Amino Acid and Protein Metabolism

Amino acid and protein metabolism covers how dietary and body proteins are turned over, how the twenty amino acids are synthesized and degraded, and how their nitrogen and carbon are handled. Because amino acids carry nitrogen that the body cannot store as such, their metabolism is closely tied to disposal of nitrogen through the urea cycle and to the use of carbon skeletons for energy or biosynthesis.

Definition

Amino acid and protein metabolism is the integrated set of pathways governing the synthesis and degradation of proteins and amino acids, including the transfer and disposal of amino nitrogen and the use of amino acid carbon skeletons for energy or biosynthesis.

Scope

This entry covers protein turnover and nitrogen balance, amino acid catabolism with transamination and deamination, the urea cycle, the synthesis of non-essential amino acids, and the distinction between essential and non-essential amino acids. It is a reference topic within nutritional biochemistry, not clinical or dietary guidance.

Key concepts

• Protein turnover and nitrogen balance
• Essential and non-essential amino acids
• Transamination and deamination
• Urea cycle and ammonia disposal
• Glucogenic and ketogenic amino acids
• Amino acid biosynthesis
• One-carbon and nitrogen flux

Mechanisms

Body proteins are continuously synthesized and degraded, and the resulting free amino acids join those from the diet in a common pool. When amino acids are catabolized, their amino group is removed by transamination and oxidative deamination, releasing ammonia that is detoxified in the liver through the urea cycle and excreted as urea. The remaining carbon skeletons are classified as glucogenic, ketogenic, or both, entering gluconeogenesis or the citric acid cycle accordingly. Non-essential amino acids can be synthesized from common intermediates, whereas the essential amino acids must be supplied by the diet. Overall flux through these pathways is governed by nitrogen balance and by hormonal and nutritional state, with the liver as the principal site that integrates amino acid disposal with the rest of fuel metabolism.

Clinical relevance

Amino acid and protein metabolism underpins concepts such as protein requirements, nitrogen balance, and the consequences of impaired ammonia disposal. The entry presents these mechanisms as background knowledge and does not offer individualized dietary targets or treatment advice.

History

Hans Krebs and Kurt Henseleit described the urea cycle in 1932, providing the first metabolic cycle and explaining how the body disposes of waste nitrogen. Rudolf Schoenheimer's isotope studies in the 1930s revealed the dynamic, continuous turnover of body proteins, overturning the view that tissue proteins were static.

Key figures

• Hans Krebs
• Kurt Henseleit
• Rudolf Schoenheimer
• Guoyao Wu

Seminal works

• wu-2009

Frequently asked questions

Why must some amino acids come from the diet?

Essential amino acids cannot be synthesized by the human body, or not in sufficient amounts, so they have to be obtained from dietary protein; non-essential amino acids can be made from common metabolic intermediates.

How does the body get rid of the nitrogen from amino acids?

Amino groups removed during amino acid breakdown form ammonia, which the liver converts to urea through the urea cycle for excretion, because nitrogen cannot be stored like carbohydrate or fat.