Amino acid : introduction, structure, Classification, Functions

Amino acid : introduction, structure, Classification

Introduction of Amino acids

An amino acid is a bi-functional organic molecule that contains both a carboxyl group - COOH as  well as an amine group - NH2. They are classified as acidic, basic or neutral according to number of amine and carboxyl groups in a molecule. Neutral amino acids contain only one amine and one carboxyl group.

Their general formula is - (R = Alkyl, aryl or any other group)

                      H   Carboxyl group
                       |  ↙
                R - C - COOH
                   ↗  |

    α-Carbon  NH₂   ←Amino group

atom
                      

1. About 20 amino acids are commonly found in proteins, but 100 occur naturally.
2. All amino acids found in proteins are α-amino acids.
3. All amino acids found in proteins (except glycine) are optically active.

Structure of Amino acids

    Zwitterion - Dipolar Nature of amino acid

Amino acids exist as charged molecules. This is because they contain both an acidic - COOH group and a basic - NH2 group in the same molecule. Transfer of a proton takes place in a kind of internal acid-base reaction. Zwitterion - Dipolar Nature of amino acid molecules containing both a positive and a negative charge.

    Support the following point of Zwitterion - Dipolar structure

1. Amino acids are relatively non-volatile crystalline solids which melt with decomposition at fairly high temperatures.

2. They are practically insoluble in non-polar solvents like petroleum ether, benzene, etc., but fairly soluble in polar solvents like water (amines and carboxylic acids).

3. They are practically neutral substances. For example, glycine (aminoacetic acid) has an acid dissociation constant Ka, of the order of 10-¹⁰ and basic dissociation constant Kb, of the order of 10-¹².

4. Their aqueous solutions behave a high dipole moment. For example, glycine has dipole moment 1.5 D and B, y and 8-amino acids have still higher values.

5. Their spectra do not reveal characteristic bands of —NH2 and —COOH groups.


Physical Properties of Amino acids

• Amino acids are colourless, crystalline compounds having sweet taste.
• They are primarily soluble in water, but only sporadically in organic solvents.
• All except glycine, occur naturally in their optically active forms; the specific rotation depends upon pH of the solution.
• All have high melting points.
• They are amphoteric in reaction and form salts, with both acids and bases.
• They form peptides as their condensation product.

Classification of Amino Acids

Amino acids can be classified on three categories given below:

(1) On the Basis of Essential amino acid and Non-essential amino acid amino acid

• Essential amino acid: Which cannot be produced in sufficient quantity by the human body. They must be included in the diet. Examples: valine, phenylalanine, and tryptophan.
• Non-Essential amino acid: are those which can be synthesized from other compounds by the body. Examples are: glycine, alanine, and serine.

(2) Classified as α-, B-, y- amino acids: Amino acids depending upon the position of the NH2 group with respect to the carboxyl group, amino acids have been broadly classified as  amino acids etc. For example,

α 

H2NCH2COOH

2-Aminoethanoic acid

(α- Aminoacetic acid)

β ܓ

H2NCH2COOH

3- Aminopropanoic acid

(β-Aminopropionic Acid)

ܓ β α 

H2NCH2CH2COOH

4-Aminobutanoic acid 

(ܓ-Aminobutyric acid)

(3) Classification according to neutral, acidic and basic character: These natural amino acids can be classified in three types depending upon the relative number of —NH2 and -COOH groups. Most of these are neutral amino acids as these have only one –NH, and one –COOH groups. Some acidic amino acids which have one -NH2 and two - COOH groups and some are basic amino acids which have two -NH2 and one -COOH groups. As all the natural amino acids are of a-type, the general formula as:

                                 CH₂-COOH 

                                  |
                                 NH₂

                               Glycine

                           Neutral amino acids

                             HOOC- CH₂ - CH - COOH

                                                       |

                                                    NH₂

                                             Aspartic acid

                                  Acidic amino acids

                           H₂N - CH₂- CH₂ - CH - COOH

                                                          |

                                                         NH₂

                                                       Lysine

                                   Basic amino acids

Chemical Properties of Amino Acids

Since they include both amino and carboxyl groups, amino acids are Amphoteric. Their chemical properties can be discussed under three main headings:

1. Chemical reactions of the α-amino group.
2. Chemical reactions of the α-carboxyl group
3. Chemical reactions shared by the α-amino and α-carboxyl groups.

1. Chemical reactions of the α-amino group

1. Amino acids form acid salt with mineral acids.
2. Amino acids are acetylated by formaldehyde.
3. When treated with nitrous acid, amino acids get diaminated and release nitrogen.
4. Amino group of amino acids reacts with FDNB (1-fluoro 2, 4- dinitro benzene) forming a yellow coloured derivative (DNB amino acid).

2. Chemical reactions of the α-carboxyl group

• Reaction with alkalies (salt alkalies): Amino acids form salts with alkalies.
• Reaction with Alcohols: All amino acids can be esterifies.

3. Reactions Shared by the α-Amino and α-Carboxyl Groups

1. Reaction with ninhydrin: Free amino acid when heated in aqueous solution with ninhydrin it is oxidised to an aldehyde with one C-atom less with release of CO2 and NH3 and ninhydrin is reduced.
2. Peptide Linkage: Two amino acids join by the union of a-carboxyl group of one amino acid with the a-amino group of other amino acid forming a dipeptide and a molecule of water is eliminated. This joining of two amino acid molecules is known as peptide linkage. Similarly a tripeptide can be formed. A number of amino acids (even hundreds) are joined together in this fashion forming polypeptides. Hofmeister recognized the function of the peptide bond in the construction of proteins (1902).
3. Chelation: Amino acids also react with ions of certain heavy metals by the process of chelation. Such ions are Cu++, Co++, Mn++ and Ca++.
4. Biological Buffers: Because of amphoretic nature amino acids act as buffers in the body fluids. They maintain pH by donating H+ ions when pH increases or by accepting H+ ions when pH falls.
5. Synthesis of Glucose: The hepatic cells generate glucose from amino acids when the blood glucose level drops. It is known as glycogenesis.
6. Formation of Histamine: Amines in body are derived from some amino acids by loss of carboxyl group. For example, histamine is derived from amino acid histamine. Histamine is an important vasodilator. It causes constriction of bronchiole smooth muscles and stimulates gastric secretion.
7. Prokaryotic Cell Wall: Some peptides are components of peptidoglycans which form bacterial cell wall.

    Functions of Amino Acids

1. Formation of Proteins: Amino acids are linked into long chains through peptide linkage forming into polypeptides and proteins. Proteins form structural as well as functional components of the cell.

2. Formation of other Compounds: Some amino acids give rise to other compounds, such as;

• Tyrosine is part of the hormones thyroxine and adrenaline
• Glycine participates in the syrflhesis of heme.
• Tryptophan produces vitamin nictinamide and plant hormons indole-3-acetic acid (IAA).
• Amino acid B-alanine is needed in the synthesis of coenzyme-A and vitamine pantothenic acid.

3. Formation of Antibiotics: Antibiotics include nonprotein amino acids in their composition.

4. Formation of Enkephalins: These are small peptides (penta-peptides) formed in certain part of brain. These are associated with the perception of pain and pleasure.

5. Biological Buffers: The amphoteric property of amino acids causes them to function as buffers in bodily fluids. When the pH rises, they donate H+ ions, and when the pH drops, they absorb H+ ions.

6. Synthesis of Glucose: The hepatic cells generate glucose from amino acids when the blood glucose level drops. This is called gluconeogenesis.

7. Formation of Histamine: Amines are derived from some amino acids in the body by the loss of a carboxyl group. For example, histamine is derived from amino acid histidine. Histamine is an important vasodilator. It also causes constriction of bronchiole smooth muscles and stimulates gastric secretion.

8. Prokaryotic Cell Wall: Some peptides are components of peptidoglycans which form bacterial cell wall.