a, Redox reactions of cysteine with key species involved, including cysteine oxidation by reactive oxygen species (ROS) and reduction by thiol antioxidants (RSH) or enzymes (Srx). b, Mechanism of allosteric redox switch containing an intramolecular allosteric disulfide bridge. Reduction of the disulfide to the corresponding dithiol results in a structural reorganization of the protein that is propagated to the active-site altering activity. c, NgTAL crystal 1. Top left, 2mFo − DFc electron density map after final refinement contoured at 3σ. No additional positive or negative electron density was observed at ±3σ. Top right, mFo − DFc omit electron density map contoured at 5σ. Bottom, alternative refinement with a methylene bridge yields unexplained positive difference electron density at the bridging atom. The 2mFo − DFc map is shown in grey at 3σ, positive (green) and negative (red) electron density maps are shown at 3σ and −3σ, respectively. d, NgTAL crystal 2. Left, 2mFo − DFc electron density map after final refinement, contoured at 1.5σ. Right, mFo − DFc omit electron density map contoured at 5σ. e, NgTAL crystal 3. Left, 2mFo − DFc electron density map after final refinement, contoured at 1.5σ. Right, mFo − DFc omit electron density map contoured at 5σ. f, NgTAL crystal 4. Low-dose dataset of an independent crystal measured in-house at a rotating anode. The NOS bridge is also present in the corresponding NgTAL structure (left, 2mFo − DFc electron density map contoured at 1σ; right, mFo − DFc omit electron density map contoured at 3σ), thus ruling out that formation of the covalent linkage seen in crystals 1–3 results from radiation damage at the high-energy synchrotron beamline. g, Structure of the Lys8–Cys38 redox switch in the oxidized (left) and reduced (right) state, showing electron density for neighbouring waters (W1 and W2) and a presumed dioxygen molecule (O2) that is exclusively observed in the reduced state. The corresponding 2mFo − DFc electron density map is shown in blue at a contour level of 1.5σ.