Proteins

Assessed Definition: Amino acids are the monomers from which proteins are made.

There are 20 different amino acids, all sharing:

• An amino group (NH₂).

• A carboxyl group (COOH).

• A unique R group (side chain) that determines their properties, including:

  • Positively charged (basic).

  • Negatively charged (acidic).

  • Hydrophobic (non-polar).

  • Polar.

  • Sulfur-containing (important for disulfide bridges).

Proteins differ because of variations in amino acid sequences, leading to diverse structures and functions.

Assessed diagram:

The general structure of an amino acid.

Protein formation

Amino acids join through a condensation reaction:

• The amine group (NH₂) of one amino acid reacts with the carboxyl group (COOH) of another.

• A peptide bond forms, and a water molecule is released.

Types of peptide structures:

• • Dipeptide = two amino acids linked by a peptide bond.

  • Polypeptide = many amino acids linked together.

  • Functional proteins often consist of one or more polypeptide chains.

  • Some proteins, like hemoglobin, contain non-polypeptide groups (e.g., heme groups in hemoglobin).

Formation of a peptide bond.

Protein structure

The structure of proteins can be described at four levels:

1. Primary Structure

• The specific sequence of amino acids in a polypeptide chain.

• Peptide bonds hold the sequence together.

• Determined by DNA (the genetic code).

• Influences all other levels of protein structure.

Primary structure of a protein.

2. Secondary Structure

• Folding of the polypeptide chain due to hydrogen bonding between amino acids (not R groups).

• Two main types:

  • Alpha-helix (α-helix) – a coiled structure.

  • Beta-pleated sheet (β-sheet) – a folded, sheet-like structure.

Secondary structure of a protein.

3. Tertiary Structure

• The overall three-dimensional shape of the protein.

• Stabilised by different interactions between R groups:

  • Hydrogen bonds (weak but numerous).

  • Ionic bonds (between oppositely charged R groups).

  • Disulfide bridges (strong covalent bonds between sulfur-containing R groups).

• The shape of a protein is crucial for its function (e.g., enzymes must have a specific active site shape).

Tertiary structure of a protein.

4. Quaternary Structure

• Formed when more than one polypeptide chain joins together.

• Example: Hemoglobin, which consists of four polypeptide chains.

Protein function

There are two main types of proteins:

1. Fibrous Proteins

   • Long, straight chains of amino acids.

   • Have structural roles in cells.

   • Examples:

     • Collagen (in connective tissues).

     • Keratin (in hair and nails).

     • Actin fibers (in muscle contraction).

2. Globular Proteins

   • Compact, spherical shape due to folding into a complex tertiary structure.

   • Often have binding sites and can change shape when molecules bind.

   • Examples:

     • Enzymes (e.g., amylase, catalase).

     • Antibodies (for immune response).

     • Hormones (e.g., insulin).

     • Receptors (on cell membranes).

Biochemical test

Purpose: To detect the presence of peptide bonds, which indicate the presence of proteins.

Procedure:

1. Add biuret reagent (a mix of sodium hydroxide and copper sulfate) to the sample.

2. Keep at room temperature.

3. Positive result = colour change from blue to violet.

Definitions