ETHYLENE BIOSYNTHESIS - PROCESSING OF A SUBSTRATE-ANALOG SUPPORTS A RADICAL MECHANISM FOR THE ETHYLENE-FORMING ENZYME

Background: The chemical mechanism of the final step of ethylene biosy nthesis (the conversion of 1-aminocyclopropanecarboxylic acid, ACC, to ethylene by ACC oxidase, the ethylene-forming enzyme, EFE) is poorly understood. Two possibilities have been suggested: a radical mechanism and an N-hydroxylation mechanism. We investigated reaction pathways a vailable to radical intermediates in this reaction using an ACC analog , 1-aminocyclobutanecarboxylic acid (ACBC) as a substrate. Results: AC BC was converted to dehydroproline (Delta(1)-pyrroline-2-carboxylic ac id) by the EFE via a ring expansion process. The possibility that an N -hydroxyaminoacid (produced during two-electron oxidation) acts as an intermediate in this process was eliminated by control experiments. Ch emical model reactions involving two-electron oxidants, such as a posi tive halogen (X+), which presumably generate N-halo derivatives, produ ce only decarboxylation products. Radical-based oxidants, in contrast, generate dehydroproline. Model reactions involving sequential single- electron transfer mechanisms also produce dehydroproline; thus our res ults support the proposal that the EFE-catalyzed step of ethylene bios ynthesis proceeds using a radical-based mechanism. Conclusions: Our re sults provide support for a radical mechanism in the final step of eth ylene biosynthesis and refute an alternative N-hydroxylation mechanism . This work extends the idea that the intrinsic chemical reactivity of a high energy iron-ore intermediate can account for the observed prod ucts in ethylene biosynthesis.