Proteins

Proteins are large macromolecules formed from amino acids. They are essential for life, performing a wide range of functions from structural support to enzymatic catalysis.

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Amino Acids

Amino acids are the monomers (building blocks) of proteins. All amino acids share a common basic structure:

• An amine group (-NH₂)
• A carboxyl group (-COOH)
• A variable R group (side chain)
• A central carbon atom

Key Features

• There are 20 different amino acids used in proteins
• They differ only in their R groups
• The R group determines the properties of each amino acid
• All are α-amino acids (amino group is on carbon next to COOH)

Peptide Bond Formation

Amino acids join together through condensation reactions:

1. Dipeptide Formation

   • Two amino acids join together
   • Forms one peptide bond
   • Releases one water molecule

2. Polypeptide Formation

   • Many amino acids join together
   • Multiple peptide bonds form
   • One water molecule released per bond formed

Protein Structure

Proteins have four levels of structure, each contributing to their final function:

1. Primary Structure

• The sequence of amino acids in a polypeptide chain
• Determined by genes
• Forms the foundation for higher levels of structure

2. Secondary Structure

• Regular folding patterns of the polypeptide chain
• Stabilized by hydrogen bonds
• Two main types:
  • α-helix (spiral structure)
  • β-pleated sheet (zigzag structure)

3. Tertiary Structure

• Overall 3D shape of a single polypeptide
• Maintained by various bonds:
  • Hydrogen bonds between R groups
  • Ionic bonds between charged R groups
  • Disulfide bridges between cysteine residues
  • Hydrophobic interactions

4. Quaternary Structure

• Assembly of multiple polypeptide chains
• Examples include:
  • Haemoglobin (4 chains)
  • Collagen (3 chains)
  • Insulin (2 chains)

Protein Functions

Proteins have diverse roles in organisms:

1. Structural Roles
   • Collagen in connective tissue
   • Keratin in hair and nails
   • Cross-linked for strength
2. Metabolic Roles
   • Enzymes for catalyzing reactions
   • Spherical shape for active sites
3. Transport Roles
   • Haemoglobin for oxygen transport
   • Channel proteins in membranes
4. Immune Response
   • Antibodies
   • Made of light and heavy chains

Biochemical Test

Biuret Test

1. Add biuret reagent (copper sulfate in sodium hydroxide)
2. Positive result: Solution turns purple/mauve
3. Tests for peptide bonds

Learning Objectives

Test Your Knowledge

Key Takeaways

• Proteins are made from 20 different amino acids
• Four levels of structure determine protein function
• Multiple bond types stabilize protein structure
• Proteins have diverse roles in organisms
• Structure-function relationship is key to understanding proteins