Structure-function correlation of fatty acyl-CoA dehydrogenase and fatty acyl-CoA oxidase

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Abstract
A new pseudosubstrate, .beta.-(2-furyl)propionyl coenzyme A (FPCoA), was used to study the functional properties of 2 enzymes, fatty acyl-CoA dehydrogenase from porcine liver and fatty acyl-CoA oxidase from Candida tropicalis, involved in the oxidation of fatty acids. Previous studies showed that the dehydrogenase exhibits oxidase activity at the rate of dissociation of the product charge-transfer complex. This raises the question of the difference in functionality between these 2 flavoproteins. To investigate these differences, the pH dependence of product formation, the isotope effects using tetradeuterio-FPCoA, and the spectral properties and chemical reactivity of the product charge-transfer complexes formed with the 2 enzymes were compared. The pH dependencies of the reaction of FPCoA with electron-transfer flavoprotein (ETF) for the dehydrogenase and of the reaction of FPCoA with O2 for the oxidase are quite similar. Both reactions proceed more rapidly at basic pH values while substrate binds more tightly at acidic pH values. These data for both enzymes are consistent with a mechanism in which enzyme is involved in protonation of the carbonyl group of substrate followed by base-catalyzed removal of the C-2 proton from substrate. The C-2 anion of substrate may then serve as the active species in reduction of enzyme-bound flavin. The deuterium isotope effects for both enzyme systems are primary across the entire pH range, assuring that the chemically important step of substrate oxidation is rate limiting in these steady-state kinetic experiments. The 2 enzymes differ in the chemical reactivity of their product charge-transfer complexes. The complex formed during reaction of the dehydrogenase and FPCoA is unstable with a dissociation rate constant of 0.01 s-1 and reacts with ETF but not with O2. The complex formed from the reaction of oxidase with FPCoA is stable, showing no dissociation; it reacts rapidly with O2 but does not react with ETF. The functional difference of these 2 FAD containing flavoproteins resides in the difference in chemical reactivity of the product charge-transfer complexes.