The mitochondrial proton gradient generated by complexes of respiratory chain is used by F0-F1-ATP synthase to phosphorylate ADP. Another mechanism consuming the gradient and lowering ATP synthesis is proton leak (yellow
arrow). The reentry of protons in the matrix noncoupled with ATP synthesis is an energy-dissipating mechanism. The brown fat
UCP1 is an example of mitochondrial proton leak. Cyt C, cytochrome C; ΔμH+, proton electrochemical gradient; e−, electron; F0, membranous part of ATP-synthase; F1, catalytic part of ATP-synthase.