallographic study of FGE reveals that the dioxygen is not coordinated to the Cu but is bound in a hydrophilic pocket juxtaposed to the metal (complex b in Scheme 2A) [14] . This non‐coordinated complex with dioxygen was suggested as a precatalytic complex of FGE.…”

“…Considerable efforts have been made to gain a better understanding of how FGE activates O 2 and oxidizes the peptidyl‐cysteines [11–14] . To complete the 4 H + /4 e − reduction of dioxygen to water, FGE requires 2 H + /2 e − from a reducing agent (such as dithiothreitol (DTT)), in addition to the 2 H + /2 e − which are supplied by the substrate cysteine [12] .…”

“… Possible reaction pathways for FGE‐catalyzed fGly formation. A) Previous proposal [10, 12, 14] . B) Proposed mechanism based on the QM/MM calculations in this study.…”

Enzyme-Inspired Coordination Polymers for Selective Oxidization of C(sp3)–H Bonds via Multiphoton Excitation

“…Different from the strategies that irradiation of these photosensitizers in the reported coordination polymers to get radical species or ionic intermediates through PET or consecutive PET/LMCT excitation, 89 the biomimetic coordination polymers utilized the mixed-valence copper species to capture the reactive oxygen species and radical species by the Cu(II) centers, 90 allowing direct C−O coupling to form the mono-oxygenation intermediate via well-modified copper catalytic manifolds. Interestingly, compared to the reported Cu 2 (μ 2 -O) 2 unit in copper enzymes, 10 the structure of the binuclear Cu 2 (μ 2 -S) 2 site in the coordination polymer, Cu-Clbpyc, mimicked concomitantly the key structural and redox features of the Cu A center, and exhibited catalytic activity to activate O 2 and oxidize the C(sp 3 )−H bond without any additional co-catalyst.…”