An uncoupling protein is a mitochondrial inner membrane protein that can dissipate the proton

Proton

The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

 gradient before it can be used to provide the energy for oxidative phosphorylation

Oxidative phosphorylation

Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate . Although the many forms of life on earth use a range of different nutrients, almost all aerobic organisms carry out oxidative phosphorylation to produce ATP,...

.

There are five types known in mammals:

• UCP1, also known as thermogenin
  Thermogenin
  Thermogenin is an uncoupling protein found in the mitochondria of brown adipose tissue . It is used to generate heat by non-shivering thermogenesis...
  
  
• UCP2
  UCP2
  Mitochondrial uncoupling protein 2 is a protein that in humans is encoded by the UCP2 gene.-Further reading:...
  
  
• UCP3
  UCP3
  Mitochondrial uncoupling protein 3 is a protein that in humans is encoded by the UCP3 gene.-Function:Mitochondrial uncoupling proteins are members of the larger family of mitochondrial anion carrier proteins . UCPs separate oxidative phosphorylation from ATP synthesis with energy dissipated as...
  
  
• SLC25A27
  SLC25A27
  Mitochondrial uncoupling protein 4 is a protein that in humans is encoded by the SLC25A27 gene.-Further reading:...
  
  , also known as "UCP4"
• SLC25A14
  SLC25A14
  Brain mitochondrial carrier protein 1 is a protein that in humans is encoded by the SLC25A14 gene.-Further reading:...
  
  , also known as "UCP5"

Uncoupling proteins play a role in normal physiology, as in hibernation

Hibernation

Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...

, because the energy is used to generate heat (see thermogenesis

Thermogenesis

Thermogenesis is the process of heat production in organisms. It occurs mostly in warm-blooded animals, but a few species of thermogenic plants exist.-Types:...

) instead of producing ATP

Adenosine triphosphate

Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

. However, other substances such as 2,4-Dinitrophenol

2,4-Dinitrophenol

2,4-Dinitrophenol , C6H4N2O5, is a cellular metabolic poison. It uncouples oxidative phosphorylation by carrying protons across the mitochondrial membrane, leading to a rapid consumption of energy without generation of ATP....

 and CCCP

Carbonyl cyanide m-chlorophenyl hydrazone

Carbonyl cyanide m-chlorophenyl hydrazone , is a chemical inhibitor of oxidative phosphorylation. It is a nitrile, hydrazone and ionophore. In general, CCCP causes the gradual destruction of living cells and death of the organism. The CCCP affects the protein synthesis reactions in seedling...

 also serve the same uncoupling function, and are considered poisonous. Ethanol

Ethanol

Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

 and salicyclic acid are also uncoupling agents and will deplete the body of ATP and increase body temperature if taken in excess.