How is urea formed in the liver?

Urea is formed in the liver through a process called the urea cycle, which detoxifies ammonia into a less toxic form.

The urea cycle, also known as the ornithine cycle, is a series of biochemical reactions that occur in the liver. This cycle is crucial for the detoxification of ammonia, a by-product of protein metabolism, into a less toxic form, urea, which can be excreted by the kidneys.

The process begins with the conversion of ammonia and carbon dioxide into carbamoyl phosphate, catalysed by the enzyme carbamoyl phosphate synthetase I. This reaction requires two molecules of ATP, making it an energy-intensive process. The carbamoyl phosphate then combines with the amino acid ornithine to form citrulline, a reaction catalysed by the enzyme ornithine transcarbamylase.

In the next step, citrulline is combined with aspartate to form argininosuccinate, in a reaction catalysed by the enzyme argininosuccinate synthetase. This reaction also requires ATP. The argininosuccinate is then split into arginine and fumarate by the enzyme argininosuccinate lyase.

Finally, the arginine is hydrolysed to ornithine and urea by the enzyme arginase. The ornithine is recycled back into the cycle, and the urea is transported to the kidneys for excretion.

The urea cycle is a vital process in the body, as it allows for the safe disposal of excess nitrogen. Without this cycle, ammonia would build up in the body, leading to serious health problems. It's important to note that the urea cycle is regulated by the availability of substrates and by the activity of the enzymes involved. Any disruption in this cycle can lead to a variety of metabolic disorders.