Uncoupling Proteins

Uncoupling proteins (UCP) are a group of proteins that produce energy. They were first discovered in brown adipose tissue (fat cells) and were originally called thermogenin. Brown adipose tissue differs from white adipose tissue, which is the predominant type of fat in adult humans. Brown adipose tissue is much more metabolically active than white adipose tissue. It can utilize a greater amount of fatty acids compared to the white adipose tissue. The reason for this is that brown adipose tissue has a greater amount of mitochondria (to use more fatty acids) and a greater amount of blood vessel around it (to deliver fatty acids). White adipose tissue is primarily used for the storage of fatty acids.

Normally, the mitochondria are responsible for creating energy in the form ATP. It creates the majority of energy through a process known as an electron transport chain. One of the final steps in the electron transport chain involves moving hydrogen ions (H+) from the mitochondrial membrane back inside of the mitochondria. This movement of hydrogen ions is responsible for the creation of ATP. In brown adipose tissue, in addition to the electron transport chain, a UCP also sits within the mitochondrial membrane. It also moves hydrogen ions back inside of the mitochondria. This, in turn, will lead to the generation of heat. Therefore, one of the primary functions of UCP is to generate heat for the body. Catecholamines, such as epinephrine and norepinephrine, are major stimulators for UCP. They will increase the activities of the proteins.

Because UCP was originally discovered in brown adipose tissue, several other types of UCP have been discovered in the body. UCP1 is found in brown adipose tissue. UCP2 is found in many tissues throughout the body, UCP3 is found in skeletal muscle, and UCP4 is found in the brain. The function of UCP2 and UCP3 is not completely known at this time. The activity of UCP2 and UCP3 increases when a person has been fasting, probably as a way to generate energy for them. Also, animals that were fed a high-fat diet had an increase in their UCP2 activity. This is probably a way for the body to increase the amount of energy used when it takes in extra calories. The function of UCP4 remains unknown at this time.

Some people hypothesized that low levels of UCP1 may be related to obesity. However, adult humans have little to no brown adipose tissue. Rather, adult humans have mainly white adipose tissue which is less metabolically active. Human infants do contain brown adipose tissue and thus UCP1. This is a way for infants to generate extra energy while they are developing. Soon after development is completed, the brown adipose tissue no longer functions. Many researchers have had an interest in finding a way to stimulate UCP1 activity in adults. They believe that this may be useful for treating obesity. UCP2 has also been suspected to be related to [Page 764]the development of obesity. The gene that makes UCP2 is located on chromosome 11 (in humans). Defects in the expression of some genes on chromosome 11 have been linked to elevated insulin levels and obesity. However, no direct link between UCP2 and obesity has been established at this time.

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