-
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(A) Electron overflow model (considered out-of-date)
Alt
UQ pool
Alternative
oxidase
inactive.
Alt
No alternative
pathway activity
Cytochrome pathway
unsaturated
Cyt
(B) Electron distribution model (reflects current thinking)
UQ
pool
Cyt
Alternative
pathway active
Cytochrome pathway
saturated
Alt
Alternative
oxidase
active
Alt
UQ
pool
Cyt
Cyt
Figure 14.33
Two models for regulation of electron flow through
the alternative oxidase. (A) In the electron overflow
model, no appreciable electron transfer through the
alternative pathway takes place until electron flow
through the cytochrome pathway is at or near satu-
ration. This could result from the effects of respirato-
ry control, if the rate of mitochondrial ATP produc-
tion exceeds its rate of utilization in the cytosol, or
from some externally imposed stress, such as low
temperature. Under such circumstances, the UQ pool
becomes sufficiently reduced to allow electrons to
flow through the alternative oxidase, the latter re-
quiring that the UQ pool be 40% to 60% reduced to
attain significant activity. (B) In the electron distribu-
tion model, the alternative and cytochrome path-
ways both show significant activity at low levels of
UQ pool reduction, and electrons are distributed be-
tween the two pathways on the basis of the relative
activities of each pathway. The activity of the alter-
native oxidase under these circumstances is thought
to be regulated by the action of a-keto acids and by
reduction/oxidation of the intermolecular disulfide
bond, as well as by additional regulatory mecha-
nisms not yet characterized.
