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Aromatic Amino Acids Explained

Learn all about the aromatic amino acids

Aromatic amino acids: what are they exactly? What makes them aromatic? (Do they smell good, like perfume or brewing coffee? Quick answer: no.) How were they discovered and why do they matter to your health? Find the answers to these questions and more in this article.

Amino Acids: The Basics

We'll start by outlining the basics of amino acids: how they were first detected, when they became understood as important components to the chemistry of the human body, and what we know (and continue to discover about them) today.

The History of Amino Acid Discovery

The history of the discovery of amino acids dates back to the 19th century. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre-Jean Robiquet identified what they called asparagine, the first known amino acid, which they'd isolated from asparagus (thus the name).

Cystine was next recognized and isolated by English physician and chemist William Hyde Wollaston in 1810, as he studied patients with bladder stones: cystine comes from the Greek kystis, meaning "bladder pouch." While this amino acid was found early in the century, cystine was not recognized as a major protein component until the very end of the 1800s. Even later than that, scientists further broke down its composition and designated the amino acid cysteine (two cysteine molecules bound together form cystine).

According to the Historical Encyclopedia of Natural and Mathematical Sciences, Volume 2, these major units of the molecular protein structure were discovered in the following order: glycine (1820), leucine (1820), tyrosine (1849), serine (1865), glutamic acid (1866), aspartic acid (1868), phenylalanine (1881), alanine (1888), lysine (1889), arginine (1895), histidine (1896), cystine (1899). By the 20th century, experts had a pretty good understanding of amino acids.

The building blocks of protein, 20 amino acids make every protein your body needs, linking them together and folding them over in different chains and combinations. Just as the 26 letters of the alphabet can make every word in the English language, that is the level of ordered complexity with which our bodies organize amino acids.

It is interesting to note that two extremely rare amino acids have been newly discovered, selenocysteine and pyrrolysine. They are known as the 21st and 22nd amino acids in the genetic code, with selenocysteine discovered in 1986 (mainly responsible for selenium transport), and pyrrolysine discovered in 2002 (an ɑ-amino acid used in the biosynthesis of proteins and not present in humans).

While scientific advancement carries on, the main 20 amino acids can be further categorized for our purposes as essential vs. nonessential.

Amino Acids: Essential vs. Nonessential

Let's quickly name, explain, and clarify the 20 standard amino acids that are hard at work in the human body.

Nonessential

Nonessential amino acids are amino acids you absolutely still need, but because they are produced naturally in the body, you don't have to eat or otherwise acquire them. Of the main 20 amino acids, there are 12 nonessential amino acids. They are as follows (with descriptions sourced from PubChem):

  1. Alanine: used for protein construction and metabolizing tryptophan and vitamin B6 (pyridoxine).
  2. Asparagine: a carrier of residual ammonia to be eliminated from the body (in case you ever wondered why urine sometimes smells like asparagus).
  3. Aspartate: found in animals and plants (especially sugar cane and sugar beets), it may be a neurotransmitter.
  4. Cysteine: an important component of hair, skin, and nails.
  5. Glutamate: the most common excitatory neurotransmitter of the central nervous system.
  6. Glutamine: involved in many metabolic processes, it's the main carrier of nitrogen throughout the body.
  7. Glycine: a fast inhibitory neurotransmitter.
  8. Proline: synthesized from glutamic acid, an essential component of collagen important for the proper functioning of joints and tendons.
  9. Serine: involved in the biosynthesis of purines, pyrimidines, and other amino acids.
  10. Tyrosine: synthesized in the body from phenylalanine, also the precursor of epinephrine, thyroid hormones, and melanin.
  11. Arginine: a semi-essential amino acid, boosts the production of nitric oxide, also an important intermediate in the urea cycle and the detoxification of nitrogenous wastes.
  12. Histidine: an essential amino acid, necessary in the production of the neurotransmitter histamine.

Semi-Essential

Now might be a good time to discuss semi-essential or conditionally essential amino acids, such as arginine. Arginine is only essential for juveniles, but is nonessential at later stages of development, unless the body is under siege from stressors such as illness and surgery, which can compromise the production of conditionally essential amino acids like arginine.

Tyrosine too is considered semi-essential. It is synthesized only from phenylalanine, which is itself an essential amino acid. However, a lack of the enzyme phenylalanine hydroxylase (which is used in tyrosine synthesis) can cause phenylketonuria (PKU), also known as phenylalanine hydroxylase (PAH) deficiency. It is a disorder that causes a buildup of phenylalanine in the body. Without the necessary enzyme to process phenylalanine (and thus gain tyrosine), people with PKU must limit the foods they eat that contain protein, as well as the artificial sweetener aspartame. If the buildup of phenylalanine is not properly managed, it can lead to severe health problems including mental and behavioral issues, intellectual disabilities, and seizures. PKU patients have low levels of tyrosine as a result, and will sometimes need to take a tyrosine supplement. If there is an issue with processing the essential amino acid phenylalanine—which under ideal circumstances would naturally produce tyrosine—that is the condition under which tyrosine would become an essential amino acid for the individual.

Essential

The nine remaining amino acids are essential, meaning they need to be acquired from outside the body in order for the human body to function. If not by food, then possibly by supplement, but whatever the way, they must be taken in to gain their benefits.

  1. Isoleucine: an isomer of leucine, important in hemoglobin synthesis and regulation of blood sugar and energy levels.
  2. Leucine: important for hemoglobin formation and an initiator of muscle protein synthesis.
  3. Lysine: required for growth and tissue repair and supplied by foods such as red meats, fish, and dairy products.
  4. Methionine: a sulfur-containing essential L-amino acid that is important in many body functions like tissue repair, detoxification processes, protecting cells from pollutants, preventing excess fat buildup in the liver, and the tone and pliability of skin, hair, and nails.
  5. Phenylalanine: used in the biosynthesis of dopamine and norepinephrine neurotransmitters.
  6. Threonine: an important residue of many proteins, such as tooth enamel, collagen, and elastin that is found in eggs, milk, gelatin, and other proteins.
  7. Tryptophan: necessary for normal growth in infants and for nitrogen balance in adults.
  8. Valine: promotes muscle growth and tissue repair, also a precursor in the penicillin biosynthetic pathway.

That's the rundown of amino acids, so now: which ones are aromatic, and what makes them so?

Aromatic Amino Acids: The Specifics

Out of all the above listed amino acids, which ones are aromatic, and why does that matter? Let's start with what "aromatic" means in this biological context.

Aromatic Defined

To a layperson, this word means "having a pleasant and distinctive smell." Fragrant, sweet-scented, perfumed. In chemistry however, it speaks of an organic compound, a large class of unsaturated compounds characterized by one (or more) planar rings of atoms joined by covalent bonds of two different kinds.

The Aromatic Amino Acids

There are three aromatic amino acids (AAA): phenylalanine (Phe); tyrosine (Tyr), derived from phenylalanine; and tryptophan (Trp).

the structure of the aromatic amino acids

You'll notice the distinctive aromatic ring on the side chain of each of these three amino acids. Each of those rings has three double bonds, a clear characteristic of aromatic amino acids. What's even more distinctive about them is that while amino acids don't absorb light in the visible region of the electromagnetic spectrum, they do all absorb light from the infrared region thanks to their double bonds. And the aromatic amino acids specifically? They absorb ultraviolet radiation, or UV light.

Ultraviolet Rings

To a different degree, all aromatic amino acids absorb ultraviolet light.

Measured in wavelengths via nanometers (nm), tryptophan has the highest absorption of ultraviolet light (280 nm) by proteins, with tyrosine high as well (above 250 nm), and phenylalanine the lowest (but still significantly above 210 nm).

This absorption ability is used to quantify the concentration of proteins in an unknown sample. It causes the electron of each to enter an excited state, and when that electron returns to its ground state, it will either release energy or emit light. If the molecule emits light, it is called a fluorescent molecule. Tryptophan is a widely used fluorescent molecule.

Tyrosine further distinguishes itself as the only one of the aromatic amino acids with an ionizable side chain. Seven out of the 20 amino acids contain ionizable side chains, meaning that their side chains can exchange a hydrogen atom with some other biomolecule in certain circumstances.

Not Just for Humans

Animals (including us humans) get our essential aromatic amino acids from the food we eat, but plants and microorganisms (like fungi, bacteria, algae, and some protozoans), must synthesize them through the shikimate pathway.

Found only in microorganisms and plants, the shikimate pathway is a seven-step aromatic amino acids and carbohydrate metabolism route. It was studied extensively via the Arabidopsis thaliana plant (commonly known as thale cress or mouse-ear cress), the findings of which were detailed in a paper titled "The Biosynthetic Pathways for Shikimate and Aromatic Amino Acids in Arabidopsis thaliana," published in The Arabidopsis Book.

The shikimate pathway begins with the formation of chorismate in plants, the precursor of tyrosine, tryptophan, and phenylalanine. As it is for us, the aromatic amino acids are essential to a plant's biological functions. In fact herbicides often inhibit the enzymes involved in the plant's biosynthesis of aromatic amino acids, which is what makes those poisons toxic to the organism's biological functions.

What Do the Aromatic Amino Acids Do for You?

You may be saying to yourself, having read or skimmed this far, "That's all well and good, but now that I know what they are and how they're classified, what do the three aromatic amino acids actually do for me?"

That's a great question! Let's take an in-depth look at each one:

Phenylalanine

Phenylalanine exists naturally in two forms: L-phenylalanine and D-phenylalanine (there is also a laboratory-made mixture called DL-phenylalanine or DLPA). Nearly identical save for their shape (you might guess they are shaped more or less like an L and a D), the L-form is the essential amino acid found in food and used for protein-building in the body. The D-form is a non-protein amino acid still being studied for its uses and effects, particularly in the areas of pain, stress, and mood disorders like depression and anxiety.

Phenylalanine is found in soy foods, cheese, seeds, nuts, lean meats, poultry, and seafood. While it's dangerous for people with PKU or PAH deficiency (as discussed previously in this article), it is so essential to human life that phenylalanine supplements might be recommended to people in need of a therapy for depression, or to curb the effects of drug and/or alcohol withdrawal. That is because phenylalanine is the key to producing dopamine, the molecule in your brain that allows you to feel pleasure and form memories.

Phenylalanine has been studied in connection with the therapeutic treatment of Parkinson's disease, as a possible treatment for children with ADHD, and as an alternative treatment for vitiligo. Vitiligo is a disorder characterized by the patient losing pigmentation on patches of their skin, a condition that theoretically could be helped by phenylalanine's connection to proper melanin production.

Perhaps the best part about phenylalanine as a potential treatment is that, because it's an essential amino acid already irreplaceable in the human body, experimenting with it causes little-to-no side effects or adverse reactions. It's like oxygen or water—yes, it's still possible to have too much of any one, but it takes a very unusual or drastic circumstance to bring about harm, and the rest of the time there is simply no life without it.

Speaking of a bare necessity for life, phenylalanine is also crucial for the production of epinephrine and norepinephrine, the vital components of the body's "fight-or-flight" response mechanism—it could literally save your life in a crisis.

As the first ingredient for the creation of tyrosine, phenylalanine is in essence the key to two important amino acids in the body. We'll delve further into tyrosine's uses next.

Tyrosine

Tyrosine is derived naturally in the body from phenylalanine, and thus is usually considered nonessential as long as there is no issue with phenylalanine processing. However, tyrosine can also be ingested, and is found in eggs, dairy products, meats, fish, beans, nuts, and wheat.

From the Greek word tyros, meaning “cheese," which is where it was first discovered, as a supplement it is most commonly used to treat PKU, and also in situations to improve alertness, memory creation/retention, and learning ability. That is because tyrosine produces chemicals for the brain that help nerve cells communicate with one another. Tyrosine may even have a hand in regulating mood, improving focus, and influencing other facets of cognitive functioning.

Tyrosine is also a key player in protein synthesis, the fundamental function that keeps us alive. Like phenylalanine before it, tyrosine has been studied in the management of stress, the improvement of mental performance during times of stress, and issues of sleep and wakefulness.

Tyrosine also has a huge role to play with the thyroid gland, which extracts iodine from the foods we eat, combines it with tyrosine, and thus produces the two primary thyroid hormones thyroxine (T4) and triiodothyronine (T3). Those hormones are important for maintaining the body's metabolic rate, heart and muscle function, brain development, digestion, and the maintenance of bones. Supplementation with tyrosine can sometimes be used in the treatment of hypothyroidism, a condition cause by an underactive thyroid, but should not be taken casually or without expert advice from a doctor or trusted medical professional.

The benefit of tyrosine being a natural inhabitant of the body is that it's often a safe therapy for a variety of clinical conditions, including hypertension and chronic pain, but as with any supplementation or treatment, it's important to seek medical advice before experimenting independently on oneself.

Tryptophan

Tryptophan is needed in infants to facilitate normal growth, and in adults for ensuring there is a balance in the breakdown and synthesis of protein. It can be found in seeds, spinach, salmon, poultry, eggs, soy products, and nuts.

Tryptophan is well known as being a precursor of serotonin, the neurotransmitter that helps regulate anxiety, reduce depression, heal wounds, and maintain bone health. Serotonin is also a component of digestive health (and indeed is largely found in the digestive system), as well as being the chemical that stimulates the parts of the brain that control wakefulness and sleep.

Tryptophan's connection to food and sleep is so famous that it was even the topic of a Seinfeld episode, where feeding someone turkey is so effective at inducing slumber, the person falls asleep immediately after she finishes eating turkey for dinner.

There is a disorder that particularly affects tryptophan too: Hartnup disease, also known as Hartnup disorder or pellagra-like dermatitis. It is a metabolic disorder that disrupts the absorption of nonpolar amino acids (which include tryptophan and phenylalanine). Often the only sign of this disorder is aminoaciduria, or a high level of amino acids in the urine, since they are not being properly absorbed by the body. However in more extreme cases, people with Hartnup disease will exhibit possible skin rashes (hence the dermatitis designation), difficulty in coordinating their movements, and even psychiatric symptoms like depression or psychosis. Those extreme reactions are usually flare-ups caused by stress, poor diet, or illness, but it is otherwise a manageable condition. The worst symptoms could very well be due to the inhibition of tryptophan—an upset to serotonin levels in humans can lead to quite precarious mental states, and as a result, sometimes terribly dire depressive outcomes.

The Bouquet of Life

The metabolism of aromatic amino acids phenylalanine, tyrosine, and tryptophan is essential to human life. While it's tempting to know that elements so important are so easy to get access to, when seeking to boost one's intake of these amino acids, it's even more important to be well-informed and carefully balanced. Not only do all of the amino acids work in concert with one another, but the resulting byproducts and response they produce are not to be trifled with lightly.

It's been posited (mostly in jest), that due to the way our brains work, dopamine and serotonin are the only two things we truly enjoy. Everything we love doing, we love because of the chemical reaction it causes in our brains. Because the aromatic amino acids specifically are direct keys to our dopamine and serotonin, we must love them too.

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