Introduction:

Proteins are vital biological macromolecules essential for life. In Class 12 Chemistry (Chapter: Biomolecules), understanding proteins includes their structure, types, and functions. These notes help you grasp the key concepts easily for board exams and competitive preparation like NEET and JEE.

What Are Proteins?

Proteins are large, complex molecules made up of amino acids joined by peptide bonds. They play critical roles in almost all biological processes. Chemically, proteins are polymers of α-amino acids.

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Amino Acids – Building Blocks of Proteins:

Structure of α-Amino Acids:

Each amino acid has a central α-carbon, an amino group (–NH₂), a carboxylic group (–COOH), a hydrogen atom, and a variable R group. General formula: NH₂–CH(R)–COOH

Classification:

- Essential Amino Acids (e.g., Valine, Leucine)

- Non-Essential Amino Acids (e.g., Glycine, Alanine)

Formation of Peptides and Proteins:

Peptide Bond: Formed when –COOH of one amino acid reacts with –NH₂ of another, releasing water.

Types:

- Dipeptide: 2 amino acids

- Tripeptide: 3 amino acids

- Polypeptide: Many amino acids

- Protein: More than ~50 amino acids in a specific 3D structure

Structure of Proteins:

1. Primary Structure: Linear sequence of amino acids.

2. Secondary Structure: α-Helix and β-Pleated Sheet (hydrogen bonds).

3. Tertiary Structure: 3D shape formed by R-group interactions.

4. Quaternary Structure: Multiple polypeptide chains (e.g., Hemoglobin).

Classification of Proteins:

By Composition:

- Simple Proteins: Only amino acids

- Conjugated Proteins: With non-protein parts

By Function:

- Structural: Keratin, Collagen

- Enzymatic: Pepsin, Amylase

- Transport: Hemoglobin

- Hormonal: Insulin

Denaturation of Proteins:

Loss of structure and biological activity due to temperature, pH, or chemicals.

Example: Cooking an egg – Albumin protein coagulates.

Functions of Proteins:

- Structural: Build cells and tissues

- Enzymatic: Catalyze reactions

- Transport: Carry substances

- Regulatory: Hormones

- Protective: Antibodies

Protein Tests (Class 12 Practical Relevance):

Biuret Test: Purple color indicates proteins.

Xanthoproteic Test: Yellow color shows aromatic amino acids.

Important Protein Examples for Board Exams:

Hemoglobin – Oxygen transport

Insulin – Blood sugar regulation

Albumin – Osmotic pressure control

Pepsin – Protein digestion

Conclusion:

Understanding proteins in Class 12 Chemistry is crucial for exams and future studies. Focus on structure, classification, and functions. Use diagrams and tables for clarity.