process

Tyrosine synthesis

Phenylalanine is an essential amino acid. Tyrosine is synthesized by hydroxylation of phenylalanine and therefore is not essential. However, if the hydroxylase system is deficient or absent, the tyrosine requirement must be met from the diet. The phenylalanine hydroxylase reaction is complex, occurring principally in liver but also in kidney. The hydroxylating system is present in hepatocyte cytosol and contains phenylalanine hydroxylase, dihydropteridine reductase, and tetrahydrobiopterin as coenzyme. The hydroxylation is physiologically irreversible and consists of a coupled oxidation of phenylalanine to tyrosine and of tetrahydrobiopterin to a quinonoid dihydro derivative with molecular oxygen as the electron acceptor: The tetrahydrobiopterin is regenerated by the reduction of the quinonoid compound dihydrobiopterin in the presence of NAD(P)H by dihydropteridine reductase: NADH exhibits a lower Km and higher Vmax for the reductase than does NADPH. Thus, the pterin coenzyme functions stoichiometrically (in the hydroxylase reaction) and catalytically (in the reductase reaction). Deficiency of dihydropteridine reductase causes a substantial decrease in the rate of phenylalanine hydroxylation.

Ref:
N.V. Bhagavan, Chung-Eun Ha, Chapter 15 - Protein and Amino Acid Metabolism, Editor(s): N.V. Bhagavan, Chung-Eun Ha, Essentials of Medical Biochemistry (Second Edition), Academic Press, 2015, Pages 227-268, ISBN 9780124166875, https://doi.org/10.1016/B978-0-12-416687-5.00015-4. (https://www.sciencedirect.com/science/article/pii/B9780124166875000154)

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Added on Sep 26, 2022 by Barbara Van De Keer
Edited on Dec 26, 2022 by Barbara Van De Keer

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