Revision: Biomolecules >> Proteins Chemistry HSC Science (General) 12th Standard Board Exam Maharashtra State Board

Amino acids that cannot be synthesised in the human body and must be obtained through diet are known as essential amino acids.

An ∝-amino acid molecule contains both acidic carboxyl (-COOH) group as well as basic amino (-NH₂) group. Proton transfer from acidic group to basic group of amino acid forms a salt, which is a dipolar ion called zwitter ion.

Chemically, peptide linkage is an amide formed between the –COOH group and –NH₂ group. The reaction between two molecules of similar or different amino acids proceeds through the combination of the amino group of one molecule with the carboxyl group of the other. This results in the elimination of a water molecule and the formation of a peptide bond –CO–NH–. The product of the reaction is called a dipeptide because it is made up of two amino acids.

For example, when the carboxyl group of glycine combines with the amino group of alanine, we get a dipeptide, glycylalanine.

• Proteins are made up of α-amino acids linked together in a chain.
• A peptide bond (–CO–NH–) is formed between the carboxyl group of one amino acid and the amino group of another with the removal of water (condensation).
• Two amino acids form a dipeptide, three form a tripeptide, and many form polypeptides.
• Proteins are long polypeptide chains containing more than 100 amino acid residues.
• The ends of a protein chain are different:
  The N-terminal has a free amino group, and the C-terminal has a free carboxyl group.
• A peptide bond is similar to a secondary amide linkage in organic chemistry.

• The primary structure of proteins shows the sequence of amino acids in a polypeptide chain.
• Secondary structure is formed by hydrogen bonding and mainly includes α-helix and β-pleated sheet.
• In an α-helix, the chain coils into a right-handed spiral stabilised by hydrogen bonds.
• In a β-pleated sheet, chains are stretched and arranged side by side, held by intermolecular hydrogen bonds.
• Tertiary structure is the overall 3D folding of the chain due to interactions like hydrogen bonds, disulfide bonds, and van der Waals forces, while quaternary structure is the arrangement of multiple polypeptide chains.

1. Enzymes are biological (protein) catalysts that increase the rate of biochemical reactions without being consumed.

2. Most enzymes are globular proteins and show high specificity due to their unique 3D structure and active site.

3. Enzymes bind with substrate to form enzyme–substrate complex, then release product and remain unchanged.

4. Enzyme activity is affected by temperature and pH, and reactions are usually reversible.

5. Types of enzymes:

• Simple enzymes (only protein)
• Conjugated enzymes (protein + cofactor)

6. Examples of enzymes and their functions:

• Amylase: Starch → glucose
• Maltase: Maltose → glucose
• Lactase: Lactose → glucose + galactose
• Invertase: Sucrose → glucose + fructose
• Pepsin: Proteins → amino acids