Stereochemical studies on the hydration of monofluorofumarate and 2,3-difluorofumarate by fumarase

Abstract

Stereochemical and product analyses have been studied in our continuing work on the bioprocessing of fluorinated substrate analogues. The hydration pathways of the fumarase-catalyzed reaction on fluorofumarate lead to a product distribution of L-threo-beta-fluoromalate to oxalacetate of 1 to 16. The beta-fluoromalate product has not been previously reported. Oxalacetate formation from the initial product, alpha-fluoromalate, an alpha-fluorohydrin, proceeds by way of a direct nonenzymic decomposition path (as opposed to collapse to the enol of oxalacetate with subsequent tautomerization). Difluorofumarate is hydrated to an alpha-fluorohydrin, alpha, beta-difluoromalate, which decomposes to 3(S)-fluorooxalacetate trapped by in situ malate dehydrogenase mediated reduction to L-threo-beta-fluoromalate (2R,3S). L-threo-Fluoro[2-3H]-malate is a slow substrate for the reverse reaction as measured by labilization of 3H while the erythro isomer is barely detectable. The pathways responsible for this volatilization are discussed. Acetylenedicarboxylate hydration stereochemistry was also determined where the initial product of the reaction, the enol of oxalacetate, tautomerized and was trapped by enzymic reduction to L-malate.