FIG. 2.

The UCP-catalyzed protonophoretic cycle—ongoing according to the (A) FA cycling or (B) “FA shuttling.” (A) In FA cycling model, FA⁻ anion diffuses laterally within the membrane to reach a subsurface peripheral UCP binding site near the matrix (196), where it binds specifically to basic residues Arg60 and Lys271 (depicted as +) (31). The IMM potential drives the carboxylate head group through an electrostatic path composed of basic residues both inside and outside the UCP cavity (31), resulting in a transport of FA⁻ anion to the other side of the membrane (vertical arrow), that is, to the ICS-proximal lipid leaflet of ICS membranes (parts of IMM enfolded into cristae). The anion diffuses laterally (horizontal arrows) away from UCP, where it is protonated. Protonated FA diffuses rapidly back across the membrane to deliver protons electroneutrally back to the matrix by a spontaneous flip-flop mechanism, completing the cycle (182). (B) “FA shuttling” mechanism, in fact considering protein as a “carrier” where FA shuttles back and forth (wobbling) (124), actually differs so that the FA molecule cannot diffuse away from the UCP protein and stays in an unspecified way bound to the protein while exposed to the cis or trans side of the membrane either as anion or after protonation. In this case, both anionic and neutral protonated FAs are carried through the UCP protein. However, since the actual binding site was verified to face the lipid bilayer (31), this mechanism is very unlikely. FA, fatty acid; ICS, intracristal space; IMM, inner mitochondrial membrane.