Ethylene biosynthesis: processing of a substrate analog supports a radical mechanism for the ethylene-forming enzyme.

M C Pirrung, J Cao, J Chen

Department of Chemistry, P.M. Gross Chemical Laboratory, Duke University, Durham, North Carolina 27708-0346, USA.

Chemistry & biology 1998 Jan

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The chemical mechanism of the final step of ethylene biosynthesis (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 available to radical intermediates in this reaction using an ACC analog, 1-aminocyclobutanecarboxylic acid (ACBC) as a substrate. ACBC was converted to dehydroproline (delta 1-pyrroline-2-carboxylic acid) by the EFE via a ring expansion process. The possibility that an N-hydroxy-aminoacid (produced during two-electron oxidation) acts as an intermediate in this process was eliminated by control experiments. Chemical model reactions involving two-electron oxidants, such as a positive halogen (X+), which presumably generate N-halo derivatives, produce only decarboxylation products. Radical-based oxidants, in contrast, generate dehydroproline. Model reactions involving sequential single-electron transfer mechanisms also produce dehydroproline; thus our results support the proposal that the EFE-catalyzed step of ethylene biosynthesis proceeds using a radical-based mechanism. Our results provide support for a radical mechanism in the final step of ethylene biosynthesis and refute an alternative N-hydroxylation mechanism. This work extends the idea that the intrinsic chemical reactivity of a high energy iron-oxo intermediate can account for the observed products in ethylene biosynthesis.

Citation

M C Pirrung, J Cao, J Chen. Ethylene biosynthesis: processing of a substrate analog supports a radical mechanism for the ethylene-forming enzyme. Chemistry & biology. 1998 Jan;5(1):49-57