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Amino Acids

Amino acids are a set of 20 different organic molecules. The term amino acid is the short name for alpha-amino carboxylic acid. Amino acids when linked with other amino acids form proteins that are required to perform nearly all cell functions. For example, proteins play different roles such as acting as enzymes, and antibodies and also providing structural support.

Hundreds of amino acids are found in nature, but only a set of 20 amino acids forms proteins. The amino acids join with each other to make proteins. They link together like the beads on a string to make long chains, which are called polypeptide chains. So, proteins are made of one or more chains of amino acids or one or more polypeptides. However, when the chains link together, they fold to give the protein its final shape. The sequence of amino acid chains decides the shape of the protein and what a protein can do in a cell is also depends on its shape.

Structure of Amino Acid

Each amino acid is mainly made of a basic amino group (NH2), an acidic carboxyl group (-COOH), and an organic R group, which is different for different amino acids. In the chemical structure of amino acids, there is a carbon atom at the center, which is known as alpha-carbon. The amino group and the carboxyl group are attached to the alpha-carbon. Whereas, at the remaining two positions, the alpha carbon is bonded to the hydrogen (H) atom and the R group. So, generally, amino acids have the following structural components:

  • An alpha carbon
  • A hydrogen atom (H)
  • A carboxyl group (-COOH)
  • An amino group (-NH2)
  • A "variable" group or "R" group
Amino Acids

Some Exceptions:

  • Amino acid glycine does not have a side chain, so, its alpha-carbon is bonded to two hydrogen atoms.
  • Nitrogen is also a part of the ring in Proline.
  • Glycine is not optically active as it does not have an asymmetric carbon atom like other amino acids.

What makes an Amino Acid Essential?

All the amino acids (20 amino acids) cannot be synthesized by all organisms. Most of them are synthesized by some processes that occur only in some plants and bacteria.

Mammals have to obtain nine out of twenty amino acids from their food. These amino acids cannot be synthesized by the mammalian enzyme. These amino acids are known as essential amino acids, the rest of the amino acids that can be synthesized by almost all organisms are known as nonessential amino acids. These amino acids are categorized into two types: essential and non-essential amino acids.

Glycine is the simplest amino acid. It got its name due to its sweet taste as glycol means sugar. It was isolated from the protein gelatin in 1820.

Amino Acid Groups

Each amino acid has a specific R group, so, they can be classified into four groups based on the properties of their R groups. These groups are polar, non-polar, positively charged, or negatively charged.

  1. Polar amino acids: Their R group is hydrophilic, which means they are attracted towards aqueous solutions.
  2. Non-polar amino acids: Their R group is hydrophobic in nature so they are not attracted to liquids.
  3. Positively charged amino acids: These amino acids have an overall positive (basic) charge; the R-group is basic or positively charged. This positive charge arises due to the unbalanced amino ends (NH4) of the radical groups of these amino acids, at physiological pH. They are also polar and thus hydrophilic in nature.
  4. Negatively charged amino acids: They have an overall negative (acidic) charge; their R-group is acidic or has a negative charge. This negative charge is due to the unbalanced carboxyl ends (COOH) of the radical groups of these amino acids, at physiological pH.
Polar Amino Acids Nonpolar Amino Acids Polar Acidic Amino Acids (negatively charged) Polar Basic Amino Acids (positively charged)
Cys: Cysteine Ala: Alanine Asp: Aspartate His: Histidine
Ser: Serine Gly: Glycine Glu: Glutamate Lys: Lysine
Thr: Threonine Ile: Isoleucine Arg: Arginine
Tyr: Tyrosine Leu: Leucine
Asn: Asparagine Met: Methionine
Gln: Glutamine Trp: Tryptophan
Phe: Phenylalanine
Pro: Proline
Val: Valine

Physical properties of amino acids

  • They are colorless and crystalline substances.
  • Most of them are tasteless and some are sweet such as glycine and alanine and some are bitter such as arginine.
  • Their melting point is high, which ranges from 200 to 300 degrees centigrade.
  • Their solubility depends upon polarity, nature of solvent, temperature, and iso-electric point. For example, they are soluble in polar solvents like water and ethanol and insoluble in non-polar solvents like ether, benzene, etc. And, Tyrosine is soluble in hot water.
  • They tend to decompose when heated to high temperatures.
  • They are optically active, except for glycine.

Chemical properties of amino acids

  • They can act as acid and base as they are zwitterions. A zwitterion is a molecule that has at least one functional group with a positive electric charge and at least one functional group with a negative electric charge. So, amino acids are zwitterions as they have a basic amino group and an acidic carboxylic group.
  • They are amphoteric in nature as they can act as acid as well as a base.
  • In Ninhydrin test, when 1 ml protein solution is heated after adding 1 ml of Ninhydrin solution the color changes to violet which shows the presence of ?-amino acids.
  • The aromatic amino acids can be detected in a protein solution with the xanthoproteic test.
  • A free amino group in the peptide chain can react with Sanger?s reagent (1-fluoro-2, 4-dinitrobenzene) in a mild alkaline medium under cold conditions.
  • When the amino group reacts with nitrous acid, the nitrogen is released and the corresponding hydroxyl is formed.

Classification of amino acids based on nutrition

  • Essential amino acids: It refers to amino acids that cannot be synthesized in the body. There are nine essential amino acids that must be obtained from food. For example, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
  • Non-essential amino acids: These are the amino acids that can be synthesized in the body and hence don?t need to be obtained from the diet. They include arginine, glutamine, tyrosine, cysteine, glycine, proline, serine, alanine, aspartate, ornithine, asparagine.

Classification of amino acids based on their metabolic fate

  1. Glucogenic amino acids: As the name suggests, these amino acids act as precursors of gluconeogenesis for glucose formation. They include glycine, serine, alanine, aspartic acid, glutamic acid, asparagine, glutamine, valine, proline, cysteine, histidine, arginine, and methionine.
  2. Ketogenic amino acids: They break down to form ketone bodies. For example, Leucine and Lysine.
  3. Both Ketogenic and Glucogenic: They form precursors for ketone bodies as well as glucose. They are isoleucine, phenylalanine, tryptophan and tyrosine.

Functions of amino acids:

They are building blocks of proteins and are also required for the synthesis of hormones and neurotransmitters. They are necessary for a healthy human body. They play important role in the production of hormones, the structure of muscles, the functioning of the nervous system, and in maintaining the health of vital organs and cellular structures. Let us see the functions of essential and non-essential amino acids separately.

Functions of Essential Amino Acids

  • Phenylalanine: It acts as a precursor for the neurotransmitters tyrosine, dopamine, epinephrine, and norepinephrine. It is required for the structure and function of proteins and enzymes and to produce other amino acids.
  • Valine: It is one of the three branched-chain amino acids. It helps promote muscle growth and regeneration and also takes part in energy production.
  • Threonine: It is a major component of structural proteins like collagen and elastin. These proteins are needed in building skin and connective tissues. Further, it is also required in fat metabolism and immune function.
  • Tryptophan: Although it is believed to be a cause for drowsiness, it performs many other functions. It acts as a precursor for serotonin and helps maintain proper nitrogen balance.
  • Methionine: It is required for metabolism and detoxification. It also helps in tissue growth and absorption of zinc and selenium and other minerals.
  • Leucine: Leucine is also a branched-chain amino acid that is needed for protein synthesis and repair of muscles.
  • Isoleucine: It is also a branched-chain amino acid. It takes part in muscle metabolism and is found abundantly in muscle tissue. It is also needed for immune function, hemoglobin formation, and regulation of energy.
  • Lysine: It is important for protein synthesis, for production of enzymes and hormones, and calcium absorption. It is also needed for immune function and production of energy and collagen and elastin.
  • Histidine: It is important the production of histamine, a neurotransmitter needed for immune response, digestion, sexual health and sleep-wake cycle. Besides this, it is important for the myelin sheath that covers the never cells.

Functions of Non-Essential Amino Acids

  • Alanine: It aids in the production of glucose and other amino acids. It also removes toxins from the body.
  • Cysteine: It has antioxidant properties. It gives resistance to our body and promotes hair growth.
  • Glutamine: It is important for the synthesis of nucleic acids (DNA and RNA) and for healthy brain functions.
  • Glycine: It helps ensure proper cell growth. Further, it acts as a neurotransmitter and helps heal wounds.
  • Glutamic acid: It is important for the development and functioning of the human brain and also plays the role of a neurotransmitter.
  • Arginine: It promotes the synthesis of hormones and proteins. It also helps in the detoxification process that occurs in the kidneys and in healing wounds and improving the immune system.
  • Tyrosine: It is important for the production of thyroid hormones: T3 and T4. It also helps in the synthesis of neurotransmitters and melanin, a pigment found in eyes, hair and skin.
  • Serine: It helps promote the growth of muscles and is needed for the synthesis of proteins of the immune system.
  • Asparagine: It helps transport nitrogen to body cells, and in the formation of purines and pyrimidine needed for the DNA synthesis. Further, it improves the nervous system and stamina of our body.
  • Aspartic Acid: It is important for the synthesis of other amino acids and body metabolism.
  • Proline: It helps in the formation of collagen, prevents thickening and hardening of the arteries? wall and promotes regeneration of new skin.

Deficiency of Amino Acids

We should include all amino acids in our diet to maintain a healthy body and mind. The deficiency of amino acids may cause the following symptoms:

  • Anemia
  • Insomnia
  • Edema
  • Diarrhea
  • Depression
  • Loss of Appetite
  • Fat deposition in the liver
  • Weakness and fatigue
  • Skin and hair problems

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