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INTRODUCTION

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OBJECTIVES

After studying this chapter, you should be able to:

  • Explain why the absence from the diet of certain amino acids that are present in most proteins is not deleterious to human health.

  • Appreciate the distinction between the terms “essential” and “nutritionally essential” amino acids, and identify the amino acids that are nutritionally nonessential.

  • Name the intermediates of the citric acid cycle and of glycolysis that are precursors of aspartate, asparagine, glutamate, glutamine, glycine, and serine.

  • Illustrate the key role of transaminases in amino acid metabolism.

  • Explain the process by which the 4-hydroxyproline and 5-hydroxylysine of proteins such as collagen are formed.

  • Describe the clinical presentation of scurvy, and provide a biochemical explanation for why a severe deprivation of vitamin C (ascorbic acid) results in this nutritional disorder.

  • Appreciate that, despite the toxicity of selenium, selenocysteine is an essential component of several mammalian proteins.

  • Define and outline the reaction catalyzed by a mixed-function oxidase.

  • Identify the role of tetrahydrobiopterin in tyrosine biosynthesis.

  • Indicate the role of a modified tRNA in the cotranslational insertion of selenocysteine into proteins.

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BIOMEDICAL IMPORTANCE

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Amino acid deficiency states can result if nutritionally essential amino acids are absent from the diet, or are present in inadequate amounts. Examples in certain regions of West Africa include kwashiorkor, which results when a child is weaned onto a starchy diet poor in protein, and marasmus, in which both caloric intake and specific amino acids are deficient. Patients with short bowel syndrome unable to absorb sufficient quantities of calories and nutrients suffer from significant nutritional and metabolic abnormalities. Both the nutritional disorder scurvy, a dietary deficiency of vitamin C, and specific genetic disorders are associated with an impaired ability of connective tissue to form hydroxyproline and hydroxylysine. The resulting conformational instability of collagen results in bleeding gums, swelling joints, poor wound healing, and ultimately in death. Menkes syndrome, characterized by kinky hair and growth retardation, results from a dietary deficiency of copper, an essential cofactor for the enzyme lysyl oxidase that functions in formation of the covalent cross-links that strengthen collagen fibers. Genetic disorders of collagen biosynthesis include several forms of osteogenesis imperfecta, characterized by fragile bones, and Ehlers-Danlos syndrome, a group of connective tissue disorders that result in mobile joints and skin abnormalities due to defects in the genes that encode enzymes including lysyl hydroxylase.

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NUTRITIONALLY ESSENTIAL & NUTRITIONALLY NONESSENTIAL AMINO ACIDS

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While often employed with reference to amino acids, the terms “essential” and “nonessential” are misleading since all 20 common amino acids are essential to ensure health. Of these 20 amino acids, 8 must be present in the human diet, and thus are best termed “nutritionally essential.” The other 12 amino acids are “nutritionally nonessential” since they need not be present in the diet (Table 27–1). The distinction between these two classes of amino acids was established in the 1930s by ...

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