Nutritionally Essential & Nutritionally Nonessential Amino Acids
المؤلف:
Peter J. Kennelly, Kathleen M. Botham, Owen P. McGuinness, Victor W. Rodwell, P. Anthony Weil
المصدر:
Harpers Illustrated Biochemistry
الجزء والصفحة:
32nd edition.p274
2025-08-11
443
While often employed with reference to amino acids, the terms “essential” and “nonessential” are misleading, since for human subjects 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 “nutritionally nonessential” amino acids, while metabolically essential, need not be present in the diet (Table 1). The dis tinction between these two classes of amino acids was established in the 1930s by feeding human subjects a diet in which purified amino acids replaced protein. Subsequent biochemical investigations ultimately revealed the reactions and intermediates involved in the biosynthesis of all 20 amino acids. Amino acid deficiency disorders are endemic in certain regions of West Africa where diets rely heavily on grains that are poor sources of tryptophan and lysine. These nutritional disorders 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.
Table1. Amino Acid Requirements of Humans
Biosynthesis of the Nutritionally Essential Amino Acids Involves Lengthy Metabolic Pathways
The existence of nutritional requirements suggests that dependence on an external source of a specific nutrient can be of greater survival value than the ability to biosynthesize it. Why? Because if the diet contains ample quantities of a nutrient, retention of the ability to biosynthesize it represents information of negative survival value, because ATP and nutrients are not required to synthesize “unnecessary” DNA—even if specific encoded genes are no longer expressed. The number of enzymes required by prokaryotic cells to biosynthesize the nutritionally essential amino acids is large relative to the number of enzymes required for the formation of the nutritionally nonessential amino acids (Table2). This suggests a survival advantage for humans to retain the ability to biosynthetize “easy” amino acids while losing the ability to make others more difficult to biosynthesize. The reactions by which organ isms such as plants and bacteria, but not human subjects, form certain amino acids are not discussed. This chapter addresses the reactions and intermediates involved in the biosynthesis of the 12 nutritionally nonessential amino acids in human tissues and emphasizes selected medically significant disorders associated with their metabolism.
Table2. Enzymes Required for the Synthesis of Amino Acids From Amphibolic Intermediates
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