How are proteins catabolised down? How are proteins used by the body for energy and other metabolites? By what pathway are proteins removed by the body?

Protein catabolism, or the breakdown of proteins, occurs in several steps:

1. Protein Digestion: Proteins are first broken down into their constituent amino acids through the process of digestion. This begins in the stomach, where the enzyme pepsin breaks proteins down into smaller polypeptides. In the small intestine, these polypeptides are broken down further into amino acids by enzymes such as trypsin and chymotrypsin.

2. Deamination: Once the amino acids are absorbed into the bloodstream, they are transported to the liver. Here, the process of deamination occurs, where the amino group is removed from the amino acid. This results in ammonia, which is converted into urea and excreted by the kidneys, and a keto acid.

3. Keto Acid Modification: The remaining keto acid can be modified further to enter various metabolic pathways. For example, it can be used in the Krebs cycle for energy production, or it can be used to synthesize new proteins or other molecules.

Proteins are not typically the body's first choice for energy production. The body prefers to use carbohydrates and fats for energy, turning to proteins only when these other sources are not sufficient. When proteins are used for energy, they are broken down into their constituent amino acids, which can then be converted into glucose or fat to be used in energy production.

The body removes excess proteins primarily through the kidneys. The liver breaks down amino acids and produces urea as a waste product, which is then transported to the kidneys. The kidneys filter the blood, removing urea and other waste products, which are then excreted in the urine. This is the main pathway by which proteins are removed from the body.