Chapter 30

After studying this chapter, you should be able to:

• Describe how amino acids participate in a variety of biosynthetic processes other than protein synthesis.
• Outline how arginine participates in the biosynthesis of creatine, nitric oxide (NO), putrescine, spermine, and spermidine.
• Indicate the contribution of cysteine and of β-alanine to the structure of coenzyme A and of cysteine to the structure of taurocholic acid.
• Discuss the role of glycine in drug catabolism.
• Document the role of glycine in the biosynthesis of heme, purines, creatine, and sarcosine.
• Identify the reaction that converts an amino acid to the neurotransmitter histamine.
• Document the role of S-adenosylmethionine as a source of methyl groups in metabolism.
• Recognize the tryptophan metabolites serotonin and melatonin, and the conversion of tryptophan to tryptamine and subsequently to indole 3-acetate.
• Indicate the role of tyrosine in the formation of norepinephrine, epinephrine, triiodothyronine, and thyroxine.
• Illustrate the key roles of peptidyl serine, threonine and tyrosine in metabolic regulation and signal transduction pathways.
• Outline the roles of glycine, arginine, and S-adenosylmethionine in the biosynthesis of creatine.
• Describe the role of creatine phosphate in energy homeostasis.
• Describe the formation of γ-aminobutyrate (GABA) and the rare metabolic disorders associated with defects in GABA catabolism.

Certain proteins contain amino acids that have been post-translationally modified to permit them to perform specific functions. Examples include the carboxylation of glutamate to form γ-carboxyglutamate, which functions in Ca2+ binding, the hydroxylation of proline for incorporation into the collagen triple helix, and the hydroxylation of lysine to hydroxylysine, whose subsequent modification and cross-linking stabilizes maturing collagen fibers. In addition to serving as the building blocks for protein synthesis, amino acids serve as precursors of biologic materials such as heme, purines, pyrimidines, hormones, neurotransmitters, and biologically active peptides. Histamine plays a central role in many allergic reactions. Neurotransmitters derived from amino acids include γ-aminobutyrate, 5-hydroxytryptamine (serotonin), dopamine, norepinephrine, and epinephrine. Many drugs used to treat neurologic and psychiatric conditions act by altering the metabolism of these neurotransmitters. Discussed below are the metabolism and metabolic roles of selected α- and non-α amino acids.

### Alanine

Alanine serves as a carrier of ammonia and of the carbons of pyruvate from skeletal muscle to liver via the Cori cycle (see Figure 20–4), and together with glycine constitutes a major fraction of the free amino acids in plasma.

### Arginine

Figure 30–1 summarizes the metabolic fates of arginine. In addition to serving as a carrier of nitrogen atoms in urea biosynthesis (see Figure 28–13), the guanidino group of arginine is incorporated into creatine, and following conversion to ornithine, its carbon skeleton becomes that of the polyamines putrescine and spermine.

###### Figure 30–1

Arginine, ornithine, and proline metabolism. Reactions with solid arrows all occur in mammalian tissues. Putrescine and spermine synthesis ...

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