J. Biol. Chem., Vol. 261, Issue 17, 7733-7741, 06, 1986
Energetics of beta-oxidation. Reduction potentials of general fatty acyl-CoA dehydrogenase, electron transfer flavoprotein, and fatty acyl- CoA substrates
WG Gustafson, BA Feinberg and JT McFarland
We have determined reduction potentials for porcine mitochondrial general
fatty acyl-CoA dehydrogenase (GAD) and electron transfer flavoprotein (ETF)
using an anaerobic spectroelectrochemical titration method. Computer
simulation techniques were used to analyze the absorbance data. Nernst
plots of the simulated data gave E'0, 7.1, quinone/semiquinone = -0.014 V
and E'0, 7.1, semiquinone/hydroquinone = -0.036 V for ETF and E'0, 7.1,
quinone/semiquinone = -0.155 V and E'0, 7.1, semiquinone/hydroquinone =
-0.122 V for GAD. Using these techniques we have also determined a
conditional reduction potential of -0.156 V for the chromophore producing
fatty acyl-CoA substrate beta-2- furylpropionyl-CoA. From this value and
our previous determination of the equilibrium constant for the
transhydrogenation reaction between beta-2-furylpropionyl-CoA and the
oxidized substrate crotonyl-CoA (Keq = 10.4), we have determined a
reduction potential of -0.126 V for the butyryl-CoA/crotonyl-CoA couple. In
light of the structural similarity between butyryl-CoA and octanoyl-CoA,
the optimal substrate for GAD, the reduction potential for octanoyl-CoA
should be similar to that for butyryl-CoA; i.e. fatty acyl-CoA substrates
and GAD are essentially isopotential. The ability of octanoyl-CoA to reduce
GAD quantitatively (Keq = 9.0) poses a dilemma in light of the nearly equal
reduction potentials. We postulate that the stable charge-transfer complex
formed between enzyme and optimal product is significantly lower in energy
than enzyme and product and thus is responsible for pulling the reaction
toward completion.
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