How is ammonia converted to urea in the liver?

Ammonia is converted to urea in the liver through a process called the urea cycle.

The urea cycle is a series of biochemical reactions that occur in the liver. This process is crucial for the detoxification of ammonia, which is a by-product of protein metabolism. High levels of ammonia in the body can be toxic, so it's essential that it's converted into a less harmful substance, such as urea.

The urea cycle begins with the formation of carbamoyl phosphate from ammonia and bicarbonate, catalysed by the enzyme carbamoyl phosphate synthetase I. This reaction occurs in the mitochondria of liver cells. The carbamoyl phosphate then combines with ornithine to form citrulline, a reaction catalysed by the enzyme ornithine transcarbamylase.

Citrulline is then transported out of the mitochondria and into the cytosol, where it combines with aspartate to form argininosuccinate. This reaction is catalysed by the enzyme argininosuccinate synthetase. The argininosuccinate is then split into arginine and fumarate by the enzyme argininosuccinate lyase.

Finally, the arginine is hydrolysed to form urea and ornithine by the enzyme arginase. The ornithine is then transported back into the mitochondria, ready to start the cycle again. The urea produced is water-soluble and can be easily excreted by the kidneys in urine.

In summary, the conversion of ammonia to urea in the liver involves a series of enzymatic reactions known as the urea cycle. This process is vital for the detoxification of ammonia, a potentially harmful by-product of protein metabolism.