STEMPO regeneration rates as a function of substrate concentration (14, 92, 137 and 162 M s -1 for 1, 5, 10 and 20 mM MBA, respectively; the colours of the data points match the data in a and in Supplementary Figure 14 ), deduced from a first-order relaxation fit of the variation of STEMPO concentration over time, as quantified from the EPR data. The solid line shows a fit using the MichaelisMenten model with MichaelisMenten constant K M = 6.8 1.3 and rate constant for chemical catalysis k cat = 0.22 s -1 (see main text). Extended Data Fig. 8 provides results with glycerol as the substrate at pH 8.0 and Extended Data Fig. 9 the corresponding data for MBA at pH 7.3 (also Supplementary Section 6 ).