The formation of ATP

The formation of ATP by condensation between ADP and inorganic phosphate is a highly endothermic reaction. The ATP synthetase is located in the primary particles on the inner face of the crista membrane; isolated primary particles catalyse hydrolysis of ATP to ADP and phosphate, and you will sometimes see ATP synthetase referred to as an ATPase.

ATP synthetase is a multi-subunit enzyme with three equivalent catalytic sites.

At any given time, each site is at a different stage in the reaction:

• One site is binding ADP and phosphate
• One site is catalysing the condensation of ADP and phosphate, and expelling and water
• One site is expelling ATP, ready to accept ADP and phosphate

As protons enter through the stalk of the primary particle that spans the crista membrane, so they cause a rotation of the central part of the ATP synthetase, forcing each site in turn to proceed to the next step in the reaction.

Electron transport and substrate oxidation are controlled by the availability of ADP

If there is no ADP available to bind to the empty site, then rotation of the central part of the ATP synthetase is not possible, and protons cannot cross the stalk of the primary particle. This leads to an accumulation of protons in the crista space, which, in turn, inhibits any further expulsion of protons by the electron transport chain, so that electron transport (and therefore substrate oxidation) ceases.