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Enzymatic Redox Chemistry 133
H
O
ENZ
SCoA
FADH 2 FADH
H
O
SCoA
FAD
O
recombination
with FADH
SCoA
O
SCoA
Figure 6.16 Inactivation of acyl CoA dehydrogenase by hypoglycin A.
R
R
H
AEnz
N
N
O
N
N
O
N
NH
substrate
deprotonation
N
NH
2 x 1e −
external
oxidants
β
α
R
H
EnzB −
O
O
SCoA
1e − transfer O
O
R
SCoA
1-electron transfer
to flavin
R
N
H
N
O
H atom transfer
at β position
R
N
H
N
O
N
H
O
O
NH
N
H
O
O
NH
R
SCoA
R
SCoA
Figure 6.17 Probable mechanism for acyl CoA dehydrogenase.
formed in the mechanism via single electron transfer to FAD. This amine
radical cation undergoes a further one-electron oxidation to give an iminium
ion, which is then hydrolysed to give the corresponding aldehyde, as shown in
Figure 6.18. The second one-electron transfer could proceed either by hydrogen
atom transfer (i.e. loss of H . from the a-position, route a) or by loss of a proton
from the a-carbon followed by single electron transfer (route b). There is
evidence from further ‘radical trap’ inhibitors for the existence of an a-radical
intermediate, consistent with route b.