Urea cycle | An overview

The urea cycle (also known as the 'ornithine cycle') is a cycle of biochemical reactions that produces urea from ammonia (NH3). This cycle occurs in ureotelic organisms. The urea cycle converts highly toxic ammonia to urea for excretion.This cycle was the first metabolic cycle to be discovered (Hans Krebs and Kurt Henseleit, 1932), five years before the discovery of the TCA cycle. The urea cycle takes place primarily in the liver and, to a lesser extent, in the kidneys.

Fig. Urea cycle reactions takes place in mitochondria and cytoplasm

The urea cycle comprises 6 enzymes expressed in cells of the liver. The first three are intramitochondrial and the others are cytosolic. Liver is the only organ which can generate urea from ammonia. Ammonia is delivered to the liver by the portal vein from the intestines and is also produced within mitochondrial.