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10 Biomolecules

10.1 Carbohydrates 10.2 Proteins 10.3 Enzymes 10.4 Vitamins 10.5 Nucleic Acids 10.6 Hormones

Proteins

NCERT Reference: Chapter 10 – Biomolecules – Page 272–275

Quick Notes

  • Proteins are polymers of α-amino acids.
  • Amino acids contain both –NH2 (basic) and –COOH (acidic) groups.
  • Protein structure is modelled as primary, secondary, tertiary, quaternary.
  • Two types of protein: fibrous (structural) and globular (functional).
  • Denaturation alters protein structure but not primary sequence.

Full Notes

Proteins are large biomolecules made up of amino acids, joined by peptide bonds. They perform a wide range of structural and functional roles in living organisms and are fundamental to life processes.

Amino Acids

Amino acids are organic compounds containing an amino (–NH2) group and a carboxyl (–COOH) group attached to the same carbon (α-carbon).

Chemistry Class 12 Biomolecules diagram showing the general structure of an α-amino acid with NH2, COOH, H and side chain R attached to the α-carbon.

where R = side chain

The amine (-NH₂) group acts as a base (accepts a H+ ion) and the carboxylic acid (-COOH) group acts as an acid (donates a H+ ion).

Chemistry Class 12 Biomolecules illustration highlighting acidic carboxyl and basic amine groups in amino acids.

Zwitterions: Isoelectric Point

At a specific pH (isoelectric point), amino acids exist as zwitterions.

IB Chemistry Class 12 Biomolecules diagram showing amino acid proton transfer to form the zwitterion NH3+ and COO− at the isoelectric point.

The zwitterion contains both a positive (–NH3+) and a negative (–COO) charge but is overall neutral.

They form at a specific pH when the amine (NH2) group accepts a H+ ion and the carboxylic acid (COOH) group loses a H+ ion.

Classification of Amino Acids

Amino acids can be classified based on the number of amino and carboxyl groups:

Also classified based on origin:

There are 20 standard amino acids commonly found in proteins.

Structure of Proteins

Proteins are naturally occurring polymers of amino acids linked by peptide bonds (–CONH–).

Chemistry Class 12 Biomolecules schematic of a protein polymer made from amino acid residues.

Peptide bonds form by condensation reactions between amino acids, releasing water:

Chemistry Class 12 Biomolecules diagram of dipeptide formation via condensation and peptide bond formation.

The process can keep happening between more amino acids, forming a polypeptide chain (polymer).

Chemistry Class 12 Biomolecules illustration of peptide polymerisation to form polypeptides and proteins.

Peptide bonds can be broken apart in hydrolysis reactions. The hydrolysis of proteins (with acid or alkali) releases the amino acids that made up the protein.

Chemistry Class 12 Biomolecules schematic of peptide bond hydrolysis regenerating amino acids.
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The conditions used for hydrolysis determine the form of the amino acids released. If acidic conditions are used, then the amino acids may exist as positively charged ions with the NH2 group accepting a H+ ion to form NH3+. Equally, if alkaline conditions are used, then the carboxylic acid groups may be forced to lose H+ ions and exist as carboxylate ions (COO).

Types of Protein

Proteins can be fibrous or globular.

Fibrous Proteins

Globular Proteins

Levels of Protein Structure

Proteins can have very complicated shapes that are difficult to analyse.

To help, we describe the structure of proteins in terms of a Primary, Secondary and Tertiary structure. (Proteins made up of more than one polymer chain also have a quaternary structure).

Primary Structure

NCERT Chemistry Class 12 Biomolecules depiction of a polypeptide primary structure as an amino acid sequence.

Secondary Structure

Folding of the polypeptide due to hydrogen bonding between C=O and N–H bonds from different amino acids in the polymer chain.

NCERT Chemistry Class 12 Secondary Structure of protein formed by hydrogen bonding between N-H and C=O groups.

The hydrogen bonding can form two shapes or patterns in the protein:

NCERT Chemistry Class 12 Biomolecules diagrams of α-helix and β-pleated sheet secondary structures stabilised by hydrogen bonds.

Tertiary Structure

3D folding of the protein due to interactions between R groups.

NCERT Chemistry Class 12 Biomolecules representation of tertiary protein folding into a compact 3D shape by R groups interacting together.

The interactions between different R groups causes the protein to ‘fold’ into complicated shapes, with the final shape between determined by the locations (and type) of amino acids in the chain.

NCERT Chemistry Class 12 Biomolecules diagram highlighting hydrogen bonds, disulfide bridges, ionic bonds and hydrophobic interactions in tertiary structure.

Quaternary Structure

Association of two or more polypeptide chains.

Example: Haemoglobin (4 subunits)

NCERT Chemistry Class 12 Biomolecules illustration of quaternary structure with multiple polypeptide subunits as in haemoglobin.

Denaturation of Proteins

Denaturation is the loss of the biological activity of a protein due to disruption of its conformation and shape.

Summary