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Amino Acids Structure Classification and Their Properties

In this amino acids structure classification and their properties post we have briefly explained about amino acids classification, amino acids structures, and physical and chemical properties.

Amino Acids Structure Classification and Their Properties

Proteins are of paramount importance for biological systems. All the major structural and functional aspects of the body are carried out by protein molecules. All proteins are polymers of amino acid. Proteins are composed of a number of amino acids linked by peptide bonds.

Although about 300 amino acid occur in nature, only 20 of them are seen in human body. Most of the amino acid (except proline) are alpha amino acids, which mean that the amino group is attached to the same carbon atom to which the carboxyl group is attached.


The general amino acids structures is H2NCH RCOOH and it can be written as:



H2N – – C – – H



There are 20 amino acids found in nature, all of which have the same structural features: an amino group (-NH3+), a carboxylate group (-COO-), and a hydrogen-bonded to the same carbon atom. Their R group side-chain distinguishes them from one another. The carbon of each amino acid is connected to four distinct groups.

  1. Amino group
  2. COOH
  3. Hydrogen atom
  4. Side chain (R)

Proline is an exception because it has a secondary amino group (-NH-), for uniformity it is also treated as alpha-amino acid. Glycine is an amino acid that contains, in its side chain, only a single hydrogen atom. Glycine is known to be a protein-genic amino acid.


Based on the Amino Acids Structures

The amino acids structures and chemical composition of amino acids are used to create a comprehensive classification system. A three-letter or one-letter symbol is allocated to each amino acid. The amino acids in protein structure are typically represented by these symbols. Proteins include 20 amino acids that are classified into seven categories.

Aliphatic side chains: Monoamino monocarboxylic acids are what they’re called. Glycine, alanine, valine, leucine, and isoleucine are the most basic amino acids in this category. Because the last three amino acids (Leu, Ile, and Val) have branched aliphatic side chains, they are known as branched chain amino acids.

Hydroxyl group: Serine, threonine, and tyrosine are amino acids with hydroxyl groups. Because tyrosine is aromatic in character, it is frequently classified as an aromatic amino acid.

Sulfur: The two amino acids incorporated during protein synthesis are cysteine with a sulfhydryl group and methionine with a thioether group. Condensation of two cysteine molecules produces cystine, another essential sulfur-containing amino acid.

Amides: Asparagine and glutamine are dicarboxylic monoamino acids, while aspartic acid and glutamic acid are dicarboxylic monoamino acids. For integration into proteins, each of these four amino acids has its own codon.

Basic amino acids: The dibasic monocarboxylic acids lysine, arginine (with guanidino group), and histidine (with imidazole ring) are the three amino acids. They have a really simple character.

Aromatic amino acids: Aromatic amino acids include phenylalanine, tyrosine, and tryptophan (which have an indole ring). Histidine, in addition to these, may be included in this group.

Imino acids: An unusual amino acid is proline, which has a pyrrolidine ring. Instead of the amino group (NH2) found in other amino acids, it possesses an imino group (NH). As a result, proline is classified as a D-imino acid.

Based On Polarity

Amino acids are classified into 4 groups based on their polarity. Polarity is important for protein structure.

Non-polar amino acids: These amino acids are also known as hydrophobic amino acids (water hating). They are not responsible for the ‘R’ group. Alanine, leucine, isoleucine, valine, methionine, phenylalanine, tryptophan, and proline are among the amino acids in this category.

Polar amino acids with no charge on ‘R’ group: As a result, the ‘R’ group of these amino acids is uncharged. They do, however, have groups like hydroxyl, sulfhydryl, and amide, and they contribute in protein structure hydrogen bonding. This group also includes the simple amino acid glycine (where R = H). Glycine, serine, threonine, cysteine, glutamine, asparagine, and tyrosine are the amino acids in this category.

Polar amino acids with positive ‘R’ group: This group includes the three amino acids lysine, arginine, and histidine.

Polar amino acids with negative ‘R’ group: This category includes the dicarboxylic monoamino acids aspartic acid and glutamic acid.

Nutritional classification

Apart from various biological roles, the 20 amino acids are essential for the creation of a range of proteins. All 20 amino acids, however, are not required in the diet. Amino acids are divided into two categories based on their dietary requirements: essential and non-essential.

Essential amino acids: Essential amino acids are amino acids that cannot be produced by the organism and must thus be obtained through the food. They are essential for the individual’s appropriate development and maintenance. Humans require 10 amino acids, which are mentioned below. Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Arginine, Valine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan.

Non-essential amino acids: To meet biological needs, the body can synthesis about 10 amino acids; thus, they are not required to be ingested in the food. Glycine, alanine, serine, cysteine, aspartate, asparagine, glutamate, glutamine, tyrosine, and proline are the amino acids in question.


Purely Ketogenic: Amino acid Leucine is purely ketogenic because it is converted to ketone bodies.

Ketogenic and Glucogenic: Lysine, Isoleucine, Phenylalanine, Tyrosine and Tryptophan are partially ketogenic and partially glucogenic. However in humans lysine is predominantly ketogenic. During metabolism, part of the carbon skeleton of these amino acids will enter the ketogenic pathway and the other part to glucogenic pathway.

Purely Glucogenic: All the remaining 14 amino acids are purely glucogenic as they enter only into the glucogenic pathway.


Amino acids can exist as ampholytes or zwitterions (German word “zwitter” = hybrid) in solution, depending on the pH of the medium.

The pH at which the molecule carries no net charge is known as iso-electric point or isoelectric pH (pI).

In acidic solution they are cationic in form and in alkaline solution they behave as anions (Fig. 3.13). At iso-electric point the amino acid will carry no net charge; all the groups are ionized but the charges will cancel each other. Therefore at isoelectric point, there is no mobility in an electrical field. Solubility and buffering capacity will be minimum at iso-electric pH.

To such a solution if we add hydrochloric acid drop by drop, at a particular pH, 50% of the molecules are in cation form and 50% in zwitterion form. This pH is pK1 (with regard to COOH). If more HCl is added, more molecules become cationic in nature and solubility increases.

On the other hand, if we titrate the solution from iso-electric point with NaOH, molecules acquire the anionic form. When 50% of molecules are anions, that pH is called pK2 (with respect to NH2)

Amino Acids Structure Classification and Their Properties

Optical Activity

Amino acids having an asymmetric carbon atom exhibit optical activity. Asymmetry arises when 4 different groups are attached to the same carbon atom.

Glycine is the simplest amino acid and has no asymmetric carbon atom and therefore shows no optical activity. All others are optically active.

The mirror image forms produced with reference to the alpha carbon atom, are called D and L isomers.

The L-amino acids occur in nature and are therefore called natural amino acids. D-amino acids are seen in small amounts in microorganisms and as constituents of certain antibiotics such as Gramicidin-S, Polymyxin, Actinomycin-D and Valinomycin, as well as bacterial cell wall peptidoglycans.

Isoleucine and threonine have 2 optically active centers and therefore each has 4 diastereo isomers.

Chemical Properties

Ninhydrin Reaction: All amino acids when heated with ninhydrin can form complexes; pink, purple or blue in color. The color complex is called Ruhemann’s purple. Proline and hydroxy proline will give yellow color with ninhydrin.

Biuret Reaction: Cupric ions in an alkaline medium form a violet color with peptide bond nitrogen (Schiff, 1896). This needs a minimum of two peptide bonds, and so individual amino acids and di-peptides will not answer this test.

Xanthoproteic test: The ring systems in phenyl alanine, tyrosine and tryptophan undergo nitration on treatment with concentrated nitric acid when heated (Salkowski, 1888). The end product is yellow in color which is intensified in strong alkaline medium.

Millon’s Test: The phenol group of phenylalanine and tyrosine containing proteins, when heated with mercuric sulphate in sulphuric acid and sodium nitrite form red colored mercury phenolate (Millon, 1849).

Aldehyde tests for tryptophan: In the Hopkins-Cole test, tryptophan containing protein is mixed with glyoxylic acid, and the mixture is layered over concentrated sulphuric acid. A violet ring at the interface of liquids shows the presence of the indole ring.

Sakaguchi’s test for arginine: Free arginine or arginyl residues in proteins react with alpha-naphthol and alkaline hypobromite to give bright red color. This is due to the guanidinium group.

Sulphur test for cysteine: When cysteine or cysteine containing proteins are boiled with strong alkali, organic sulphur splits and forms sodium sulphide, which on addition of lead acetate produces lead sulphide as a black precipitate. Methionine does not answer this test because sulphur in methionine is in the thio-ether linkage which is difficult to break. Albumin and keratin will answer sulphur test positively; but casein will give a negative test.

Nitroprusside reaction for SH groups: Proteins with free sulfhydryl groups give a reddish color with sodium nitroprusside, in ammoniacal solution. Many proteins give a negative, reaction in the native state, but when denatured, reaction will be positive, showing the emergence of free SH groups.

Pauly’s test for histidine or tyrosine: Diazo benzene sulfonic acid reacts with imidazole group of Histidine to form a cherry-red colored diazotised product under alkaline conditions. The same reagent will give an orange red colored product with phenol group of Tyrosine.


Essential Amino Acids

1. Phenylalanine aids in the maintenance of a healthy neurological system as well as memory enhancement. Valine is a crucial component in increasing muscle growth. The amino acid threonine aids in the immune system’s function.

2. Vitamin B3 and the serotonin hormone are both made from tryptophan. This serotonin hormone is important for hunger control, sleep regulation, and mood enhancement.

3. Isoleucine is necessary for the synthesis of haemoglobin, stimulating the pancreas to produce insulin, and carrying oxygen from the lungs to various regions of the body.

4. Methionine is used to cure kidney stones, maintain healthy skin, and prevent pathogenic microorganisms from invading the body. Protein synthesis and growth hormones are both aided by leucine.

5. Lysine is required for the creation and fixation of calcium in bones, as well as the synthesis of antibodies, hormones, and enzymes.

6. Histidine is involved in a variety of enzymatic processes as well as the formation of both red and white blood cells (erythrocytes) (leukocytes).

Non-Essential Amino Acids

1. Alanine aids in the elimination of toxins as well as the creation of glucose and other amino acids. Cysteine is an antioxidant that gives our bodies with resistance and prevents the growth of hair, nails, and other bodily tissues.

2. Glutamine is required for the creation of nucleic acids DNA and RNA and promotes healthy brain function. Glycine aids in the normal growth and function of cells, as well as the healing of wounds. It has the function of a neurotransmitter.

3. Glutamic acid is a neurotransmitter that plays an important role in the development and function of the human brain. Arginine aids in the production of proteins and hormones, kidney cleansing, wound healing, and the maintenance of a healthy immune system.

4. Tyrosine is required for the formation of thyroid hormones T3 and T4, as well as the synthesis of a group of neurotransmitters and melanin, a natural pigment found in our eyes, hair, and skin. Serine aids in the growth of muscles and the production of immune system proteins.

5. Asparagine is primarily involved in the transportation of nitrogen into our body cells, the creation of purines and pyrimidine for DNA synthesis, nervous system development, and body stamina. Aspartic acid is a key component of metabolism and the synthesis of other amino acids.

6. Proline is primarily engaged in tissue healing, the synthesis of collagen, the prevention of artery wall thickening and hardening (arteriosclerosis), and the regeneration of new skin.

Further Readings