The "proton buffering" model proposes that UCP1 is intrinsically a proton carrier, and that fatty acid acts as a prosthetic group during proton transport. Fatty acid penetrates from the lipid phase, with its carboxyl group oriented to the proton translocation path. Here, it works as a donor-acceptor of protons between the residual carboxyl groups of UCP1. Ultimately, protons are extruded to the matrix side of the membrane.
Rial et al (2004) suggest fatty acids are inducers of proton transport by UCP by allowing themselves to become substrates for UCP and activation of the proton buffering mechanism itself. Binding of nucleotides to UCP inhibits it's proton transport capability. UCP accepts purine ribose tri- and di- nucleotides; GTP, ATP, GDP and ADP. The monophosphates GMP and AMP are poor ligands for UCP binding.