Escherichia colifatty acid cyclopropane synthase (CFAS) was overproduced and purified as a His6 -tagged protein. This recombinant enzyme is as active as the native enzyme with aKmof 90lMforS-AdoMet and a specific activity of 5·10 )2 lmolÆmin )1 Æmg )1 . The enzyme is devoid of organic or metal cofactors and is unable to catalyze the wash-out of themethyl protons ofS-AdoMet to the solvent, data that do not support the ylidemechanism. Inactivationof the enzyme by 5,5¢-dithiobis-(2-nitrobenzoic acid) (DTNB), a pseudo first-order process with a rate constant of )1 Æs )1 , is not protected by substrates. Graphical analysis of the inactiva-tion byDTNB revealed that only one cysteine is responsible for the inactivation of the enzyme. .
