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Amino Acids

What are amino acids?

Amino acids are types of organic compounds that contain amino and carboxyl groups in their side chain are essential for the growth of young animals. Carbon, hydrogen, oxygen, and nitrogen are the key elements found in the amino acid structure. Other elements are also found in the side chain of certain amino acids. When proteins are hydrolyzed by acids, alkalis, or enzymes, they yield different types of amino acids. All the acids obtained from protein are α-amino acid but proline and hydroxyproline are imino-acids. Nine amino acids are essential for the growth of our body. The deficiency in anyone prevents the growth of young animals and may even cause death.

Structure and examples of amino acids that contain amino and carboxyl groups in their side chain

Amino acids structure

Amino acid molecules are the basic building blocks of proteins that contain amino (-NH2) and carboxyl (-COOH) groups in their structure. Each amino acid has a definite structure. According to the attachment of the amino and carboxyl groups, these are classified as α, β, and γ amino acids. The names, structure, formula, and examples of some common amino acids are given below the picture,

Amino acids structure, examples, names and formula

Types of amino acid

They can be classified in several ways. They are described by latter g, l, and e followed by the name of amino acid. The latter g, l, and e indicate the general occurrence, lesser occurrence, and essential. Different types of amino acids can be placed in the following groups,

Nonessential amino acids

Nonessential amino acids are those acids that are required for normal health and growth. They are normally synthesized in our body and cannot be essential to eat extra food to acquire them. Alanine, asparagine, aspartic acid, and glutamic acid are examples of nonessential amino acids.

Essential amino acids

Essential or indispensable amino acids are acids that cannot be synthesized in our body and are essential to eating food to get them. There are nine amino acids like histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine are essential for the growth of a our body. A deficiency in anyone prevents the growth of our body and may even cause death. Therefore, it is essential to eating food that contains these acids.

Sources of essential amino acids

Animal-based foods such as meat, milk, fish, and eggs are the main sources of essential amino acids. They are also obtained from plant-based food like soy, beans, nuts, and grains.

Conditional amino acids

Conditional amino acids are normally not essential but are important in times of illness, injury, or stress. Arginine, cysteine, glutamine, tyrosine, glycine, ornithine, proline, and serine are examples of conditional amino acids.

Classification

Based on the number of amino groups and carboxyl groups, these are classified as neutral, acidic, and basic amino acid.

Neutral

A neutral amino acid contains one amino group and one carboxyl group in their side chain. Systematic name and chemical formula of some common neutral amino acids are given below the table,

Examples of neutral amino acids
Name Systematic name Formula
Glycine Aminoacetic acid C2H5NO2
Alanine α-aminopropanoic acid C3H7NO2
Valine α-aminoisovaleric acid C5H11NO2
Leucine α-aminoisoacproic acid C6H13NO2
isoleucine α-amino-β-methyl-n-valeric acid C6H13NO2
Norleucine α-amino-n-caproic acid C6H13NO2
Phenylalanine α-amino-β-phenyl-propanoic acid C9H11NO2
Tyrosine α-amino-β-(p-hydroxyphenyl)-propanoic acid C9H11NO3
Serine α-amino-β-hydoxypropanoic acid C3H7NO3
Cysteine α-amino-β-mercaptopropanoic acid C3H7NO2S
Cystine Bis(α-aminopropanoic acid)-β-disulfide C6H12N2O4S2
Threonine α-amino-β-hydroxy-n-butyric acid C4H9NO3
Methionine α-amino-γ-methylthio-n-butyric acid C5H11NO2S
Iodogorgic acid 3,5-di-iodotyrosine C9H9I2NO3
Thyroxine β-3,5-di-iodo-4-(3,5-di-iodo-4-hydoxy)-phynyl-α-aminopropanoc acid C15H11I4NO4
Tryptophan α-amino-β-indolepropanoic acid C11H12N2O2
Proline pyrrolidine-α-carboxylic acid C5H9NO2
Hydroxyproline γ-hydoxypyrrolidine-α-carboxylic acid C5H9NO3

Acidic

An acidic amino acid contains one amino group and two carboxyl groups in thair side chain. Systematic name and chemical formula of some common acidic amino acids are given below the table,

Examples of acidic amino acids
Name Systematic name Formula
Aspartic acid α-aminosuccinic acid C4H7NO4
Asparagine α-aminosuccinamic acid C4H8N2O3
Glutamic acid α-aminoglutaric acid C5H9NO4
β-hydoxyglutamic acid α-β-hydroxyglutaric acid C5H9NO4
Glutamine α-aminoglutaramic acid C5H10N2O3

Basic

A basic amino acid contains two amino groups and one carboxyl group in its structure. Systematic name and chemical formula of some common basic amino acids are given below the table,

Examples of basic amino acids
Name Systematic name Formula
Ornithine α, γ-diamino-n-valeric acid C5H12N2O2
Arginine α-amino-δ-guanidino-n-valeric acid C6H14N4O2
Lysine α, ε-aminocaproic acid C6H14N2O2
Histidine α-amino-β-imidazolepropanoic acid C6H9N3O2

Properties of amino acids

In a neutral solution, they exist as dipolar ions. For each amino acid, there is a particular pH scale value at which the concentration of dipolar ion is maximum. In acid solution, the conjugate acid is predominating but in basic solution, the conjugate is predominating. The dipolar ion is electrically neutral because the net charge is zero. In this condition, they do not migrate when placed in an electric field. The pH at which migration does not occur is called the isoelectric point of amino acid. Some common properties of amino acid are given below,

  • They are colourless crystalline solid.
  • They are generally soluble in water but sparingly soluble in organic solvents.
  • Most of the acids are melt with decomposition but sublimation is also possible with a number of acids.
  • They contain one chiral center.
  • Except for glycine, all the acids are optically active.
  • They have properties to form zwitterion due to the presence of amino and carboxyl groups in their side chain.
  • They are amphoteric in nature due to the presence of acidic and basic groups in their side chain.
  • They have properties to form peptide linkage.

Ninhydrin reaction

Ninhydrin (indane 1, 2, 3-trione hydrate) reacts with amino acids to form a coloured product. It is used as a sparing reagent in the identification and qualitative estimation. Most of the acids react with ninhydrin to give blue coloured products. However, proline and hydroxyproline give yellow coloured products.

Peptide linkage in proteins

Proteins are hydrolyzed by acids, alkalis, or enzymes to form a mixture of amino acids. Therefore, amino acids in proteins are joined in a linear fashion by peptide linkages. The atoms in the group of -CONH- are planners. The peptide linkages of amino acids in a protein molecule are given below the picture,

Peptide linkage in protein molecule