Mar 27

Lecture Overview

  • Copyright Notice
      - This lecture presentation and the accompanying PowerPoint slides are exclusive copyrights of Professor Omri.
      - Intended for students enrolled in Biochemistry I (CHMI-2227 E) at Laurentian University in Winter term 2026.
      - Unauthorized or commercial use, including uploading to external sites, is prohibited.

Immunoglobulins (Antibodies)

  • Definition
      - Immunoglobulins, also known as antibodies, are glycoproteins essential for the body’s defense against invading organisms and foreign compounds, such as viruses and bacteria.

  • Synthesis
      - Synthesized by lymphocytes, specifically β-lymphocytes.
      - Antibodies recognize and bind to foreign substances called antigens (Ag).
      - Antigens may include proteins, nucleic acids, and polysaccharides.

  • Binding Function
      - The binding of antibodies to antigens aggregates these foreign substances, marking them for destruction by macrophages.

Immune Responses

  • Primary Immune Response
      - Occurs upon first exposure to an antigen.
      - Takes several days for β-lymphocytes to produce antibodies, yielding a slower response and fewer antibodies.

  • Secondary Immune Response
      - A faster and stronger response occurs upon subsequent exposure to the same antigen.
      - Memory B cells facilitate this rapid defense.
      - This principle is fundamental to vaccinations, where initial doses prime the immune system and booster shots enhance the response.

Classes of Immunoglobulins

  • Five Major Classes
      1. Immunoglobulin M (IgM)
      2. Immunoglobulin A (IgA)
      3. Immunoglobulin G (IgG)
      4. Immunoglobulin E (IgE)
      5. Immunoglobulin D (IgD)

  • Classification Based on Heavy Chains
      - The constant region of the heavy chains determines the class of an antibody.

Structure of Antibodies

  • IgG Structure
      - Most abundant immunoglobulin in the bloodstream, characterized by a Y-shaped structure.
      - Composed of:
        - Heavy Polypeptide Chains
          - 2 identical chains (blue).
        - Light Polypeptide Chains
          - 2 identical chains (red).
      - Chains are connected by disulfide bonds (yellow).

  • Domains of Chains
      - Light chains consist of 2 domains (red).
      - Heavy chains consist of 4 domains (blue).
      - N-terminal domains are variable and interact with antigens, with sequence variations defining antibody specificity.

  • Fragment Production via Digestion
      - Papain digestion produces:
        - 2 identical Fab fragments (antigen-binding capacity).
        - 1 Fc fragment (crystallizable fragment).
      - Pepsin digestion yields F(ab)2 fragment.

Enzymes

  • Definition and Function
      - Enzymes are globular proteins that catalyze biochemical transformations.
      - Derived from the Greek word meaning "in yeast."
      - Enzymes act as catalysts, altering reaction rates without undergoing permanent changes themselves.
      - A catalyst accelerates the attainment of reaction equilibrium.

  • Substrates
      - Specific reactants that enzymes act upon, highly specific interactions.

  • Catalytic Efficiency
      - Enzymes can enhance reaction rates up to 10^16 times over uncatalyzed reactions.
        - Example: Urease.
          - Catalyzed rate: 3imes104/sec3 imes 10^{4}/sec
          - Uncatalyzed rate: 3imes10−10/sec3 imes 10^{-10}/sec
          - Ratio: 1imes10141 imes 10^{14}

Activation Energy and Specificity

  • Energy of Activation
      - Enzymes lower the energy required to reach the activated complex, leading to reactions requiring less energy.

  • Enzyme Specificity
      - Some enzymes are highly specific and catalyze reactions for only one stereoisomer.
        - Example: Tryptophan synthetase specific for L-Serine.
        - Example: Asparaginase converts Asparagine to Aspartic acid but does not work on Glutamic acid.

  • Models of Enzyme-Substrate Interaction
      - Lock and Key Model
        - Active site has a specific shape that matches the substrate like a key in a lock.
        - Rigid active site does not change shape upon substrate binding.
      - Induced Fit Model
        - Active site is flexible, undergoing a conformational change upon substrate binding for a tighter fit.
        - Both enzyme and substrate can change shape to achieve optimal interaction.

Active Site of Enzymes

  • Significance
      - Active site frequently located in a cleft, where substrate binds and catalysis occurs.
      - Contains reactive groups necessary for the reaction.

  • Induced Fit Implications
      - The enzyme is flexible and can accommodate various substrates, allowing multiple substrates to bind.

Units of Enzyme Activity

  • Measurement Units
      - Katal (Kat)
        - Defined as the amount of enzyme converting 1 mole of reactant to product in 1 second under standard conditions.
      - International Unit (IU)
        - Amount of enzyme producing 1 µmol of product per minute.
        - Conversion: 1extIU=1.67imes10−8extKat1 ext{ IU} = 1.67 imes 10^{-8} ext{ Kat}

Nomenclature and Classification of Enzymes

  • Naming Conventions
      - Enzymes can have recommended names (based on substrates or reactions) and systemic names assigned by the Enzyme Commission (EC).

  • Six Classes of Enzymes
      1. Oxidoreductases: catalyze oxidation-reduction reactions.
         - Example: Lactate dehydrogenase.
      2. Transferases: catalyze the transfer of functional groups.
         - Example: Alanine transaminase.
      3. Hydrolases: catalyze cleavage by the addition of water.
         - Example: Pyrophosphatase.
      4. Lyases: catalyze cleavage of C-C, C-S, and certain C-N bonds.
         - Example: Pyruvate decarboxylase.
      5. Isomerases: catalyze racemization of isomers.
         - Example: Alanine racemase.
      6. Ligases: catalyze bond formation by joining molecules with hydrolysis of ATP.
         - Example: Glutamine synthetase.

  • Subclassification
      - Each class is classified into subclasses based on the nature of the catalyzed reaction, represented by a four-number EC code.

Enzyme Sensitivity and Composition

  • Environmental Sensitivity
      - Enzymes are sensitive to temperature and pH.
      - Each enzyme has an optimum temperature and pH for activity.

  • Enzyme Composition
      - Simple Enzymes: protein-only.
      - Holoenzymes: composed of protein (apoenzyme) plus non-protein components (cofactor).
        - Co-factors can be metal ions or organic molecules (coenzymes).
        - Vitamins may serve as coenzyme precursors.

  • Metal Ions
      - Certain enzymes require metal ions for activity:
        - Metal-activated enzymes: require stimulation from metal ions.
        - Metalloenzymes: contain tightly-bound metal ions at the active site.

Enzyme Kinetics

  • Mechanism of Enzyme Action
      - The reaction involves the conversion of substrate (S) to product (P) via an enzyme (E):
        E+S<br>ightleftharpoonsES<br>ightarrowE+PE + S <br>ightleftharpoons ES <br>ightarrow E + P
      - The enzyme and substrate concentrations influence the formation of the enzyme-substrate complex (ES) and product formation.

  • Reaction Rates
      - Rate increases with substrate concentration until saturation, reaching maximal velocity (Vmax).
      - First Order Kinetics: At low substrate concentration, velocity is proportional to substrate concentration.
      - Zero Order Kinetics: At high substrate concentration, enzyme saturation results in minimal changes in velocity.

  • Michaelis-Menten Equation
      - Describes velocity dependence on substrate concentration:
        Vo=racVmax[S]Km+[S]Vo=rac{V_{max}[S]}{K_{m}+[S]}
      - Parameters
        - Km: Substrate concentration at which reaction velocity is half of Vmax, is a constant indicating binding strength.
          - Small Km indicates tight binding.
          - High Km indicates weak binding.
        - Vmax: The theoretical maximal rate of the reaction.

  • Lineweaver-Burk Plot
      - A double reciprocal plot can simplify the determination of Km and Vmax:
        - rac1V=racKmVmaximesrac1[S]+rac1Vmaxrac{1}{V} = rac{K_m}{V_{max}} imes rac{1}{[S]} + rac{1}{V_{max}}
      - Provides a straight line with slope of racKmVmaxrac{K_m}{V_{max}} and intercepts of −rac1Km- rac{1}{K_m} and rac1Vmaxrac{1}{V_{max}}.

Enzyme Deficiencies and Defects

  • Phenylketonuria (PKU)
      - Defective Process: Impaired metabolism of phenylalanine due to deficient phenylalanine hydroxylase.
      - Symptoms: Intellectual disability, developmental delays, seizures, behavioral problems.

  • Tay-Sachs Disease
      - Defective Process: Accumulation of gangliosides due to deficient hexosaminidase A.
      - Symptoms: Neurological deterioration, muscle weakness, loss of motor skills, blindness, death usually by age 4.

  • Albinism
      - Defective Process: Impaired melanin synthesis from tyrosine due to deficient tyrosinase.
      - Symptoms: Lack of pigmentation, sensitivity to sunlight, vision problems.

  • Argininemia
      - Defective Process: Impaired urea synthesis due to deficient arginase.
      - Symptoms: Mental retardation, developmental delays, spasticity, seizures, failure to thrive.