Protein Metabolism

Dietary proteins are digested in the intestine and produced their constituent amino acids that are absorbed into the blood stream. Amino acids are also obtained in the blood stream from normal body protein degradation or from de novo synthesis.

Amino acids are safely locked by the α-amino group. Removal of α-amino group is essential for generating energy from amino acids, and it is an obligatory step in amino acid metabolism.

The first stage of amino acid metabolism involves the removal of the α-amino group from amino acids by transamination and subsequent deamination forming ammonia and corresponding α-ketoacid. A small portion of ammonia is excreted through the urine, but large portion is used in the formation of urea that is the most important route for removing of nitrogen from the body.

In second stage of amino acid metabolism, the α-ketoacids also called carbon skeletons of amino acids are converted into common intermediate products. These intermediate products can (1) be metabolized to carbon dioxide and water with generation of energy via citric acid cycle; (2) be used to form glucose via gluconeogenesis; or (3) synthesize ketone bodies.

Overall, protein metabolism can be divided into two categories:

A) Catabolic pathway of protein metabolism:

• Catabolism of body protein to amino acids – our body proteins are converted to amino acids via a series of catabolic reactions.
• Transamination and Deamination – α-amino group is removed from the amino acids by transamination and deamination, forming ammonia and corresponding α-ketoacid.
• Catabolism of carbon skeleton of amino acid – Carbon skeleton of amino acid is catabolised into seven common intermediate products such as pyruvate, fumarate, oxaloacetate, α-ketoglutarate, acetyl CoA, acetoacetyl CoA and succinyl CoA.

B) Anabolic pathway of protein metabolism:

• Protein synthesis – Protein is synthesized from the amino acids through a series of anabolic reactions.
• Biosynthesis of non-essential amino acids – Non-essential amino acids are formed from the intermediates of metabolism or, from the carbon skeletons of essential amino acids.
• Urea cycle – Most of the toxic ammonia synthesized from the deamination of amino acids in the liver are converted into non-toxic urea by the process of urea cycle. From the liver urea is transported to kidney for disposal.