Mitochondrial ATP synthases (ATPases) transduce the energy contained in membrane electrochemical proton gradients into the energy required for synthesis of high-energy phosphate bonds. ATPases contain two linked complexes: F1, the hydrophilic catalytic core; and F0, the membrane-embedded protein channel. F1 consists of three Alpha chains and three Beta chains, which are weakly homologous, as well as one Gamma chain, one Delta chain and one Gamma chain. F0 consists of three subunits: a, b and c. A mitochondrial F1-ATPase inhibitor protein, ATPIF1 (ATPase inhibitory factor 1), also known as IP, IF1, ATPI or ATPIP (ATPase inhibitor protein), binds to the C-terminal region of a Beta subunit of the F1-ATPase at low pH values and, via interference of the Beta and Gamma subunit interaction, ATPIF1 regulates the activity of the F1F0-ATPase. This reversible ATPIF1 binding to F1F0-ATPase also occurs on the surface of endothelial cells.