Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Metabolism of Simple Proteins
Specific Pathways of Metabolism of Certain Amino Acids
Metabolism of Phenylalanine and Tyrosine

Phenylalanine is classified as an essential amino acid because animal Tissues lack The ability to synthesize its benzene ring. At the same time, Tyrosine is entirely non-essential provided that an adequate amount of phenylalanine is supplied through the diet. This is explained by the fact that the primary pathway of phenylalanine transformation begins with its oxidation (specifically, hydroxylation) into tyrosine (Fig. 12.6). The hydroxylation reaction is catalyzed by a specific phenylalanine 4-monooxygenase, which, like all other hydroxylases, contains tetrahydrobiopterin as a coenzyme. A block in this reaction, caused by impaired synthesis of phenylalanine 4-monooxygenase in the Liver, leads to The Development of a severe inherited disorder known as phenylketonuria (Phenylpyruvic Oligophrenia). During Transamination, tyrosine is converted into p-hydroxyphenylpyruvic acid, which, under the action of a specific oxidase, undergoes oxidation, decarboxylation, hydroxylation, and intramolecular rearrangement of the side chain to form homogentisic acid; this reaction requires the presence of ascorbic acid, the exact role of which remains unclear. The subsequent conversion of homogentisic acid into maleylacetoacetic acid is catalyzed by homogentisate oxidase. Under the action of a specific isomerase in the presence of Glutathione, maleylacetoacetic acid is converted into fumarylacetoacetic acid, which undergoes Hydrolysis to yield fumaric and acetoacetic acids, whose further transformations are already well established.

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Fig. 12.6. Main Metabolic pathways of Phenylalanine and Tyrosine transformations.

The numbers in circles indicate the sites of metabolic blocks in phenylketonuria (1), tyrosinemia (2), albinism (3), and alkaptonuria (4).

The Role of the tyrosine molecule in the Biosynthesis of Thyroid Hormones and catecholamines is detailed in Chapter 8. Phenylalanine and tyrosine also serve as precursors for Melanins. The enzyme tyrosinase plays an active role in this crucial biological process, which is responsible for the pigmentation of the Skin, eyes, and Hair.