Heat generation in brown fat

The production of ATP in the mitochondria has been long known to depend on the proton gradient that is setup by the electron transport chain. Briefly, electrons from energy-rich molecules like glucose and fatty acids are transported to the inner mitochondrial membrane, which contains specialized proteins that can receive and donate electrons to the next electron acceptor in the chain. The flow of electrons eventually combine with oxygen to form water, but this flow of electrons is also coupled to the energy-requiring process of pumping protons from the inner mitochondrial matrix to the inter-membrane space. ATP Synthase, a multi-unit protein situated in the inner mitochondrial membrane, is the only passage for protons back into the mitochondrial matrix. When protons flow down their gradient through the ATP Synthase, sufficient energy is available to catalyze the phosphorylation of ADP to yield ATP.

In addition to ATP Synthase, researchers recently discovered another protein channel situated in the inner mitochondrial matrix that allows the passage of protons back into the mitochondrial matrix. They named these channels “uncoupling proteins” (UCPs) because they do not have any enzymatic activity to catalyze the production of ATP and therefore allow protons to flow down their gradient back into the mitochondrial matrix without utilizing this energy for the synthesis of ATP. In an animal model, researchers noted the following about the UCP to ATP Synthase ratio in brown fat and white fat:

Brown fat is most abundant in infants to generate body heat since newborns cannot shiver to stay warm. White fat is used to store energy in the form of triglycerides and is usually $20$‍-$25\mathrm{\%}$‍ of an adult human’s body weight.