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INTRODUCTION

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OBJECTIVES

After studying this chapter, you should be able to:

  • Cite examples of 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.

  • Discuss the role played by glycine in drug catabolism and excretion.

  • Document the role of glycine in the biosynthesis of heme, purines, creatine, and sarcosine.

  • Identify the enzyme and the reaction that convert an amino acid to the neurotransmitter histamine.

  • Document the role of S-adenosylmethionine in metabolism.

  • Recognize the tryptophan metabolites serotonin, melatonin, tryptamine, and indole 3-acetate.

  • Indicate the role of tyrosine in the formation of norepinephrine and epinephrine.

  • 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.

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BIOMEDICAL IMPORTANCE

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Certain proteins contain amino acids that have been posttranslationally 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 5-hydroxy-lysine, 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 diverse 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.

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l-α-AMINO ACIDS

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Alanine

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Alanine serves as a carrier of ammonia and of the carbons of pyruvate from skeletal muscle to liver via the Cori cycle (see Chapter 19), and together with glycine constitutes a major fraction of the free amino acids in plasma.

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Arginine

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Figure 30–1 summarizes the metabolic fates of arginine. In addition to serving as a carrier of nitrogen atoms in urea biosynthesis (see Figure 28–16), 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 (see below).

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FIGURE 30–1

Arginine, ornithine, and proline metabolism. Reactions with solid arrows all occur in mammalian tissues. Putrescine and spermine synthesis occurs in both mammals and bacteria. Arginine phosphate of invertebrate muscle functions as a phosphagen analogous ...

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