Abbreviated: Phe or F
Molecular Formula: C6H5CH2CH(NH2)COOH
pKa: 1.83 (carboxyl), 9.13 (amino)
Physiological role:
Due to its benzyl side chain, phenylalanine is considered an aromatic and hydrophobic amino acid. Its start codons are UUU and UUC.
Phenylalanine is naturally produced by microorganisms. Genetic engineering of Escherichia coli has resulted in increased production of phenylalanine, making commercial use possible. It is largely used for medical, feed, and
nutritional applications (such as aspartame).
Carboxy-terminal phenylalanine has been shown to be essential to the correct assembly of bacterial outer membrane protein (Struyve et al., 1991). Further, the bacterium Bacillus anthracis, which produces the deadly anthrax toxin, utilizes a phenylalanine clamp to catalyze protein translocation through the anthrax toxin pore (Krantz et al., 2005). The phenylalanine residues are present within the beta-barrel stem of the pore, and are thought to mediate membrane insertion and maintain the overall stability of the channel.
Biosynthesis and metabolism:
Enzymes featured in this pathway:
1. chorismate mutase
2. prephenate dehydratase
3. tyrosine aminotransferase
Phenylalanine biosynthesis begins with chorismate, which is a branch-point intermediate common to the synthesis of all aromatic amino acids. The enzyme chorismate mutase catalyzes its transformation into prephenate. This occurs by a Claisen rearrangement reaction, and does not necessarily occur through an intermediate. Instead, it has a chair-like transition state.
Prephenate is then decarboxylated with loss of the carboxyl group to give to phenylpyruvate. Prephenate is an intermediate in the shikimate pathway. The shikimate pathway is involved in chorismate synthesis. Chorismate is essential for the production of aromatic amino acids phenylalanine, tyrosine, and tryptophan.
Prephenate is then decarboxylated with loss of the carboxyl group to give to phenylpyruvate. Prephenate is an intermediate in the shikimate pathway. The shikimate pathway is involved in chorismate synthesis. Chorismate is essential for the production of aromatic amino acids phenylalanine, tyrosine, and tryptophan.
Phenylpyruvate is subsequently transaminated using glutamate as the nitrogen source. Alpha-ketoglutarate and phenylalanine are the finished products of this pathway.
In addition, plants and microorganisms may employ an alternative pathway for phenylalanine and tyrosine biosynthesis called the pretyrosine or arogenate pathway. As shown, depending on the terminal enzyme used either amino acid may be formed from arogenate.
Degradation of phenylalanine using the enzyme phenylalanine hydroxylase (PAH) leads to the production of tyrosine:
PAH catalyzes the hydroxylation of phenylalanine to tyrosine. This enzyme is a monooxygenase because one molecule of O2 is found on the product and the other on the released water molecule. This can lead to further degradation (catabolism) of phenylalanine, resulting in fumarate and acetylacetate.
Additional resources:
In humans, large quantities of phenylalanine in the body may result in the amino acid crossing the blood-brain barrier, which can interfere with serotonin production. Phenylalanine is essential for neural development. Phenylketonuria, an autosomal recessive inborn error of phenylalanine metabolism results in sever intellectual impairment (Williams et al., 2008).
For more on phenylalanine synthesis, see KEGG.
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