This preliminary and, in the absence of molecular dynamics simulations of nitroarene dioxygenases, purely static evaluation of structural features of O 2 transport tunnels in 2NTDO and its variants underscores the well-known relevance of tunnels. Tunnels are considered critical for the selectivity of O 2 transport to the active site but not necessarily for O 2 activation and reactivity. (Figure ) whereas alterations to the substrate tunnel following adaptation to 3-nitrotoluene decreased oxygenation efficiencies of other substrates. Evidence from engineering O 2 channels in lipoxygenase showed that two-fold larger bottleneck radii increased oxygenation efficiencies and specific protein activities. Here, we do not observe that changes in kinetic parameters associated with O 2 consumption (e.g., k cat /K m -values in Table ) of wt 2NTDO, variants, and substrates correlate with their O 2 uncoupling behaviour.