Krebs, << krehbz, >> cycle is a series of chemical reactions through which tissues use carbohydrates, fats, and proteins to produce energy. The Krebs cycle takes place in all cells that require oxygen to live. It is also called the citric acid cycle or tricarboxylic acid cycle. Hans Krebs, a German-born British biochemist, shared the 1953 Nobel Prize in physiology or medicine for discovering the process. The cycle is an important part of metabolism, in which cells change food into energy.
The chemical reactions in the cycle change acetyl-coenzyme A, a form of acetic acid, to carbon dioxide and water. Acetyl-coenzyme A (abbreviated acetyl-CoA) forms in the body from the partial breakdown of carbohydrates, fats, and proteins.
The Krebs cycle begins when acetyl-CoA combines with oxaloacetic acid, which is an end product of the cycle itself. This combination of acids forms citric acid. Rearrangement of the citric acid molecules produces isocitric acid. The isocitric acid then passes through a series of stages during which six intermediate acids are formed: (1) a-ketoglutaric, (2) succinic, (3) succinyl-coenzyme A, (4) fumaric, (5) malic, and then, finally, (6) oxaloacetic. The oxaloacetic acid is then ready to combine with acetyl-CoA to begin another cycle. Each step of the cycle provides energy.
See also Metabolism (Catabolism) ; Respiration (Cellular respiration) .
