The accurate determination of catalytic parameters (in particular the turnover number, kcat) from the fitting of the Michaelis-Menten equation to the experimental profiles shown in Figure is impaired in many cases by the available substrate concentration range, which is in turn limited by substrate solubility. Therefore, we additionally determined rate vs. substrate concentration profiles in the presence of 5% acetonitrile, which increases substrate solubility by about 3-fold.