biosciences
TRANSCRIPT
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1/2www.ruf.rice.edu/~bioslabs/studies/mitochondria/oxphos.html
Introduction/training [organization of the study] [ polarography] [calibrating] [research paper ]
Mitochondria theory: [overview] [structure] [Krebs reactions] [electron transport] [the
gradient] [oxidative phosphorylation]
Mitochondria in vitro: [ preparation] [fate of substrates] [state IV] [state III] [metabolic
poisons] [mitotraces] [rationale] [experiments]
Additional topics: [glossary of terms ] [Hans Krebs] [origin of mitochondria] [other functions]
Oxidative phosphorylation
The relationship between synthesis (phosphorylation) of ATP and electron transport (the last
part of oxidative metabolism) often confuses students. Here are some facts that may help
dispel misconceptions about oxidative phosphorylation.
ATP synthase is not part of the electron transport system (ETS)
Oxygen consumption results from electron transport and does not require ATP
synthesis
Protons entering the matrix through ATP synthase do not reduce oxygen
The ETS cannot transport electrons if protons cannot be translocated into the
intermembrane space
The number of protons translocated by proton pumps do not corrrelate directly with
number of ATP molecules synthesized
The ETS does not "want" to maintain or restore a chemiosmotic gradient – electron
transport is driven by the proximity of reduced and oxidized carriers, facilitating
exchange of electrons and free energy
As long as substrate is present a chemiosmotic gradient is maintained (unless
mitochondria are poisoned)
Activation of ATP synthase does not "lower" the gradient – it increases the rate at
which energy is removed from the gradient; the ETS maintains the gradient at a constant
level
ATP synthase consists of two functional units, one that conducts the passage of protons (F0)
and one that catalyzes the phosphorylation of ADP (F1). Both units must be functional for
ATP synthesis to take place. The F0 subunit (actually "F sub-zero," the zero is a subscript)
can be pictured as a rotor while the F1 subunit remains stationary. F0 includes a "gamma"
subunit that rotates as protons are driven through the channel created by the c ring component
of F0.
The following is a simplistic description of the Boyer model for proton-driven ATP synthesis.
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conformation of each beta subunit changes as the gamma subunit of F0 rotates. At any given
time one beta subunit is in the "loose" (L) conformation, which binds ADP and inorganic
phosphate. In that conformation the subunit is constrained so that it does not release either
molecule. A beta subunit in "tight" (T) conformation binds ATP with such tenacity that it readily
converts ADP and inorganic phosphate to ATP. A subunit in the T conformation cannot
release ATP, however. In the "open" (O) conformation a beta subunit releases bound
nucleotides.
Even if there is no proton gradient one beta subunit will be in the T form with bound ATP,
which forms spontaneously even in the absence of a proton gradient. The role of the gradient is
to cause the release of bound ATP, not to cause its synthesis. Once ATP is released, binding
of ADP and inorganic phosphate is spontaneous. Of course, both reactants must be present
for the system to operate. A complete cycle takes place as follows. As protons are driven
through the c ring their passage causes rotation of the gamma subunit. As the subunit rotates it
causes conversion of the T form (with bound ATP) to the O form. ATP is then released.
Meantime, the subunit in L form (which holds bound ADP and inorganic phosphate) is
converted to the T form which results in conversion of ADP and phosphate to ATP. The
subunit that had been in the O conformation is converted into the L form, binding and "locking"
ADP and inorganic phosphate in place.
For ATP synthesis to continue ADP and inorganic phosphate must both be available and the
ETS must be capable of conducting electron transport and storing energy as a chemiosmotic
gradient. Each proton pump translocates a specific number of protons (from 2 to 4) with each
passage of an electron pair. A specific number of protons must be driven through the F0
subunit of ATP synthase to accomplish one complete cycle. Because some energy stored in
the gradient is always lost as heat or exploited for processes other than ATP synthesis, there is
not a one to one correspondence between number of protons translocated by the ETS and
number of protons entering the matrix through ATP synthase.
Copyright and Intended Use
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Created by David R. Caprette ( [email protected] ), Rice Unive rsity 31 May 05