Amino Acids- Definition, Properties, Structure, Classification, Functions

What are Amino Acids?

Amino acids are a type of chemically and biochemically neutral, readily identifiable compounds from other natural chemicals due to their ampholytic capabilities and their function as building blocks of proteins, respectively. An amino acid includes a carboxylic acid having a main amino group that is aliphatic, positioned opposite the carboxyl group, and has a distinctive stereochemistry.

Twenty amino acids are used in the biosynthetic process that creates proteins under stringent genetic control. Consequently, amino acids are the fundamental components of proteins. There are more than 300 amino acids present in nature, yet only 20 of them are universally recognised by genes and found in proteins. Non-protein amino acids are several types of modified amino acids. Some are posttranslational changes made to residues after a protein has been created, whereas others are amino acids found in live organisms but not proteins.

Properties of Amino acids

Physical Properties

  1. Amino acids are colourless, crystalline solids.
  2. All amino acids have a melting point greater than 200 °.
  3. They dissolve easily in water, just slightly in alcohol, and more slowly in methanol, ethanol, and propanol. The amino acid R-group and the solvent’s pH both have a significant impact on solubility.
  4. When heated to a high temperature, they disintegrate.
  5. Except for glycine, all amino acids have optical activity.
  6. Peptide bond formation: The amino and carboxylate groups of amino acids may join to create a peptide bond.A covalent bond was formed between the alpha-amino group of one amino acid and the alpha-carboxyl group of another,creating the -CO-NH-linkage. Planar and partly ionic, peptide connections.

Chemical Properties

1.Zwitterionic property

A zwitterion is a molecule with active groups, at least each one of which possesses a positive and negative electrical charge. The entire molecule bears a net charge of zero. Amino acids are the most prominent zwitterions.

They include both a carboxylic and an amine group (acidic). Since -NH2 is the more powerful base, it accepts H+ from the -COOH group and produces a zwitterion in the process. The usual structure of amino acids seen in solutions is the (neutral) zwitterion.

2.Amphoteric property

Because they include both amine and carboxylic groups, amino acids are amphoteric, acting as both an acid and a base.

3. Ninhydrin test

The presence of -amino acids is detected by the liquid is heated after adding 1 ml of Ninhydrin solution to 1 ml of protein solution.

4. Xanthoproteic test.

Tyrosine, tryptophan, and phenylalanine are three aromatic amino acids that are looked for using the xanthoproteic test in a protein solution. The amino acid chain’s benzoid radicals are nitrated as a result of a reaction using nitric acid, which gives the solution its yellow colour.

5. Reaction with Sanger’s reagent

In a mildly alkaline medium at room temperature, Sanger’s reagent (1-fluoro-2, 4-dinitrobenzene) reacts with a free amino group in the peptide chain.

6. Reaction with nitrous acid

The amino group and nitrous acid react, releasing nitrogen and creating the corresponding hydroxyl.

Structure of Amino acids

Alpha-amino acids make up all 20 of the common amino acids. They have a R group side chain, an amino group, and a carboxyl group all connected to the -carbon.

Exceptions are:

  • Glycine, a compound without a side chain. It has two hydrogen atoms in its -carbon.
  • Proline is a ring-shaped molecule with a nitrogen atom.
  • In addition, toward an amine group at one terminal and also an acid group at another, each amino acid also has a particular side chain. All amino acids have a common backbone, however each amino acid has a unique side chain.
  • With the exception of glycine, all 20 of the amino acids have the L-configuration because all but one of their -carbons are asymmetric. Glycine is neither D nor L because it lacks an asymmetric carbon atom, which prevents it from being optically active.

Classification of amino acids on the basis of R-group

1, Nonpolar, aliphatic amino acids: These amino acids have nonpolar, hydrophobic R groups. Leucine, Isoleucine, Glycine, Alanine, Valine, Methionine, and Proline.

2. Aromatic amino acids:Tyrosine, tryptophan, and phenylalanine are highly nonpolar amino acids due to their aromatic side chains (hydrophobic). Interactions between hydrophobic molecules are open to everybody.

3. Polar, uncharged amino acids:These amino acids include active groups that form hydrogen bonds involving water, making their R groups more water-soluble or hydrophilic than those of the nonpolar amino acids.The serine, threonine, cysteine, asparagine, and glutamine are members of this family of amino acids.

4. Acidic amino acids:Acidic amino acids are those with an acidic or negatively charged R-group. Aspartic acid with glutamic acid

5. Basic amino acids:Those are amino acids having a basic or positively charged R-group, Histidine, Lysine, and Arginine

Classification of amino acids on the basis of nutrition

Essential amino acids (Nine)

Nine amino acids must be consumed in the diet for protein synthesis to take place since they cannot be produced by the body.

Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine are the necessary amino acids.

Non-essential amino acids (Eleven)

Since the body is capable of producing certain amino acids, dietary intake is not always required.

Arginine, glutamine, tyrosine, cysteine, glycine, proline, serine, ornithine, alanine, asparagine, and aspartate are some of the non-essential amino acids.

Classification of amino acids on the basis of the metabolic fate

  1. Glucogenic amino acids:Amino acids that promote the production of glucose are known as gluconogenic amino acids. Arginine, proline, valine, methionine, cysteine, aspartic acid, asparagine, glutamic acid, and the amino acids glycine, alanine, serine, and asparagine.
  2. Ketogenic amino acids:Ketone bodies are created when these amino acids are broken down. Lysine and leucine
  3. Both glucogenic and ketogenic amino acids:Amino acids that are both glucogenic and ketogenic: These amino acids degrade to provide the building blocks for both ketone bodies and glucose. Tryptophan, Tyrosine, Phenylalanine, and Isoleucine.

Functions of Amino acids

  1. 20 essential amino acids in particular, which are believed to be the building blocks of all proteins and peptides, are essential for life.
  2. A protein’s three-dimensional shape is determined by the linear arrangement of its amino acid residues in a polypeptide chain, and a protein’s function is determined by its structure.
  3. The maintenance of the human body’s health depends on amino acids. They mostly support the

Production of hormones

  • Muscle structure
  • The proper operation of the human nervous system
  • The condition of critical organs
  • Typical cellular composition
  1. The amino acids are either oxidised to provide energy or utilised by different tissues to build proteins and nitrogen-containing substances (such as purines, heme, creatine, and adrenaline).
  2. Proteins from food and tissue degrade to produce carbon skeletons and nitrogen-containing substrates.
  3. The biosynthesis of purines, pyrimidines, neurotransmitters, hormones, porphyrins, and non-essential amino acids uses the nitrogen-containing substrates.
  4. The carbon skeletons are used in gluconeogenesis, the citric acid cycle, and the production of fatty acids.


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  • Rodwell, V. W., Botham, K. M., Kennelly, P. J., Weil, P. A., & Bender, D. A. (2015). Harper’s illustrated biochemistry (30th ed.). New York, N.Y.: McGraw-Hill Education LLC.
  • John W. Pelley, Edward F. Goljan (2011). Biochemistry. Third edition. Philadelphia: USA.
  • http://www-plb.ucdavis.edu/courses/bis/105/lectures/AminoAcids.pdf
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  • https://www.biologyexams4u.com/2012/09/amino-acids html#.W2MViTozbIV
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