Introduction

Amino acids are organic compounds and form the building components for the living system necessary for the growth and development of human beings. About 300 amino acids occur in nature. The key elements of amino acids consist of carbon (C), hydrogen (H), oxygen (O) and nitrogen (N). Sulfur (S) is also present in the side chains of cysteine and methionine. The major functional groups present in the amino groups include amine (-NH2), carboxyl (-COOH) and side chain (R group). A chain of amino acids forms a protein. In other words, proteins are polymers of amino acids. Chains of amino acids are bound together through peptide bonds.

Altogether, there are a total of 20 amino acids that combine together to form proteins of different structures and functions. On the basis of the position of the amino group with respect to the carboxyl group, amino acids are classified as α, β, ץ, δ and so on. On hydrolysis of proteins, α-amino acids are obtained. Residues of amino acids form the second-largest component of human muscles and tissues. Besides this, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis.   

Body Structure of Amino Acid

All 20 naturally occurring amino acids have common structural features – an amino group (-NH3+), carboxylate (-COO-) group and a hydrogen-bonded to the same carbon atom. The difference lies in the side-chain called R group. 4 different groups are attached to α-carbon.

These 4 groups are:

• Amino group (-NH2)
• COOH
• Hydrogen atom
• Side-chain (R)

Zwitterion: Zwitter ion acts as a dipolar ion in the solution. It is neutral as it contains both positive and negative charges. In an aqueous solution, the carboxyl group loses a proton and the amino group accepts a proton. This gives rise to a dipolar ion called a zwitterion. Amino acids show amphoteric behaviour as they react with both acids and bases.

Structure of Proteins

Amino acids are linked together via peptide bonds or peptide linkage. Peptide bonds are formed between -COOH and -NH2 groups.

When two same or different amino acids combine, this results in the elimination of a water molecule and hence the formation of peptide bonds occurs. The end product obtained is called dipeptide as it is made up of two amino acids. Similarly, when third amino acids combine to dipeptide then the product obtained is called tripeptide. In the same way, tetrapeptide, pentapeptide or hexapeptide products can be obtained. More than ten amino acids combine to form a product called a polypeptide. 

On the basis of structure proteins can be classified into the following 2 types:

• Fibrous proteins: In this structure of the protein, the polypeptide chain runs parallel and is held together with the help of hydrogen and disulphide bonds and thus forming a fibre-like structure called fibrous proteins
• These are insoluble in water

For e.g.- keratin (present in hair, silk) and myosin (muscle protein) etc.

• Globular proteins: When polypeptide coils resulting in a spherical shape. These are soluble in water

For e.g. Insulin and albumin.

The structure and shape of proteins can be studied at 4 levels. Level of protein structure includes- primary, secondary, tertiary and quaternary.

• Primary structure of protein: Consists of one or more polypeptide chains
• Amino acids are linearly arranged and linked with each other via peptide bonds
• Any change in arrangement forms a different protein
• Secondary structure of protein: characteristics of this structure are the presence of alpha-helix and beta-pleated sheet Amino acids are linked together via peptide bond as well as hydrogen bond
• The tertiary structure of protein- Further folding of secondary structure results in the formation of the tertiary structure of the protein
• This structure is characterized by hydrogen bonds, disulphide linkages, van der Waals and electrostatic forces of attraction
• Quaternary structure of protein: Proteins made up of two or more polypeptides are called sub-units
• The spatial arrangement of these subunits with each other is called quaternary structure

Characteristics of Peptide Bond

Characteristics of peptide bonds are as follows:

1. Peptide bonds have partial double bond character.
2. Peptide bonds cannot be broken by the high salt concentration or by heat.
3. Peptide bonds are rigid and hence provide a definite shape to the protein structure.
4. They contain partial positive charge and negative charge groups.

General properties of amino acids

1. Amino acids have a high melting point and boiling point.
2. Amino acids are white crystalline solid substances.
3. With respect to taste, amino acids taste sweet, tasteless and bitter.
4. Amino acids are soluble in water and insoluble in organic solvents.

Classification of Amino Acids

Amino acids can be classified on the basis of the following bases, these are:

1. On the basis of R-group.
2. Polarity and R-group.
3. Nutritional requirements.

Based on R- group: 

Further in this group amino acids are categorized into:

• Simple amino acids: They have no functional group in their side chain. 

For e.g. glycine, valine, alanine, leucine, isoleucine.

• Hydroxyl amino acids: Comprise a hydroxyl group in their side chain. 

For e.g.serine, threonine.

• Sulfur-containing amino acids: Contain sulfur in their side chain.

For e.g. cysteine, methionine

• Aromatic amino acids: Benzene rings are present in their side chain. 

For e.g. phenylalanine, tyrosine

• Heterocyclic amino acids: A side chain ring present containing at least an atom other than carbon.

        For e.g. Tryptophan, histidine, proline

• Amine group-containing amino acids: In this derivative of amino acids, the carboxyl group gets transformed into an amide group. 

For e.g., Asparagine, glutamine etc. 

• Branched-chain amino acids (BCAA): Amino acids contain aliphatic side chains with a branch.

            For e.g. leucine, isoleucine, valine etc.

• Acidic amino acids: Carboxyl group is present in the side chain.

            For e.g. Aspartic and Glutamic acid.

• Basic amino acids: Amino group is present in the side chain.

For e.g. Lysine and Arginine.

• Imino acid: They contain a secondary amine group.

For e.g. Proline.

Based on Polarity and R-group:

The following can be categorized:

• Amino acids containing non-polar group: Mainly hydrocarbons in nature, hydrophobic and have aliphatic and aromatic groups [aliphatic R groups].

For e.g. Alanine, Valine, Leucine, Isoleucine, and Proline form an aliphatic R group.

For e.g.Phenylalanine, Tryptophan, and Methionine(sulfur) consist of aromatic groups.

• Amino acids containing polar uncharged R group: Amino acids are polar and have a neutral pH value. 

For e.g. Glycine, Serine, Threonine, Cysteine, Tyrosine, Glutamine, and Asparagine.

• Negatively charged amino acids: The side chain i.e. R-group contains an extra carboxyl group. 

For e.g.Aspartic acid and Glutamic acid.

• Positively charged amino acids: An extra amino group is present in the side chain providing basic nature to protein. 

For e.g.-Lysine, Arginine, Histidine

Based on Nutritional Requirements

Amino acids can be categorized into the following types:

1. Essential amino acids: Those amino acids which cannot be synthesized by our body and are obtained from outside sources. These include arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
2. Non- essential amino acids: Those amino acids can be synthesized by our body. These include alanine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine and tyrosine. 

Sources of Amino Acids  

 Amino acids play an immense role in performing certain biological functions and important chemical reactions in our bodies. Food rich in amino acids includes beans, broccoli, beetroots, pumpkin, cabbage, nuts, dry fruits, chia seeds, oats, peas, carrots, cucumber, green leafy vegetables, onions, soybeans, whole grain, peanuts legumes, lentils, etc. Fruits rich in amino acids include apples, bananas, berries, figs, grapes, melons, oranges, papaya, pineapple and pomegranates. Animal based products include dairy products, eggs, seafood, chicken, meat, pork etc.

Conclusion 

In general terms, we can say that amino acids or proteins are important for a human being’s everyday life whether it is synthesized inside the body or obtained from outside sources as it forms the building blocks for the human body system. Chain of acids forms proteins of different functions. All 20 amino acids are essential for human life. Peptide linkage binds amino acids together providing rigid shape to protein. Source of protein is abundant either from plant or animal sources. Protein in the diet helps to maintain the balanced diet of the person.