CH 20 Lecture: Proteins and Their Functions pt.1 (4/29)

Overview of Proteins

  • Proteins can be classified into two categories: Fibrous and Globular.

Important Concepts

  • Fibrous Proteins: Insoluble in water, structural functions.

    • Characteristic: Nonpolar R groups on the outside. Examples: Alpha keratin, collagen.

  • Globular Proteins: Soluble in water, functional roles such as enzymes and transport proteins.

    • Characteristic: Polar or charged R groups on the outside. Examples: Myoglobin, hemoglobin, immunoglobulins.

Classification of Proteins

  1. Alpha Keratin:

    • Type: Fibrous.

    • Structure: Mainly composed of alpha helices.

    • Location: Found in hair, nails, feathers, scales.

    • Characteristics: Contains many disulfide bonds for rigidity. Nails have more disulfide bonds than hair.

  2. Beta Keratin:

    • Type: Fibrous.

    • Structure: Composed of beta-pleated sheets.

    • Location: Found in silk.

    • Characteristics: Stronger than iron wire of the same diameter; derived from silkworms and spiders.

  3. Collagen:

    • Type: Fibrous.

    • Structure: Three-stranded helix, not a true alpha helix due to proline content.

    • Location: Present in skin, blood vessels, and teeth.

    • Characteristics: Most abundant protein, involves modified amino acids (hydroxyproline, hydroxylysine). The formation of cross-links leads to rigidity as we age. Requires vitamin C for synthesis; deficiency leads to scurvy (e.g., teeth falling out).

  4. Myoglobin:

    • Type: Globular.

    • Structure: Primarily alpha helices, monocentric (one subunit).

    • Function: Stores oxygen in muscle tissues.

    • Characteristics: Contains one heme group; binds one molecule of oxygen.

  5. Hemoglobin:

    • Type: Globular.

    • Structure: Composed of four subunits (two alpha and two beta).

    • Function: Transports oxygen from lungs to tissues.

    • Characteristics: Each molecule binds four oxygen molecules; higher structural complexity (quaternary structure). Can bind carbon monoxide more readily than oxygen, which can lead to poisoning.

    • Fetal Hemoglobin: Contains gamma subunits for better oxygen binding during fetal development.

  6. Immunoglobulins (Antibodies):

    • Type: Globular, conjugated (sugar part).

    • Function: Part of immune response; bind antigens.

    • Structure: Four subunits (two heavy, two light chains) connected by disulfide bonds.

    • Characteristics: Each antibody can bind two antigens; variable region dictates specificity.

Denaturation of Proteins

  • Denaturation refers to the loss of secondary, tertiary, and quaternary structures while retaining primary structure (peptide bonds remain intact).

  • Factors affecting denaturation:

    • Temperature: Increases kinetic energy, affects weak interactions.

    • pH: Alters ionic bonds between acidic and basic amino acids.

    • Detergents: Disrupt hydrophobic interactions.

    • Heavy Metals: Disrupt disulfide bonds.

    • Organic Solvents (e.g., alcohol): Target hydrogen bonds.

    • Mechanical Action (e.g., violent whipping): Affects all weak interactions.

Summary of Protein Functions

  • Fibrous Proteins: Structural roles (e.g., Collagen in skin, Alpha/Beta Keratin in hair/nails)

  • Globular Proteins: Functional roles (e.g., transport in Hemoglobin, Myoglobin in muscle, Immunoglobulins in immune response)