Aromatic amino acids

Aromatic Amino Acids

Aromatic amino acids are a class of amino acids that include an aromatic ring in their structure. These amino acids are essential components of proteins and play crucial roles in various biological processes. The primary aromatic amino acids are phenylalanine, tyrosine, and tryptophan.

Structure

Aromatic amino acids are characterized by the presence of an aromatic ring, which is a planar, cyclic structure with alternating double bonds that follow Huckel's rule of aromaticity. This structure imparts unique chemical properties to these amino acids, such as the ability to absorb ultraviolet light.

Phenylalanine

Phenylalanine is an essential amino acid, meaning it must be obtained through the diet. It contains a benzyl side chain, which is a phenyl group attached to a methylene group. Phenylalanine is a precursor to tyrosine, which in turn is a precursor to several important neurotransmitters.

Tyrosine

Tyrosine is a non-essential amino acid that can be synthesized from phenylalanine. It contains a phenol group, which is a hydroxyl group attached to a benzene ring. Tyrosine is a precursor to catecholamines, such as dopamine, norepinephrine, and epinephrine, as well as thyroid hormones.

Tryptophan

Tryptophan is an essential amino acid with an indole side chain, which consists of a benzene ring fused to a pyrrole ring. Tryptophan is a precursor to the neurotransmitter serotonin and the hormone melatonin.

Biological Functions

Aromatic amino acids are involved in a variety of biological functions:

• Protein Synthesis: As building blocks of proteins, aromatic amino acids are incorporated into polypeptide chains during translation.
• Precursor Molecules: They serve as precursors for the synthesis of neurotransmitters and hormones.
• UV Absorption: The aromatic rings allow these amino acids to absorb ultraviolet light, which is useful in protein quantification assays.

Metabolism

The metabolism of aromatic amino acids involves several pathways:

• Phenylalanine Hydroxylation: Phenylalanine is hydroxylated to form tyrosine by the enzyme phenylalanine hydroxylase.
• Tyrosine Catabolism: Tyrosine is further metabolized into fumarate and acetoacetate, which enter the citric acid cycle.
• Tryptophan Catabolism: Tryptophan is broken down into kynurenine and subsequently into nicotinamide adenine dinucleotide (NAD+).

Clinical Significance

Disorders in the metabolism of aromatic amino acids can lead to various diseases:

• Phenylketonuria (PKU): A genetic disorder caused by a deficiency in phenylalanine hydroxylase, leading to the accumulation of phenylalanine.
• Alkaptonuria: A disorder resulting from the accumulation of homogentisic acid due to a defect in tyrosine metabolism.
• Tryptophan Deficiency: Can lead to decreased serotonin levels, affecting mood and sleep.

Also see

• Amino acid
• Protein structure
• Enzyme
• Metabolic pathway