HO - 02.4 Proteins

BIOL 107: The Molecules of Life - Proteins

Overview of Proteins

  • Proteins are fundamental biological macromolecules essential to all forms of life.

  • Key structural components include:

    • Amino acids (monomers)

    • Polypeptides (polymers of amino acids)

    • Proteins (folded polypeptides)

  • Proteins function as:

    • Molecular machines of life

Amino Acids

  • All amino acids share a common structure:

    • Central alpha-carbon (Cα)

    • Carboxyl group (−COOH)

    • Amino group (−NH2)

    • Side-chain (R group) that varies between different amino acids.

  • Classification of amino acids based on their side chains:

    1. Non-polar (Hydrophobic)

    2. Polar (Hydrophilic)

    3. Negatively charged (Acidic)

    4. Positively charged (Basic)

  • Total of 20 plus 1 proteinogenic amino acids that are the building blocks of proteins.

Peptide Bonds

  • Amino acids link together via peptide bonds to form polypeptides:

    • Formed through dehydration synthesis (condensation reaction).

    • Dipeptide formed when two amino acids join.

  • The structure of polypeptides includes:

    • N-terminus (amino group) and C-terminus (carboxyl group).

    • Peptide backbone formed as amino acids are linked.

Polypeptides and Protein Structure

  • Average Human Protein Properties:

    • Contains approximately 450 amino acid residues.

    • Weighs around 50 kDa (50,000 daltons).

  • Chains of amino acids that are not folded do not constitute a fully functional protein. Folding is critical for function.

    • Example: Polypeptide structures can adopt various shapes based on energy considerations.

  • Levels of protein structure include:

    • Primary structure: Sequence of amino acids.

    • Secondary structure: Local folding patterns, including alpha helices and beta sheets, stabilized by hydrogen bonds.

    • Tertiary structure: Overall folding due to interactions between side chains (R groups).

    • Quaternary structure: Combination of multiple polypeptide subunits.

  • Examples of proteins with different quaternary structures (e.g., hemoglobin has different structural variations in fetal and adult forms).

Secondary Structure Details

  • Common Structures:

    • Alpha helix: Formed by hydrogen bonds between amino groups and carboxyl groups in the backbone.

    • Beta sheets: Formed in antiparallel or parallel configurations, also stabilized by hydrogen bonds.

    • Additional structures include coils and loops (hairpins).

Tertiary Structure Details

  • Folding Patterns: Involves interactions among R groups, determining the three-dimensional shape.

  • This structure is crucial for the protein's functionality due to the spatial arrangement of its active sites.

Functional Roles of Proteins

  • General Classification and Examples:

    • Enzymes: Catalysts for biochemical reactions (e.g., amylase, alcohol dehydrogenase).

    • Transport proteins: Facilitate the movement of molecules across membranes (e.g., sodium-potassium pump, hemoglobin).

    • Structural proteins: Provide support and shape to cells (e.g., actin, tubulin, keratin).

    • Hormonal proteins: Coordinate bodily functions (e.g., insulin).

    • Defense proteins: Protect against pathogens (e.g., immunoglobulins).

    • Motor proteins: Involved in movement (e.g., myosin, dynein).

    • Storage proteins: Supply nutrients during development (e.g., albumin).

Mixed Macromolecules: Peptidoglycans

  • Composition: Peptidoglycans consist of peptides and sugars.

  • Example: Serve as the structural component of bacterial cell walls, forming a mesh-like structure crucial for cell integrity.