In addition to their roles as building blocks for peptides and proteins, and as precursors of neurotransmitters and hormones, amino acids are a source of energy from the diet and during fasting. The carbon skeletons of some amino acids can be used to produce glucose through gluconeogenesis, thereby providing a metabolic fuel for tissues that require or prefer glucose; such amino acids are designated as glucogenic or glycogenic amino acids. The carbon skeletons of some amino acids can also produce the equivalent of acetyl-CoA or acetoacetate and are termed ketogenic, indicating that they can be metabolized to give immediate precursors of lipids or ketone bodies (Fig. 18.1). In an individual consuming adequate amounts of protein, a significant quantity of amino acids may also be converted to carbohydrate (glycogen) or fat (triacylglycerols) for storage. Unlike carbohydrates and lipids, amino acids do not have a dedicated storage form equivalent to glycogen or fat.
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| When amino acids are metabolized, the resulting excess nitrogen must be excreted. As the primary form in which the nitrogen is removed from amino acids is ammonia, and because free ammonia is quite toxic, humans and most higher animals rapidly convert the ammonia derived from amino acid catabolism to urea, which is neutral, less toxic, very soluble, and excreted in the urine. Thus the primary nitrogenous excretion product in humans is urea, produced by the urea cycle in liver. Animals that excrete urea are termed ureotelic. In an average individual, more than 80% of the excreted nitrogen is in the form of urea (25-30 g/24 hours). Small amounts of nitrogen are also excreted in the form of uric acid, creatinine, and ammonium ion.
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| The carbon skeletons of many amino acids may be derived from metabolites in central pathways, allowing the biosynthesis of some, but not all, the amino acids in humans. Amino acids that can be synthesized in this way are therefore not required in the diet (non-essential amino acids), whereas amino acids having carbon skeletons that cannot be derived from normal human metabolism must be supplied in the diet (essential amino acids). In contrast to the catabolic process, for the biosynthesis of non-essential amino acids, amino groups must be added to the appropriate carbon skeletons. This generally occurs through the transamination of an α-keto acid corresponding to that specific amino acid.
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