Figure 2. Schematic mechanism of proton transport by the UCPs: direct proton translocation, or proton release associated with cycling of fatty acid.

The inner mitochondrial membrane is shown as the two outer stippled rectangles. The matricial side is below the membrane. The UCP is represented as the two inner striped grey rectangles and ovals corresponding to membranous domain and gating domain, respectively. The right-hand side part of the figure shows a direct transport of proton by UCP; the proton is provided by the carboxyl group of either amino acid residues or fatty acid. The left-hand side of the figure illustrates the free diffusion of protonated fatty acid through the membrane followed by release of proton on the matricial side. Then, UCP returns the anionic form of fatty acid through the membrane.