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

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 Cu^I . Isostructural complexes that contain Ag^I instead of Cu^I 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.

Structure of formylglycine-generating enzyme in complex with copper and a substrate reveals an acidic pocket for binding and activation of molecular oxygen

The formylglycine generating enzyme (FGE) catalyzes oxidative conversion of specific peptidyl-cysteine residues to formylglycine. FGE mediates O₂-activation and hydrogen-atom abstraction in an active site that contains Cu(i) coordinated to two cysteine residues. Similar coordination geometries are common among copper-sensing transcription factors and copper-chaperone but are unprecedented among copper-dependent oxidases. To examine the mechanism of this unusual catalyst we determined the 1.04 Å structure of FGE from Thermomonospora curvata in complex with copper and a cysteine-containing peptide substrate. This structure unveils a network of four crystallographic waters and two active site residues that form a highly acidic O₂-binding pocket juxtaposed to the trigonal planar tris-cysteine coordinated Cu(i) center. Comparison with structures of FGE in complex with Ag(i) and Cd(ii) combined with evidence from NMR spectroscopy and kinetic observations highlight several structural changes that are induced by substrate binding and prime the enzyme for O₂-binding and subsequent activation.