AMINO ACIDS - Definition, Types, Structure, Sources & Benefits

We know amino acids are constitutive elements of proteins. Around twenty amino acids join together in different combinations to make up all the proteins in plants and animals. An amino acid, as the name reveals, comprises an amino group that is basic, a carboxylic acid group, and a neutral side chain. This class of organic compounds being the building blocks of protein are involved in many body functions. You can find the information about amino acids you'd been looking up, below.

What are amino acids?

we can define amino acids as “a group of organic compounds that contain an amino group (-NH), a carboxylic acid group (-COOH), and a neutral side chain or R group.” These compounds join together through peptide linkages to form proteins.

As a part of proteins, amino acids are involved in all the vital functions performed by proteins. This group of compounds is taken either from the diet or as a supplement to enhance athletic performance or for other purposes.

A protein does not perform the desired function in its original state. In fact, the amino acids produced during the hydrolysis of proteins are subsequently used by the body to perform various functions. The amino acids can also join together to build different proteins in the human body.

Structure of Amino Acids

Each amino acid molecule consists of a central α-carbon, to which an amino (-NH) group and a carboxyl (-COOH) group are bonded. a hydrogen atom (H) and a neutral side chain, referred to as the R group satisfies the remaining two bonds.. The side chain or R group is variable radical and it distinguishes an amino acid from other amino acids and is responsible for its properties and functions.

Classification of amino acids

Depending on the structural and chemical properties of Group R, amino acids can be classified into basic, acid, nonpolar, and polar amino acids.

Acidic Amino Acids

This class of amino acids has the following properties

• Has carboxylic acid in the R group
• Ionizes in water and carries -1 charge
• Examples include glutamic acid and aspartic acid

Basic amino acids

This class of amino acids has the following properties

• The side chain is basic which means it accepts protons
• Arginine and lysine carry +1 charge
• Histamine has both acidic and basic properties

Polar Amino Acids

Glutamine, serine, tyrosine, threonine, asparagine, and cysteine come under the umbrella of polar amino acids. Their side chains possess different functional groups including aliphatic, aromatic sulfur-containing, and amide R groups.

• Aliphatic hydroxyl groups serve as the side chain or R group in serine and threonine
• Tyrosine, a phenol derivative, possesses an aromatic ring containing a hydroxyl group
• Cysteine contains a more reactive sulfur atom
• Asparagine and glutamine have amide R groups

Non-Polar Amino Acids

Non-polar amino acids include glycine, leucine, alanine, proline, isoleucine, phenylalanine, valine, methionine, and tryptophan. Their properties are

• They are hydrophobic in nature. The globular proteins, in aqueous solutions, fold up to form a 3D structure to conceal them inside
• They also contain aliphatic or aromatic rings as side chains
• Leucine contains one and its isomer, isoleucine contains two chiral carbon atoms
• Proline exhibits uniqueness as the nitrogen of the amino group bonds with another carbon to form a ring structure
• Phenylalanine, as obvious from the name, contains phenyl groups attached to alanine
• Methionine like cysteine contains sulfur atoms in its side chain, but sulfur of methionine is less reactive
• An indole ring is bonded to an alanyl side chain in tryptophan

Types of amino acids

Amino acids, on the basis of their requirement in diet, can be categorized into two types, essential and non-essential amino acids. The essential amino acids are not synthesized in the human body and are taken through the diet. The non-essential amino acids, on the contrary, are synthesized in your body and ought not to be present in the diet.

Essential Amino Acids

The essential acids are not made inside the human body and have to be taken through the diet. There are nine essential amino acids including, leucine, tryptophan, phenylalanine, histidine, methionine, valine, isoleucine, threonine, and lysine. All nine essential amino acids are involved in many processes, including, energy production, immune function, tissue growth, and nutrient absorption.

Histidine

It is responsible for the production of histamine which is a neurotransmitter responsible for physiological processes like an allergic response, sleep-wake cycles, digestion, and sexual function. Furthermore, it is crucial for the maintenance of the myelin sheath, for the protection of nerve cells. It also acts as an active site in several enzymes.

Isoleucine

Isoleucine is one of the branched-chain amino acids (bcaas) and is an important part of fibrin, the blood-clotting protein. More functions include immunity, hemoglobin production, energy regulation, and muscle metabolism.

Leucine

Leucine is another example of bcaas and plays an integral role in muscle repair, protein synthesis in the human body, glucose metabolism, wound healing, and growth. It is a major constituent in hemoglobin.

Lysine

It is involved in the synthesis of several other proteins including collagen, and elastin, as well as enzymes and hormones. It also participates in calcium absorption in the body.

Methionine

Major functions of methionine include tissue growth, metabolism, detoxification as well as absorption of vital antioxidant minerals including zinc and selenium.

Phenylalanine

This amino acid is responsible for the synthesis of neurotransmitters like epinephrine, norepinephrine, dopamine and tyrosine, and other enzymes and proteins. It can be converted into other amino acids by transamination reactions.

Threonine

It plays a vital role as a part of the structural proteins collagen and elastin. Other functions include fat metabolism, blood clotting, and immunity.

Tryptophan

Tryptophan is a precursor for a neurotransmitter, serotonin, which is responsible for sleep, appetite, and mood. It is also involved in maintaining correct nitrogen balance.

Valine

Valine is also another branched-chain amino acid (BCAA) and its functions include muscle repair and growth, tissue regeneration, and energy production.

Non-Essential Amino Acids

Nonessential amino acids, are essential for various body functions but can be synthesized in our bodies and therefore may not necessarily come from a food source.

These are eleven in number, out of which, eight (8) are conditional amino acids. Conditional amino acids are those which our body fails to make under stress or illness.

The non-essential amino acids include tyrosine, glutamic acid, asparagine, cysteine, proline, alanine, serine, arginine, aspartic acid, glycine, and glutamine. While ornithine, arginine, glutamine, tyrosine, glycine, proline, cysteine, and serine are conditional amino acids.

Nonessential amino acids like essential amino acids serve in the synthesis of enzymes, hormones, neurotransmitters, and antibodies. Their other functions include muscle repair, tissue regeneration, growth, red blood cell production, and energy production. 

Sources of amino acids

Natural sources

The amino acids are a part of protein and are present in all foods that contain protein. As mentioned earlier, essential amino acids are supposed to be taken from an outside source whereas, non-essential amino acids are not necessary to be present in our diets.

Almost all the essential amino acids are present in most proteins. The food sources that provide all necessary amino acids are known as complete proteins. The food items that contain complete protein include beef, mutton, poultry, seafood, eggs, and milk and milk products.  Soy and quinoa are plant-based sources of complete protein.

Other plant-based foods, like lentils, nuts, beans, and cereals, usually lack one or more essential amino acids. But you can get all the essential amino acid profiles from these sources by making different combinations in salads, sandwiches, or other meals and snacks.

Supplements

Amino acids supplementation is not required unless dietary sources are insufficient or you need it in more quantity. The amino acid supplements may provide additional health benefits like enhanced muscle growth and better athletic performance. 

Generally, amino acid supplements contain individual amino acids rather than blends. BCAAs or branched chain amino acids supplements are the most commonly taken amino acid blend. These branched-chain amino acids are involved in muscle repair after strenuous exercise or muscle growth and tissue regeneration.

The doses of supplements vary according to your requirements and reason for taking them. It is suggested to consult a doctor or dietitian who has enough knowledge about supplements and your requirements.

Benefits of amino acids

• It helps in regulating mood and improves the symptoms associated with depression-like anxiety, insomnia, anorexia nervosa, etc.
• The blend of BCAAs can help reduce muscle soreness and fatigue after exercise.
• The BCAAs also help in improving athletic performance by reducing the feeling of exertion.
• They also accelerate healing after injury or surgery and reduce post-surgery complications.
• Involved in biological processes as a precursor of neurotransmitters, acting as active sites in enzymes or production of enzymes and hormones.
• Some amino acids participate in the absorption of minerals including calcium, zinc, and selenium.
• Amino acids involved in the synthesis of collagen and elastin proteins are beneficial for the skin.

Summary

The amino acids are a group of organic compounds which contain an α-carbon atom attached to which are a basic amino group, an acidic carboxyl group, a neutral side chain, and a hydrogen atom.

It is classified as polar, non-polar, acidic, and basically based on the chemical properties of the side chain. Amino acids are also classified into essential, which are to be taken through diet, and non-essential which our body can synthesize.

Being a building block of protein, they possess the same functions and benefits as proteins. The majority of proteins have all the essential amino acids, therefore, they are called complete proteins. Sources of complete protein include beef, mutton, seafood, chicken, eggs, milk, and other dairy products. If you’re concerned about your requirements of certain amino acids being met, you can add supplements as well, on the advice of a certified and qualified professional.