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Chemistry > Biomolecule > Amino Acids
Biomolecule

Amino Acids

We all know that proteins are one of the main building blocks of life. But do you know what are the building blocks of proteins? The basic building blocks of proteins what we call Amino acids. These are absolutely essential for humans. Let us learn all about them.

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Amino Acids

There are some 20 amino acids in the proteins that we consume. These amino acids bond together to form a larger protein molecule. Amino acid being organic compound molecules can form various different links with each other due to the versatile nature of carbon. This enables the great diversity of proteins that can be found in nature. These are an essential nutrient in our diet because of the functions they perform.

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Structure of Amino Acids

amino acids(Source: Wikibooks)

There are actually thousands of amino acids occurring in nature. But only about 20 amino acids form a part of the proteins in the human body. These twenty acids will be our focus here. Although all these have varied structures, the basic structure of amino acid remains uniform.

  • All amino acids contain a carbon atom in the middle of the molecule, the alpha-carbon
  • This atom is surrounded by three chemical groups.
  • One is an amine group -NH2
  • The second one is a carboxyl group -OOOH
  • The third group is denoted by R. This is the variable radical group and is different for every amino acid. This R group makes the amino acid unique.

Classification of Amino Acids

Amino Acid can be classified based on their structure and the structure of their side chains i.e. the R chains. Now two basic subcategories are

1] Non-Polar Amino Acids

These are also known as Hydrophobic. The R group can be either of Alkyl groups (with an alkyl chain) or Aromatic groups. The acids falling in this group are stated below. Numbers one to seven are Alkyl and the last two are aromatic

  1. Glycine (H)
  2. Alanine (CH3)
  3. Valine  ( CH (CH3)2 )
  4. Methionine ( CH2CH2SCH3 )
  5. Leucine ( CH2CH(CH3)2 )
  6. Isoleucine ( -CH(CH3)CH2CH3 )
  7. Proline (special structure)
  8. Phenylalanine
  9. Tryptophan

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2] Polar Amino Acids

If the side chains of amino acid contain different polar groups like amines, alcohols or acids they are polar in nature. These are also known as Hydrophilic Acids. These are further divided into three further categories.

a) Acidic: If the side chain contains an extra element of carboxylic acid component these are acid-polar amino acids. They tend to donate their hydrogen atom. These are:

  1. Aspartic Acid ( CH2COOH)
  2. Glutamic Acid ( CH2CH2COOH )

b) Basic: These have an extra nitrogen group that tend to attract a hydrogen atom. The three basic polar amino acids are

  1. Histidine
  2. Lysine ( CH2(CH2)2NH2 )
  3. Arginine

c) Neutral: These are neither acidic nor basic. They have an equal number of amino and carboxyl groups. Also, they have at least one hydrogen component connected to electronegative atoms. Some of these neutral acids are

  1. Serine ( CH2OH )
  2. Threonine ( CH(OH)CH3 )
  3. Asparagine ( CH2OHNH2 )
  4. Glutamine ( CH2CH2CONH2 )
  5. Cysteine ( CH2SH )
  6. Tyrosine

Amino acid can also be classified on the basis of their need to the human body and their availability in the human body

1] Essential Amino Acids

These are the acids that cannot be synthesized in our bodies. We must rely on food sources to obtain these amino acids. They are

  • Leucine
  • Isoleucine
  • Lysine
  • Theorine
  • Methionine
  • Phenylalanine
  • Valine
  • Tryptophan
  • Histidine (conditionally essential)

2] Non-Essential

These acids are synthesized in our bodies itself and we need not rely on outside sources for them. They are either produced in our bodies or obtained from protein breakdowns.

Properties of Amino Acids

Now that we have seen the structure and types of amino acids. Now from this information, we can arrive at the properties of amino acids.

  • Each amino acid has both an acidic and basic group as you can see from its structure. This is the reason they behave like salts.
  • Any amino acid in the dry state is in crystalline form. They exist as a dipolar ion. The COOH group exists as an anion. And the NH2 group exists as a cation. This dipolar ion has a special name “Zwitter ions’. 
  • In aqueous solution, alpha amino acids exist in equilibrium between a cationic form, an anionic form and dipolar ion.
  • The Isoelectric point is the pH point at which the concentration of zwitter ions is the highest ad the concentration of cationic and anionic form is equal. This point is definite for every α-amino acid.
  • They are generally water soluble and also have high melting points.

Solved Example for You

Q: Proteins are condensation polymers of

  1. α amino acid
  2. β amino acid
  3. α hydroxyl acids
  4. none of the above

Sol: The correct option is “A”. α amino acid is the monomeric units of proteins.

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