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Amino Acids – Definition, Structure, Properties, Classification

  • Last Updated : 21 Jan, 2022

A biomolecule, sometimes known as a biological molecule, is a term that refers to molecules found in living things that are required for one or more biological processes, such as cell division, morphogenesis, or development. Large macromolecules (or polyanions) like proteins, carbohydrates, lipids, and nucleic acids, as well as tiny molecules like primary metabolites, secondary metabolites, and natural products, are all examples of biomolecules. Biological materials is a more broad term for this type of material. Biomolecules are essential components of living organisms. While endogenous biomolecules are made within the organism, organisms typically require external biomolecules, such as specific nutrients, to exist.

Amino Acids

Proteins are complex macromolecules made up of amino acids that are found in all living cells. Amino acids, in other words, are the building blocks of proteins. There are around 500 naturally occurring amino acids that we are aware of.

Structure of Amino Acids

Amino acids are the basic components of proteins. Organic substances containing both amino and carboxylic groups are known as amino acids.

Any carbon atom other than that of the carboxyl (–COOH) group may be linked to the amino group (–NH2).

Properties of Amino Acids

  • They are crystalline, colourless compounds.
  • Their melting point is really high.
  • The nature of the side chain influences the solubility in water.
  • They are amphoteric, which means they react with acids and bases.
  • Except for glycine, all have asymmetric carbon, which causes plane polarised light to rotate. Optical activity is the name for this feature.

Classification of Amino Acids

Classification Based on the Requirement of the Body:

  • Non-essential amino acids: These amino acids are produced by the body and do not need to be consumed. Out of the twenty amino acids, ten are non-essential. Glycine, alanine, serine, cysteine, glutamine, tyrosine, proline, aspartic acid, asparagine, and glutamic acid are amino acid.
  • Essential Amino Acids: These amino acids are not synthesised by the body and must be obtained from food. Out of the twenty amino acids, ten are non-essential. Valine, leucine, isoleucine, arginine, lysine, threonine, phenylalanine, tryptophan, and histidine are the amino acids that make up the human body. These essential amino acids are necessary for our bodies to grow, and a lack of them in our diet can lead to disorders like kwashiorkor.

Classification of Naturally Occurring Amino Acids: The naturally occurring amino acids can be classified into three groups: Aliphatic, Aromatic, and Heterocyclic amino acids.

Aliphatic Amino Acids: 

They are amino acids with a lengthy chain. Amino acids are further divided into the following categories:

  • Neutral Amino Acid:  Their molecules have an equal number of carboxylic and amino groups. Example: Glycine, alanine, valine, etc.

Glycine

  • Acidic Amino Acid: In their molecules, they have a higher proportion of carboxylic groups than amino groups. Example: Aspartic acid, glutamic acid which contains two –COOH groups and one –NH2 group.

Aspartic acid

  • Basic Amino Acid:  Its molecules have a higher proportion of amino groups than carboxylic groups. Example:  Lysine, arginine, and histidine, which contains two –NH2 groups and –COOH groups.  

Lysine acid

  • Sulphur Containing Amino Acid: Its structure contains sulphydryl (–SH) groups. Example: Cysteine and Methionine

Methionine

Aromatic Amino Acid: 

They contain a benzene ring in their structure. Example: Phenylalanine and Tyrosine

Phenylalanine

Heterocyclic Amino Acids: 

They have a heterocyclic ring on their side chain with at least one element other than carbon. Example: Tryptophan, Proline, and Hydroxyproline

Proline

Some Common Amino Acids are:

S. No.

α- Amino acid

Three-letter abbreviation  

One letter abbreviation  

1.

Glycine

Gly

G

2.

Alanine

Ala

A

3.

Valine

Val

V

4.

Leucine

Leu

L

5.

Isoleucine

Ile

I

6.

Phenylalanine

Phe

F

7.

Methionine

Met

M

8.

Tryptophan

Trp

W

9.

Proline

Pro

P

10.

Serine

Ser

S

11.

Cysteine

Cys

C

12.

Asparagine

Asn

N

13.

Glutamine

Gln

Q

14.

Threonine

Thr

T

15.

Tyrosine

Tyr

Y

16.

Aspartic acid

Asp

D

17.

Glutamic acid

Glu

E

18.

Lysine

Lys

K

19.

Arginine

Arg

R

20.

Histidine

His

H

How is Protein Formed from Amino Acids?

The amino group of one amino acid molecule establishes a chemical connection with the carboxylic group of the other when they combine to make proteins. A peptide bond is the outcome of this process. This process of linking amino acids continues until all of the amino acids required for the synthesis of a protein have been linked together. Between the two amino acids, one water molecule is lost in this process. When two amino acids are joined in this way, a bigger unit known as a peptide is formed. A polypeptide is made up of many peptides that have been linked together. Polypeptides then join together to form a full protein.

Peptide bond

Role of Amino Acid in Our Body

  1. Amino acids are transformed into physiologically active molecules in a variety of ways. Tyrosine, for example, is transformed into the hormones thyroxine and adrenaline, as well as the skin pigment melanin.
  2. From the vitamin nicotinamide and the plant hormone indole acetic acid, glycine is involved in the synthesis of heme (haemoglobin protein) and tryptophan.
  3. Citrulline and ornithine are amino acids that are actively involved in the urea cycle in the liver, which helps to keep ammonia levels below hazardous levels.

Sample Questions 

Question 1: Which foods contain amino acids?

Answer:

Animal and plant sources of amino acids are available. Amino acids can be found in grains, nuts, seeds, beans, legumes, fruits, vegetables, eggs, milk, cheese, fish, and seafood, among other foods.

Question 2: Is it safe to take amino acids every day?

Answer:

In general, using a small amount of amino acids every day is safe. A high dose of amino acids, on the other hand, is detrimental. They can cause stomach pain, diarrhoea, a dangerous drop in blood pressure, gastrointestinal distress, and other problems. 

Question 3: List 21 amino acids?

Answer:

Glycine, Alanine, Valine, Leucine, Isoleucine, Phenylalanine, Methionine, Tryptophan, Proline, Serine, Cysteine, Asparagine, Glutamine, Tyrosine, Aspartic acid, Glutamic acid, Lysine, Arginine, Histidine, and Selenocysteine are the 21 amino acids.

Question 4: Do amino acids have side effects?

Answer:

Excess amino acid consumption raises the risk of hypertension, heart illness, weariness, and contraction loss.

Question 5: What do amino acids do for your body?

Answer:

Proteins are nutrients that help you create muscle. Proteins are made up of amino acids, which are the basic building components. As a result, they aid in the growth and healing of the body’s damaged cells and tissues. Some amino acids, such as tyrosine, are transformed into physiologically active substances such as the hormones thyroxine and adrenaline, as well as the skin pigment melanin.

Question 6: Do amino acids affect kidneys?

Answer:

Citrulline and ornithine are two amino acids that are involved in the urea cycle in the liver. This contributes to keeping ammonia levels below dangerous levels.


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