Spontaneous proton leak can occur in a nonspecific manner at the level of protein-lipid interface within the inner mitochondrial membrane.
This process is mediated by uncoupling protein (UCP), which can be via indirect or direct transport. UCP1 is also known as thermogenin, which is found primarily in brown adipose tissue.
Proton leak is implicated in the release of energy as protons move back into the mitochondria. This energy is released as heat in brown adipose cells and is an active process during early development and in hibernating animals.