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. The F1 subunit includes three "beta" subunits that are identical in structure but not in form. The 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.