Phenylalanine

Phenylalanine
Systematic name	2-Amino-3-phenyl- propanoic acid
Abbreviations	Phe F
Chemical formula	C9H11NO2
Molecular mass	165.19 g mol-1
Melting point	283 °C
Density	1.29 g cm-3
Isoelectric point	5.5
pKa	2.20 9.09
PubChem	994
CAS number	[673-06-3] (D) [63-91-2] (L) [150-30-1] (D/L or racemic)
SMILES	N[C@@H](Cc1ccccc1)C(O)=O
Disclaimer and references

Phenylalanine is an α-amino acid that is found in many proteins (such as hemoglobin, is essential in the human diet, and normally is readily converted to the amino acid tyrosine in the human body.

In humans, the L-isomer of phenylalanine, which is the only form that is involved in protein synthesis, is one of the 20 standard amino acids common in animal proteins and required for normal functioning in humans. Phenylalanine also is classified as an "essential amino acid" since it cannot be synthesized by the human body from other compounds through chemical reactions and thus has to be taken in with the diet. Phenylalanine, tyrosine, and tryptophan are the biggest of the standard amino acids.

The intricate coordination of systems in the human body is seen in the process, catalyzed by enzymes, by which l-phenylalanine is degraded into l-tyrosine, which in turn is converted into L-DOPA, which is further metabolized into other vitally important products: dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline). However, in the advent of the lack of a particular enzyme due to a genetic effect, the harmony is disrupted and the body loses its ability to metabolize phenylalanine, resulting in the serious disorder phenylketonuria.

The non-carbohydrate, non-nutritive artificial sweetener and flavor enhancer aspartame (aspartyl-phenylalanine-1-methyl ester) is synthesized from phenylalanine and the non-essential amino acid aspartic acid. Synthesized only from the L-isomers of these amino acids, the sweet taste of aspartame is unexpected: L-phenylalanine has a bitter taste and L-aspartic acid has a flat taste. The discovery, manufacture, and use of aspartame, which is now found in many products helps to address the human desire for sweet things in the face of deleterious health consequences traced to use of the sugar (sucrose). Applying human creativity to satisfy human desires, however, can yield solutions with both positive and negative consequences. While production of an artificial sweetener can offer the potential for health benefits, it also is alleged that aspartame can be deleterious to one's health and that its approval involved conflicts of interest and lack of sufficient research.

Phenylalanine's three letter code is Phe, its one letter code is F, its codons are UUU and UUC, and its systematic name is 2-Amino-3-phenylpropanoic acid (IUPAC-IUB 1983).

Structure

In biochemistry, the term amino acid is frequently used to refer specifically to alpha amino acids: those amino acids in which the amino and carboxylate groups are attached to the same carbon, the so-called α–carbon (alpha carbon). The general structure of these alpha amino acids is:

     R
     |
 H2N-C-COOH
     |
     H

where R represents a side chain specific to each amino acid.

Most amino acids occur in two possible optical isomers, called D and L. The L amino acids represent the vast majority of amino acids found in proteins. They are called proteinogenic amino acids. As the name "proteinogenic" (literally, protein building) suggests, these amino acid are encoded by the standard genetic code and participate in the process of protein synthesis. In phenylalanine, only the L-stereoisomer is involved in synthesis of mammalian proteins.

Phenylalanine's chemical formula is C₆H₅- CH₂- CH(NH₂)-COOH, or in general form C₉H₁₁NO₂ (IUPAC-IUB 1983).

Like tyrosine and tryptophan, phenylalanine contains a large rigid aromatic group on the side chain. Phenylalanine, tyrosine, and tryptophan—like isoleucine, leucine, and valine—are hydrophobic and tend to orient towards the interior of the folded protein molecule. Phenylalanine is classified as nonpolar because of the hydrophobic nature of the benyl side chain. It is a white, powdery solid.

Forms

L-phenylalanine

L-Phenylalanine (LPA) is an electrically-neutral amino acid, one of the twenty common amino acids used to biochemically form proteins, coded for by DNA. L-phenylalanine is used in living organisms, including the human body, where it is an Essential amino acids|essential amino acid. L-phenylalanine also can be converted into L-tyrosine, another one of the twenty protein-forming amino acids necessary for human health, and the process can lead to such important chemical compounds as dopamine, norepinephrine, and epinephrine.

D-phenylalanine

D-phenylalanine (DPA), can be synthesized artificially. D-phenylalanine can be converted only into phenylethylamine. D-phenylalanine is a non-protein amino acid, meaning that it does not participate in protein biosynthesis. D-phenylalanine and other D-amino acids are, however, found in proteins, in small amounts, particularly aged proteins and food proteins that have been processed. The biological functions of D-amino acids remain unclear. Some D-amino acids, such as D-phenylalanine, may have pharmacologic activity.

DL-phenylalanine

DL-phenylalanine is a racemic mixture of phenylalanine, meaning it contains equal amounts of the D and L enantiomers. DL-Phenylalanine is marketed as a nutritional supplement for its putative analgesic and antidepressant activities.

The putative analgesic activity of DL-phenylalanine may be explained by the possible blockage by D-phenylalanine of enkephalin degradation by the enzyme carboxypeptidase A. The mechanism of DL-phenylalanine's putative antidepressant activity may be accounted for by the precursor role of L-phenylalanine in the synthesis of the neurotransmitters norepinephrine and dopamine. Elevated brain norepinephrine and dopamine levels are thought to be associated with antidepressant effects.

Dietary sources

Phenylalanine cannot be made by animals, which have to obtain it from their diet.

Phenylalanine is contained in most protein-rich foods. Especially good sources are dairy products (curd, milk, cottage cheese), avocados, pulses and legumes (particularly peanuts and lima beans), nuts (pistachios, almonds), seeds (piyal seeds), leafy vegetables, whole grains, poultry, fish, other seafoods, and some diet beverages.

Biosynthesis

Phenylalanine is produced by plants and most microorganisms from prephenate, an intermediate on the shikimate pathway (Lehninger 2000).

Prephenate is decarboxylated with loss of the hydroxyl group to give phenylpyruvate. This species is transaminated using glutamate as the nitrogen source to give phenylalanine and α-ketoglutarate.

Biological aspects

The genetic codon for phenylalanine (UUU and UUC) was the first to be discovered. Marshall W. Nirenberg discovered that, when he inserted m-RNA made up of multiple uracil repeats into E. coli, the bacterium produced a new protein, made up solely of repeated phenylalanine amino acids.

L-phenylalanine can be converted into L-tyrosine, another one of the DNA-encoded amino acids. L-tyrosine in turn is converted into L-DOPA, which is further converted into dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline) (the latter three are known as catecholamines).

The enzyme phenylalanine hydroxylase normally converts phenylalanine into tyrosine. If this reaction does not take place, phenylalanine accumulates and tyrosine is deficient, leading to the serious disorder Phenylketonuria.

Phenylalanine is a large, neutral amino acid (LNAA). LNAAs compete for transport across the blood brain barrier (BBB) via the large neutral amino acid transporter (LNAAT). Excessive phenylalanine in the blood saturates the transporter. Thus, excessive levels of phenylalanine significantly decrease the levels of other LNAAs in the brain. But since these amino acids are required for protein and neurotransmitter synthesis, phenylalanine accumulation disrupts brain development in children, leading to mental retardation (Pietz et al. 1999)

Since phenylalanine uses the same active transport channel as tryptophan to cross the blood-brain barrier, in large quantities it interferes with the production of serotonin, which is a metabolic product of tryptophan.

Lignin is derived from phenylalanine and from tyrosine. Phenylalanine is converted to cinnamic acid by the enzyme phenylalanine ammonia lyase (Lehninger 2000).

D-phenylalanine is absorbed from the small intestine, following ingestion, and transported to the liver via the portal circulation. A fraction of D-phenylalanine appears to be converted to L-phenylalanine. D-phenylalanine is distributed to the various tissues of the body via the systemic circulation. D-phenylalanine appears to cross the blood-brain barrier with less efficiency than L-phenylalanine. A fraction of an ingested dose of D-phenylalanine is excreted in the urine. There is much about the pharmacokinetics in humans that is unknown.

Phenylketonuria

The genetic disorder phenylketonuria (PKU), also known as Folling's disease, is the inability to metabolize phenylalanine. It is a genetic disorder characterized by a deficiency in the enzyme phenylalanine hydroxylase (PAH), which is necessary to metabolize the phenylalanine to tyrosine. When PAH is deficient, phenylalanine accumulates and is converted into phenylketones, which are detected in the urine. These include phenylacetate, phenylpyruvate, and phenylethylamine (Michals and Matalon 1985). Detection of phenylketones in the urine is diagnostic.

Left untreated, this condition can cause problems with brain development, leading to progressive mental retardation and seizures (see Biological aspects above). However, PKU is one of the few genetic diseases that can be controlled by diet. A diet low in phenylalanine and high in tyrosine can bring about a nearly total cure.

Individuals with this disorder are known as "phenylketonurics." Treatment of PKU includes the elimination of phenylalanine from the diet, and supplementation of the diet with tyrosine. Babies who are diagnosed with PKU must immediately be put on a special milk/formula substitute. Later in life, the diet continues to exclude phenylalanine-containing foods. Women affected by PKU must pay special attention to their diet if they wish to become pregnant, since high levels of phenylalanine in the uterine environment can cause severe malformation and mental retardation in the child. However, women who maintain an appropriate diet can have normal, healthy children. This dietary restriction also applies to pregnant women with hyperphenylalanine (high levels of phenylalanine in blood) because they do not properly metabolize phenylalanine.

If PKU is diagnosed early enough, an affected newborn can grow up with normal brain development, but only by eating a special diet low in phenylalanine for the rest of his or her life. This requires severely restricting or eliminating foods high in phenylalanine, such as breast milk, meat, chicken, fish, nuts, cheese, and other dairy products. Starchy foods such as potatoes, bread, pasta, and corn must be monitored. Many diet foods and diet soft drinks that contain the sweetener aspartame must also be avoided, as aspartame consists of two amino acids: phenylalanine and aspartic acid. Aspartame is found in many sugarless gums, sugarless soft drinks (such as Diet Coke, and Diet Pepsi), some forms of Lipton Tea, and a number of other food products.

References

ISBN links support NWE through referral fees

• Doolittle, R. F. 1989. Redundancies in protein sequences. In G. D. Fasman, ed., Prediction of Protein Structures and the Principles of Protein Conformation. New York: Plenum Press. ISBN 0306431319.
• International Union of Pure and Applied Chemistry and International Union of Biochemistry and Molecular Biology (IUPAC-IUB) Joint Commission on Biochemical Nomenclature. 1983. Nomenclature and symbolism for amino acids and peptides: Recommendations on organic & biochemical nomenclature, symbols & terminology. IUPAC-IUB. Retrieved June 14, 2007.
• Lehninger, A. L., D. L. Nelson, and M. M. Cox. 2000. Lehninger Principles of Biochemistry, 3rd ed. New York: Worth Publishing. ISBN 1572591536.
• Michals, K., and R. Matalon. 1985. Phenylalanine metabolites, attention span and hyperactivity. American Journal of Clinical Nutrition 42(2): 361-365. PMID 4025205.
• Pietz, J., R. Kreis, A. Rupp, E. Mayatepek, D. Rating, C. Boesch, and H. J. Bremer. 1999. Large neutral amino acids block phenylalanine transport into brain tissue in patients with phenylketonuria. Journal of Clinical Investigation 103: 1169–1178. PMID 10207169.

External links

• Phenylalanine and tyrosine biosynthesis. Retrieved June 20, 2007.
• Computational Chemistry Wiki. Retrieved June 20, 2007.
• Nitrogen Order's Molecule of the Week. Retrieved June 20, 2007.
• DL-phenylalanine versus imipramine in depression. Retrieved June 20, 2007.

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