Phenylalanine Hydroxylase - Biological Function

Biological Function

PheOH is a critical enzyme in phenylalanine metabolism and catalyzes the rate-limiting step in its complete catabolism to carbon dioxide and water. Regulation of flux through phenylalanine-associated pathways is critical in mammalian metabolism, as evidenced by the toxicity of high plasma levels of this amino acid observed in phenylketonuria (see below.) The principal source of phenylalanine is ingested proteins but relatively little of this pool is used for protein synthesis. Instead, the majority of ingested phenylalanine is catabolized through PheOH to form tyrosine; addition of the hydroxyl group allows for the benzene ring to be broken in subsequent catabolic steps. Transamination to phenylpyruvate, whose metabolites are excreted in the urine, represents another pathway of phenylalanine turnover, but catabolism through PheOH predominates.

In humans, this enzyme is expressed both in the liver and the kidney, and there is some indication that it may be differentially regulated in these tissues. PheOH is unusual among the aromatic amino acid hydroxylases for its involvement in catabolism; tyrosine and tryptophan hydroxylases, on the other hand, are primarily expressed in the central nervous system and catalyze rate-limiting steps in neurotransmitter/hormone biosynthesis.