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Amino acids: Structure,Types, Source and Enantiomers/ Isomers

Which biomolecule performs the majority of the functions in our body? Yes, its proteins. However, do you know what are the constituent units of proteins? The answer is amino acids. They are the building blocks of proteins. How many proteins are there in our body? No, ofcourse, it is difficult to estimate the number. However, the interesting thing is that different proteins are made up of a fixed number of amino acids that our body possesses. The diversity in amino acids gives the diversity in the structure and function of proteins. Let’s take a deeper look at the structure, types and isomers of amino acids.

Table of Contents

Structure of Amino acids

The organic compounds which have two functional groups, i.e., amino group (NH2) and carboxyl group (COOH) on the same carbon (alpha carbon) are called α-amino acids.

structure of an amino acid

An amino acid consists of a hydrogen, carboxylic acid, amino group and one variable R group. They are the substituent of four functional groups in methane occupying all the four valency positions of carbon. Based on the difference in R groups, there are 20 standard amino acids present in living organisms.

R group in amino acid

Name of the amino acid

Hydrogen

Glycine

Methyl group

Alanine

Hydroxymethyl group

Serine

some examples of amino acid

Types of Amino acid

An amino group, a carboxyl group and a R group together decide the physical and chemical properties of an amino acid.

On the Basis of Number of Amino and Carboxyl groups

On the basis of the number of amino and carboxyl groups, amino acids are classified as:

  • Basic amino acids
  • Acidic amino acids
  • Neutral amino acids

types of amino acid

Acidic amino acids

Amino acids that have one additional carboxyl group in the side chain (R group) and possess 2 carboxyl groups and one amino group are called acidic amino acids. Examples include aspartic acid and glutamic acid.

example of acidic amino acid

Basic amino acids

Amino acids that have one additional amine group in the side chain (R group) consist of 2 amine groups and one carboxyl group. Such amino acids are known as basic amino acids. Examples include lysine, arginine and histidine.

basic amino acid

Neutral amino acids

Amino acids that have only one amino group and one carboxyl group. Out of a total 20 standard amino acids, there are 15 neutral amino acids. Examples include glycine, alanine, leucine, isoleucine, valine, phenylalanine, proline, methionine, serine, threonine, tyrosine, cysteine, glutamine, asparagine and tryptophan.

example of neutral amino acid

On the basis of R group 

Aromatic amino acids

Aromatic amino acids are those amino acids which have a benzene ring in their R group. Examples of aromatic amino acids include tryptophan, tyrosine and phenylalanine.

aromatic amino acid

Non-aromatic amino acids

Non-aromatic or aliphatic amino acids are those in which the R groups are aliphatic straight chains, e.g, glycine, alanine, lysine, arginine, etc.

example of non aromatic amino acids

Source of Amino acids

Based on whether amino acids are synthesised in the body or obtained from external sources, they can be classified as essential and nonessential amino acids. Out of 20 standard amino acids, 10 are essential amino acids and the other 10 are nonessential amino acids.

Essential amino acids

These are the amino acids which are obtained from dietary proteins and cannot be synthesised by the human body.

Non- essential amino acids

These are the amino acids which are not obtained from the diet but can be synthesised by the human body.

Essential amino acids

Non essential amino acids

Phenylalanine

Aspartate

Valine

Alanine

Threonine

Glycine

Tryptophan

Glutamate

Isoleucine

Cysteine

Methionine

Asparagine

Histidine

Glutamine

Arginine

Glycine

Lysine

Proline

Leucine

Serine

Histidine and Arginine are semi-essential amino acids. This is because during specific circumstances, such as illness, pregnant women, children, and lactating mothers require them in greater quantities.

Enantiomers/isomers of amino acid

Amino acids have two types of isomers on the basis of their resemblance to glyceraldehyde. These isomers are D and L isomers or enantiomers. The enantiomers differ in their optical activity.

  • L isomers have their amine group on the left side of alpha carbon. They are the most common form of isomers found in living organisms. They rotate plane polarised light in the clockwise direction.
  • D isomers have their amine group on the right side of alpha carbon. They are the rarest form of isomers. D-alanine is present in bacterial cells. They rotate plane polarised light in the anti-clockwise direction.

Both of these isomers form mirror images to each other. 

enantiomers

Charge of amino acids

In water, amino acids ionise into amino and carboxyl groups. By losing one H+ ions, the carboxyl group attributes acidic character to the amino acid. Similarly the amine group receives a H+ ions and becomes basic or alkaline. Therefore, amino acids are amphoteric in nature and can exhibit both acidic and basic properties.
Amino acids can exist as zwitter ions in solution at specific pH levels, depending on the R group. A zwitterion is a molecule with one functional group having positive charge and the other functional group having negative charge. Thus, a zwitterion is neutral and has no net charge.
When the carboxyl group loses H+ ion, it becomes negatively charged whereas the amino group picks up the H+ ions and becomes positively charged.

formation of zwitterion in amino acid

different state of amino acid at different ph

Practice Problems of Amino acids

1. Match the following.

Column I

Column II

I. Aromatic amino acid

A. Lysine

II. Basic amino acid

B. Phenylalanine

III. Neutral amino acid

C. Serine

a. I - B, II - C, III - A
b. I - C, II - C, III - B
c. I - A, II - B, III - C
d. I - B, II - A, III - C

Solution: The correct match is as follows:
 

Column I

Column II

1

Aromatic amino acid

B

Phenylalanine

2

Basic amino acid

A

Lysine

3

Neutral amino acid

C

Serine

Hence, option d is correct.

2. Read the given assertion and reason and select the correct option.

Assertion: Amino acids can be essential as well as non-essential.

Reason: Amino acids can only be obtained from the diet.

a. Both assertion and reason are correct, reason is the correct explanation of assertion.
b. Both assertion and reason are correct but reason is not the correct explanation of assertion.
c. Assertion is correct, reason is incorrect.
d. Both assertion and reason are incorrect.

Solution: Based on whether amino acids are synthesised in the body or obtained from external sources, they can be classified as essential and nonessential amino acids. Thus, the assertion is correct. 
The amino acids which are obtained from the dietary proteins and cannot be synthesised by the human body are called essential amino acids. But, there are some amino acids which our body can synthesise and need not be obtained from the diet. These are called non-essential amino acids. Thus, the reason is incorrect. Hence, option c is correct.

FAQs of Amino acids

Question 1. Define amino acids.

Solution: Amino acids are organic compounds which have two functional groups, i.e., amino group (NH2) and carboxyl group (COOH) on the same carbon (alpha carbon). An amino acid consists of a hydrogen, carboxylic acid, amino group and one variable R group. They are the substituent of four functional groups in methane occupying all the four valency positions of carbon. 

Question 2. Mention the two isomers of amino acid.

Solution: Amino acids have two types of isomers on the basis of their resemblance to glyceraldehyde. These isomers are D and L isomers or enantiomers. The enantiomers differ in their optical activity.

  • L isomers have their amine group on the left side of alpha carbon. They are the most common form of isomers found in living organisms. They rotate plane polarised light in the clockwise direction.
  • D isomers have their amine group on the right side of alpha carbon. They are the rarest form of isomers. D-alanine is present in bacterial cells. They rotate plane polarised light in the anti-clockwise direction.

Question 3. Mention the difference between essential and non-essential amino acids.

Solution: 

Essential amino acids

Non-essential amino acids

Essential amino acids are obtained from the diet because they cannot be synthesised by the body.

Non-essential amino acids can be synthesised by the body.

Question 4.  What are acidic amino acids? Give examples

Solution: Acidic amino acids have an additional carboxylic group as R group. Examples include glutamic acid and aspartic acid.

Question 5. How many standard amino acids are constituents of proteins in living organisms?

Solution: In living organisms, 20 standard amino acids are present which link to form proteins.

Question 6. What are aromatic amino acids? Name some examples.

Solution: Aromatic amino acids have a benzene ring as their R group. Examples include tryptophan, tyrosine and phenylalanine. 

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