BETA-HALOETHANOL SUBSTRATES AS PROBES FOR RADICAL MECHANISMS FOR GALACTOSE-OXIDASE

Ketyl radical anions with a halogen substitutent on the carbon adjacen t to ketyl are known to rapidly rearrange by halide anion ejection. Su ch a rearrangement is an ideal probe for possible ketyl radical anion intermediates in the catalytic mechanism of the monocopper/tyrosine ra dical enzyme galactose oxidase (GOase). Turnover of beta-fluoro-, beta -chloro-, beta-bromo-, and beta-iodoethanol by GOase leads to mechanis m-based inactivation of the enzyme by trapping the enzyme in a catalyt ically inactive one-electron-reduced form. Presuming that mechanism-ba sed inactivation and turnover proceed through the same reactive interm ediates, the data reported here narrow down the possible mechanisms fo r the substrate oxidation step (the two electron transfer from substra te to enzyme) to two similar possibilities. Either the reaction procee ds through a short-lived ketyl radical anion intermediate or it procee ds through a closely related concerted E2R mechanism with considerable ketyl radical anion character in the transition state.