The proton is delivered to the matrix side FA spontaneously flip-flops back to the matrix side FA anion picks up a proton FA anion diffuses laterally to UCP The FA anion diffuses away laterally from UCP FA anion flip-flops to the opposite surface PathwayStep2193 PathwayStep2192 PathwayStep2191 PathwayStep2190 PathwayStep2189 PathwayStep2188 The fatty acid cycling model Pubmed10653472 Pubmed8576230 Pubmed9821950 Reactome Database ID Release 43167826 Reactome, http://www.reactome.org ReactomeREACT_6258 The "fatty acid cycling" hypothesis proposes that protonated fatty acids flip-flop in the membrane and deliver a proton to the matrix side. UCP1 catalyses the return of the fatty acid anion to the cytosolic side of the membrane, resulting in net proton transport catalysed by the protein.