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 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

Visitors: to e nsure that your mess age is not mistaken for S PAM, please include the acronym "Bios211" in the subject line

of e-mail communications

Created by David R. Caprette ( caprette@rice.edu ), Rice Unive rsity 31 May 05