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Chapter VI - Proteins

Lesson Objectives

  • Defining Amino Acids

  • Classifications of Proteins

    • Based on function

    • Based on structure

  • Relating Protein Activities

    • Real-life scenarios involving proteins

Introduction to Proteins

  • Etymology: The term "protein" derives from the Greek word "proteios" meaning "of first importance".

  • Definition: Biomolecules that contain numerous amide bonds, formed by linking amino acids together.

Functions of Proteins

  • Proteins serve various roles, including:

    • Structural: Provide support and structure to tissues and cells (e.g. collagen, keratin).

    • Regulatory: Involved in the regulation of biological processes.

    • Contractile: Enable muscle movements (e.g. myosin, actin).

    • Protective: Involved in defense mechanisms (e.g. antibodies, clotting factors).

    • Transport: Carry substances (e.g. hemoglobin for oxygen transport).

    • Storage: Serve as reserves of nutrients (e.g. casein, ovalbumin).

    • Toxins and Enzymes: Serve as active agents in various metabolic processes.

Structural Proteins

  • Provide support and structure to tissue and cells.

  • Key examples include:

    • Collagen: Major protein in connective tissues.

    • Keratin: Found in skin, hair, nails.

Enzymatic Catalysis

  • Definition: Proteins called enzymes catalyze nearly all reactions in living organisms.

  • Importance: Without enzymes, reactions would occur too slowly to sustain life.

Contractile Proteins

  • Function: Enable muscle contraction and expansion.

  • Key proteins involved: Myosin and Actin.

Storage Proteins

  • Roles: Biological reserves of amino acids and metal ions.

  • Examples:

    • Casein: Found in milk, provides nutrients to newborns.

    • Ovalbumin: Found in eggs, serves a similar function.

    • Ferritin: In the liver, stores iron.

Transport Proteins

  • Function: Carry vital substances in the blood.

  • Examples:

    • Hemoglobin: Carries oxygen from lungs to cells, and carbon dioxide from cells back to lungs.

Defense Proteins

  • Function: Protect body from foreign proteins (antigens).

  • Examples:

    • Antibodies: Produced by the immune system.

    • Prothrombin and Fibrinogen: Involved in blood clotting.

Proteins in the Human Body

  • Immune System Proteins:

    • Antibodies: Fight infections.

    • Complement System: 20 proteins activated during infection.

  • Muscle Proteins:

    • Actin and Myosin: Facilitate muscle movement.

    • Myoglobin: Stores oxygen in muscle tissues.

  • Blood Proteins:

    • Hemoglobin: Transports oxygen.

    • Fibrinogen: Clots blood.

    • Albumin: Maintains blood fluid levels.

  • Structural Proteins:

    • Cytoskeleton: Maintains cell shape.

    • Collagen and Keratin: Provide tensile strength and structure.

  • Enzymatic Proteins:

    • Digestive enzymes assist in food breakdown.

  • Membrane Proteins:

    • Function as channels, receptors, or enzymes for substance movement through membranes.

Amino Acids

  • Definition: Organic compounds with an amino group (NH2) and a carboxyl group (COOH).

    • Building blocks of proteins.

Classification of Amino Acids

  1. Essential Amino Acids: Must be obtained through diet.

    • Examples: Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine.

  2. Non-Essential Amino Acids: Synthesized by the body.

    • Examples: Alanine, Asparagine, Cysteine, Glutamic Acid, Glycine, Proline.

Properties of Amino Acids

  • Physical State: Colorless, crystalline solids.

  • Melting Point: Generally high (>200˚C).

  • Solubility: Soluble in polar solvents, insoluble in nonpolar solvents.

  • Taste: Varies from tasteless to sweet or bitter based on the amino acid.

Structure of Amino Acids

  • All amino acids have the following features:

    • A carboxyl group (COOH).

    • An amino group (NH2).

    • A side chain (R group) that distinguishes each amino acid.

Classification of Amino Acids

  • Nonpolar, Aliphatic Amino Acids: R groups are nonpolar/hydrophobic.

  • Polar, Uncharged Amino Acids: More soluble in water.

  • Aromatic Amino Acids: Contain aromatic side chains.

  • Acidic Amino Acids: Negatively charged at physiological pH (e.g., aspartic acid, glutamic acid).

  • Basic Amino Acids: Positively charged at physiological pH (e.g., lysine, arginine, histidine).

Acid-Base Properties of Amino Acids

  • Amphoteric Nature: Can act as either an acid or a base.

  • Protonation/Deprotonation: Depends on pH of the surrounding environment.

Zwitterion

  • An amino acid that possesses both positive and negative charges, resulting in a net charge of zero.

Isoelectric Point (pI)

  • The pH at which an amino acid predominantly exists in its neutral form.

  • Ranges of pI values:

    • Basic amino acids: 7.8 to 10.8

    • Neutral amino acids: 4.8 to 6.3

    • Acidic amino acids: 2.8 to 3.3

Peptides

  • Definition: Formed when amino acids join via amide bonds.

    • Dipeptide: Two amino acids.

    • Tripeptide: Three amino acids.

  • Peptide Bonds: Covalent bonds formed between the carboxyl group of one amino acid and the amino group of another.

Levels of Protein Structure

Primary Structure

  • Specific sequence of amino acids in a polypeptide chain.

  • The sequence is read from the amino-terminal (N) to the carboxyl-terminal (C).

Secondary Structure

  • Localized folding of the polypeptide chain.

  • Common forms: α-helix and β-pleated sheets.

Tertiary Structure

  • 3D structure resulting from side chain interactions within a single polypeptide chain.

Quaternary Structure

  • Formed by the interactions of multiple polypeptide chains.

  • Example: Hemoglobin consists of two alpha and two beta subunits.

Glycoproteins

  • Composed of proteins and carbohydrate chains.

  • Function in various physiological activities, including immunity.

Lipoproteins

  • Carry cholesterol and triglycerides to cells.

  • Formed of fat droplets surrounded by phospholipid layers.

    • LDL: Low-density lipoprotein (bad cholesterol).

    • HDL: High-density lipoprotein (good cholesterol).