A TRANSITION FROM IONIC TO FREE-RADICAL MECHANISMS IN CHEMISTRY AND ENZYMOLOGY

1) Examples were presented of enzymes which catalyse different generic reactions at the same active site. These enzymes which belong to the P-450 class are aromatase, 14alpha-demethylase, 17alpha-hydroxylase-17,20-lyase and nitric oxide synthase. 2) The multicatalysis is due to a selection process in which the functional group at the sensitive C-atom of the substrate chooses a compatible iron-oxygen species (Fe(IV)O. or Fe(III)-OOH) for further reaction. 3) The P-450 dependent hydroxylation and C-C bond cleavage reactions occur via a radical mechanism and the enzymes participating in these processes have evolved to deal with situations where the ionic processes are deemed energetically unfavourable.