Alanine cycle
Encyclopedia
The alanine cycle is quite similar to the Cori cycle
. When muscles produce lactate
during times of decreased oxygen, they also produce alanine
. This alanine is shuttled to the liver where it is used to make glucose
.
The alanine cycle is less productive than the Cori Cycle, which uses lactate, since a byproduct of energy production from alanine is production of urea
. Removal of the urea is energy-dependent,requiring four "high-energy" phosphate bonds (3 ATP hydrolyzed to 2 ADP and one AMP), thus the net ATP produced is less than that found in the Cori Cycle. However, unlike in the Cori Cycle, NADH is conserved because lactate is not formed. This allows for it to be oxidized via the electron transport chain
. This pathway requires the presence of alanine aminotransferase, which is restricted to tissues such as muscle
, liver
, and the intestine
. Therefore, this pathway is used instead of the Cori Cycle only when an aminotransferase is present and when there is a need to transfer ammonia to the liver.
Alanine cycle also serves other purposes:
Cori cycle
The Cori cycle , named after its discoverers, Carl Cori and Gerty Cori, refers to the metabolic pathway in which lactate produced by anaerobic glycolysis in the muscles moves to the liver and is converted to glucose, which then returns to the muscles and is converted back to lactate.-Cycle:Muscular...
. When muscles produce lactate
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
during times of decreased oxygen, they also produce alanine
Alanine
Alanine is an α-amino acid with the chemical formula CH3CHCOOH. The L-isomer is one of the 20 amino acids encoded by the genetic code. Its codons are GCU, GCC, GCA, and GCG. It is classified as a nonpolar amino acid...
. This alanine is shuttled to the liver where it is used to make glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
.
The alanine cycle is less productive than the Cori Cycle, which uses lactate, since a byproduct of energy production from alanine is production of urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
. Removal of the urea is energy-dependent,requiring four "high-energy" phosphate bonds (3 ATP hydrolyzed to 2 ADP and one AMP), thus the net ATP produced is less than that found in the Cori Cycle. However, unlike in the Cori Cycle, NADH is conserved because lactate is not formed. This allows for it to be oxidized via the electron transport chain
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...
. This pathway requires the presence of alanine aminotransferase, which is restricted to tissues such as muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
, liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
, and the intestine
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
. Therefore, this pathway is used instead of the Cori Cycle only when an aminotransferase is present and when there is a need to transfer ammonia to the liver.
Alanine cycle also serves other purposes:
- Recycles carbon skeletons between muscle and liver
- Transports ammonium to the liver and is converted into urea.
External links
- Diagram at colorado.edu
- http://themedicalbiochemistrypage.org/amino-acid-metabolism.html#alaninecycle at indstate.edu
