Amino Acid Degradation and the Urea Cycle
30.1 Nitrogen Removal Is the First Step in the Degradation of Amino Acids
30.2 Ammonium Ion Is Converted into Urea in Most Terrestrial Vertebrates
30.3 Carbon Atoms of Degraded Amino Acids Emerge As Major Metabolic Intermediates
This fourteenth-century hand-colored woodcut from Germany depicts a wheel that classifies urine samples according to their color and consistency. In the middle of the wheel, a doctor inspects a patient’s urine by sight, smell, and taste. The vials on the wheel aided physicians in diagnosing diseases.
Dietary protein is a vital source of amino acids. Proteins ingested in the diet are digested into amino acids or small peptides that can be absorbed by the intestine and transported in the blood. Another source of amino acids is the degradation of defective or unneeded cellular proteins. Protein turnover—the degradation and resynthesis of proteins—takes place constantly in cells. Although some proteins, such as the proteins that make up the lens of the eye, are very stable, many proteins are short-lived, particularly those that are important in metabolic regulation. Altering the amounts of these proteins can rapidly change metabolic patterns. In addition, cells have mechanisms for detecting and removing damaged proteins. A significant proportion of newly synthesized protein molecules are defective because of errors in translation. Even proteins that are normal when first synthesized may undergo oxidative damage or be altered in other ways with the passage of time. What is the fate of the amino acids released when proteins are degraded? Amino acids are not stored like carbohydrates or fats, and so they must be processed biochemically. Excess amino acids are first used as building blocks for anabolic reactions, such as protein synthesis and nucleotide synthesis. If these needs are met, the amino acids are degraded and the carbon skeletons are used in catabolism or anabolism. What happens to the nitrogen of amino acids? Its safe removal is important because excess nitrogen in the form of ammonia is toxic.
In this chapter, we examine how nitrogen is removed from amino acids and subsequently eliminated as urea, the excreted form of NH4+ in most vertebrates. We then consider the fates of the carbon skeletons of the amino acids.