Amino Acids Physical Properties, Structure, Classification, Functions

Sourav Bio

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Definition of Amino Acids

The group of neutral compounds known as amino acids is distinguished chemically from all other natural compounds mainly due to their ampholytic properties and biochemically mainly because they are protein constituents. An amino acid is a carboxylic acids that contain an aliphatic primary ammonium group in the same position as the carboxyl group. It also has a distinctive stereochemistry. Biosynthesis of proteins is done using 20 amino acids, which are subject to strict genetic control. Amino acids are the fundamental unit of protein. There are more than 300 amino acids found in nature, but only 20 are present in proteins because they are encoded by genes. Modified amino acids, also known as non-protein amino acid, are other amino acids. Some amino acids are modified residues after a protein is synthesized using posttranslational modification; others are amino acid present in living organisms, but not as components of proteins.

Properties of Amino acids

Physical properties of amino acids

  1. Solubility: The majority of amino acids are soluble in water, but insoluble in organic solvents.
  2. Melting points:  Amino acid melts at higher temperatures, usually above 200°C.
  3. Taste: Amino acids can be sweet(Gly, Ala, Val), tasteless (Leu) or bitter (Arg, Ile). Monosodium glutamate, or ajinomoto, is used in the food industry as a flavoring agent. It can also be used to enhance taste and flavor of Chinese foods. Chinese restaurant syndrome is a short-term flulike condition that can be seen in some people who are not sensitive to MSG.
  4. Optical properties: All amino acids, except glycine, have optical isomers because of the presence of an asymmetric carbon. Some amino acids also contain a second asymmetrical carbon, e.g. isoleucine, threonine. It has been shown that the structure of L-and D-amino acid is different from glyceraldehyde.
  5. Amino acids as ampholytes: Amino acid can contain both basic (NH2) and acidic (COOH) groups. Ampholytes are amino acids that can either donate or accept protons.

Chemical Properties

1. Zwitterionic property

A zwitterion molecule is one that has functional groups. At least one of these groups has a positive electrical charge and at most one has one with a negative. The molecule’s net charge is zero. The most well-known examples are amino acids. They have an amine (basic) as well as a carboxylic (acidic) group. The -NH2 is the stronger base. It picks up H+ form the -COOH to create a zwitterion. The neutral zwitterion refers to the most common form of amino acids found in the solution.

2. Amphoteric property

Amino acids can be amphoteric, meaning they can act as both an acid and a base because of the presence of two amine or carboxylic groups.


3. Ninhydrin test

If 1 ml Ninhydrin solution is mixed with 1 ml protein and heated, the violet color will indicate the presence of a-amino acid.

4. Xanthoproteic test

The xanthoproteic is used to detect aromatic amino acids (tryptophan, tyrosine and phenylalanine in a protein solution). A reaction with nitric acids results in the yellowing of the solution. This causes the nitration and formation of benzoid radicals within the amino acid chain.

5. Reaction with Sanger’s reagent

Sanger’s reagent (1,fluoro-2 and 4-dinitrobenzene), reacts with an amino group free in the peptide chains in mild alkaline media under cold conditions.

6. Reaction with nitrous acid

The amino group reacts with nitrogen acid to release nitrogen and create the corresponding hydroxyl.

Structure of Amino acids

Alpha-amino acid is the name of all 20 common amino acids. Each one of the common amino acids has a carboxyl, an amino, and a sidechain (R) attached to the a carbon.

Structure of Amino acids
Structure of Amino acids

Exceptions are:

  • Glycine does not contain a sidechain. Its a carbon contains two hydrogens.
  • Proline is a nitrogen-containing ring that contains the proline.
  • Each amino acid therefore has an amine at one end, an acid at the other, as well as a distinct side chain. All amino acids have the same backbone, but each amino acid has a different side chain.
  • Each of the 20 amino acids, except glycine, are L-configured. All but one amino acid is an asymmetrical carbon. Glycine is not optically active because it does not contain an unasymmetrical carbon atom.
Amino acid classification based on the structure
Amino acid classification

Amino acid classification based on the structure

Based on their chemical structure and physical nature, an exhaustive classification of amino acids can be made. Each amino acid is given a symbol or 3 letter name. These symbols are used to represent amino acids in the protein structure. Seven distinct groups are made up of the 20 amino acids in proteins.

  1. Amino acids with aliphatic side chains: Monoamine monocarboxylic acid is an amino acid that has aliphatic side chain. This group includes the most basic amino acids, such as glycine (alanine), valine, leucine, leucine, and valine. The branched side chains of the last three amino acids (Leu and Val) are what we call branched chain amino acid.
  2. Hydroxyl group containing amino acids:  Tyrosine, threonine, and serine are examples of hydroxyl groups containing amino acid. Tyrosine, which is aromatic in nature, is usually considered to be under aromatic amino acids.
  3. Sulfur containing amino acids: Cysteine containing a sulfhydryl and methionine containing a thioether are two of the amino acids that were incorporated into protein synthesis. Cystine is another importa nt, sulfur-containing amino acid. It is made by the condensation of two cysteine molecules.
  4. Acidic amino acids and their amides: Aspartic and glutamic acid are dicarboxylic monoamino acid acids, while asparagine or glutamine are the respective amide derivatives. These four amino acids have distinct codons that allow them to be incorporated into proteins.
  5. Basic amino acids: The dibasic monocarboxylic acid dibasic lysine (with guanidino) and arginine with imidazole (with imidazole) are the three basic amino acids. They are very basic in nature.
  6. Aromatic amino acids: Phenylalanine (with an indole-ring), tyrosine, and tryptophan are all aromatic amino acids. Histidine could also be included in this category.
  7. Imino acids: A unique amino acid is proline containing the pyrrolidine rings. Instead of the amino group (NH2) found in other amino acid, it has an imino (NH) instead. Proline is a D-imino acid.
  8. Heterocyclic amino acids: Histidine, tryptophan and proline.

Classification of amino acids based on polarity

Based on their polarity, amino acids can be divided into four groups. For protein structure, polarity is crucial.

  1. Non-polar amino acids: Also known as hydrophobic (water hateing), these amino acids are non-polar. They do not have a charge on the R’ group. This group includes the following amino acids: alanine (leucine), isoleucine; valine; methionine; phenylalanine; tryptophan, proline.
  2. Polar amino acids with no charge on ‘R’ group: These amino acid have no charge on their “R” group. However, they possess groups like hydroxyl or sulfhydryl and are involved in hydrogen bonding proteins. Glycine, where R = H, is also included in this category. This group includes glycine (where R = H), serine, threonine and cysteine as well as glutamine, asparagine, tyrosine and threonine.
  3. Polar amino acids with positive ‘R’ group: This group includes the three amino acids histidine, arginine, and lysine.
  4. Polar amino acids with negative ‘R’ group: This group includes the dicarboxylic monoamino acid– aspartic and glutamic acids.

Nutritional classification of amino acids

These 20 amino acids are necessary for the synthesis and other biological functions. All 20 amino acids are not required to be consumed in the diet. Amounts of amino acids can be divided into two groups based on nutritional requirements: essential and non-essential.

1. Essential or indispensable amino acids

Essential amino acids are those amino acids that cannot be made by the body. They must be obtained through diet. These amino acids are essential for the proper growth and maintenance. These ten amino acids are vital for humans and rats: Arginine (Valine), Histidine (Isoleucine), Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan. [The code A.V. HILL, MP. T. T. (first letter in each amino acid) can be remembered to recall essential amino compounds. H. VITTAL and LMP are also useful codes. H. VITTAL, LMP; PH. VILLMA, TT, PVT TIM HALL and MATTVILPhLy.]

Two amino acids, namely histidine and arginine, can be synthesized by adults, but not by growing children. These are semi-essential amino acid (remember Ah?, to remember). 8 amino acids are essential, while 2 are optional.

2. Non-essential or dispensable amino acids

There are about 10 amino acids that the body can make to meet its biological needs. They should not be eaten in the diet. These include glycine (glycine), alanine (serine), cysteine, asparagine(asparagine), glutamate, glutamine and tyrosine (proline).

Amino acid classification based on their metabolic fate

The carbon skeleton in amino acids can be used as a precursor to the synthesis of fat (ketogenic), glucose (glycogenic), or both. The metabolic point of view divides amino acids into three groups.

  1. Glycogenic amino acids: These amino acids may be used as precursors to the formation of glucose and glycogen. E.g. alanine, aspartate, glycine, methionine etc.
  2. Ketogenic amino acids: They can be used to make fat. Only two amino acids, lysine and leucine, are ketogenic.
  3. Glycogenic and ketogenic amino acids: The four amino acids, isoleucine (phenylalanine), tryptophan, and tyrosine, are precursors to the synthesis of both glucose, as well as fat.

Synthesis of amino acids

Chemical synthesis

Mutant bacteria is often used to produce amino acids commercially. Enzymatic conversions are possible with synthetic intermediates. 2-Aminothiazoline-4-carboxylic acid is an intermediate in one industrial synthesis of L-cysteine for example. The addition of ammonia fumarate to fumarate via a lyase produces aspartic acid.


The first form of nitrogen in plants is glutamate. This organic compound is formed from alphaketoglutarate in the mitochondrion. Transaminases are used by plants to transfer the amino group of other amino acids from glutamate into another alpha-keto. Aspartate aminotransferase, for example, converts glutamate to oxaloacetate and alpha-ketoglutarate to aspartate. Transaminases are also used in other organisms for amino acid synthesis.

Modifications to standard amino acids can often create nonstandard amino acids. For example, homocysteine is formed through the transsulfuration pathway or by the demethylation of methionine via the intermediate metabolite S-adenosylmethionine, while hydroxyproline is made by a post translational modification of proline.

Many uncommon amino acids are synthesized by plants and microorganisms. Some microbes produce 2-aminoisobutyric and lanthionine which are sulfide-bridged alanine derivatives. These amino acids can be found in peptidic-lantibiotics like alamethicin. [126] However, in plants, 1-aminocyclopropane-1-carboxylic acid is a small disubstituted cyclic amino acid that is an intermediate in the production of the plant hormone ethylene.

Functions of Amino acids

  • Particularly, 20 important amino acids are vital for life because they contain peptides as well as proteins. They are also known to be the building blocks of all living things.
  • The three-dimensional structure of a protein is determined by its structure. It is determined by the linear sequence of amino acids in a polypeptide chain.
  • For maintaining the health and well-being of the body, amino acids are essential. They are essential for the following: Normal cellular structure, Production of hormones, Structure and functioning of the nervous system, Health of vital organs, Healthy functioning of the human nervous system.
  • Different tissues use amino acids to synthesize proteins, or to make nitrogen-containing compounds (e.g. purines, heme and creatine, epinephrine), as well as to oxidize them to create energy.
  • Both dietary and tissue protein are broken down to produce nitrogen-containing substrates as well as carbon skeletons.
  • These nitrogen-containing substrates are used for the biosynthesis and synthesis of pyrimidines (neurotransmitters), hormones, hormones, hormonal compounds, porphyrins and nonessential amino acid.
  • Carbon skeletons can be used to fuel the citric acid cycle, for gluconeogenesis or in fatty acid synthesis.
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