Enzymes-04.12.2024

Enzymes

  • Definition: Usually proteins acting as catalysts in specific biochemical reactions.

  • Characteristics:

    • Catalysts: Increase reaction rates without being consumed.

    • Most are globular proteins; some are made of RNA (ribozymes).

    • Thousands present in human cells; specific enzymes required for each reaction.

Advantages of Biocatalysis over Inorganic Catalysts

  • Greater Reaction Specificity: Avoidance of side products.

  • Milder Reaction Conditions: Ideal for cellular conditions (pH ~ 7, 37°C).

  • Higher Reaction Rates: Operate within biologically relevant times.

  • Capacity for Regulation: Control biological pathways.

  • Example: Carbonic Anhydrase enhances reaction by a factor of 107, facilitating CO₂ transfer.

Acetazolamide

  • Description: First-generation carbonic anhydrase inhibitor; decreases ocular fluid and intraocular pressure.

  • Uses: Treatment of glaucoma, epilepsy, altitude sickness, periodic paralysis, and idiopathic intracranial hypertension.

  • Characteristics:

    • Enzymes named with suffix "ase"; reactants are substrates.

    • Enzymes are catalysts that control rates of biochemical reactions.

    • Specific enzymes promote desired pathways among multiple potential ones.

Major Classes of Enzymes

  • EC Number: Classifies enzymes based on function.

    1. Oxidoreductases: Catalyze oxidation-reduction reactions.

    2. Transferases: Transfer chemical groups.

    3. Hydrolases: Catalyze hydrolytic cleavage.

    4. Lyases: Non-hydrolytic cleavage of bonds or addition across double bonds.

    5. Isomerases: Change geometrical arrangement of molecules.

    6. Ligases: Join two molecules with hydrolysis of a high-energy bond.

Enzyme Structure

  • SIMPLE ENZYMES: Composed only of proteins.

  • CONJUGATED ENZYMES: Include an apoenzyme (protein part) and coenzymes (non-protein parts).

Coenzymes and Cofactors

  • Coenzymes add reactive functional groups to enzymes; generally small organic molecules or metal ions like Zn²⁺, Mg²⁺.

  • Enzymes bind specific substrates; named based on reactions catalyzed, suffix "-ase". Examples: lactase, amylase.

Enzyme-Substrate Complex

  • Apoenzyme + Cofactor: Forms a functional conjugated enzyme.

  • Holoenzyme: Biochemically active form of the enzyme.

Enzyme Specificity

  • Absolute Specificity: Catalyzes reaction for one specific substrate (e.g., catalase for H₂O₂).

  • Group Specificity: Acts on substrates with specific functional groups.

  • Linkage Specificity: Acts on particular chemical bonds regardless of molecular structure.

  • Stereochemical Specificity: Distinguish between stereoisomers (e.g., L-amino acids).

Chymotrypsin

  • A protease that cleaves dietary proteins at specific peptide bonds adjacent to aromatic amino acids during digestion.

Enzyme-Substrate Binding

  • Binding Energy: Drives formation of the enzyme-substrate complex, lowering activation energy.

  • Active Site: The region of the enzyme where substrate fits and reacts.

Enzyme Kinetics

  • Kinetics Definition: Study of the rate of reactions.

  • Factors affecting enzymatic reaction rates:

    • Substrate concentration

    • Enzyme concentration

    • Temperature

    • pH

    • Activators and inhibitors.

    • Michaelis-Menten Plot: Relationship between V (velocity) and [S] (substrate concentration).

Michaelis-Menten Kinetics

  • Michaelis Constant (Km): Indicates substrate concentration at which half of the enzyme's active sites are occupied; reflects enzyme affinity.

  • Low Km: High affinity for substrate, saturation at low substrate concentrations.

  • High Km: Low affinity, requires higher concentrations.

Enzyme Models

  • Lock and Key Model: Substrate fits perfectly into the enzyme's active site.

  • Induced Fit Model: Enzyme changes shape upon substrate binding, creating a near-perfect fit.

Enzyme Inhibition

  • Types of Inhibitors:

    • Irreversible Inhibitors: Permanently shut off enzyme activity (e.g., toxins).

    • Reversible Inhibitors: Bind and can dissociate; used as drugs to modulate enzyme activity.

Enzyme Inhibitors in Medicine

  • Examples include antibacterials (like penicillin) and anticancer agents. Roughly 30% of FDA-approved drugs target enzymes.

COX1 Inhibition by Aspirin

  • Aspirin irreversibly inhibits COX1 responsible for converting arachidonic acid into prostaglandins, affecting platelet function.

Regulation of Enzyme Activity

  • Can be controlled by noncovalent modification (allosteric) or covalent modification (irreversible or reversible).

Allosteric Regulation

  • Allosteric enzymes have multiple binding sites; effector molecules can modulate activity positively or negatively.

Zymogens and Activation

  • Zymogens: Inactive enzyme precursors activated by cleavage (e.g., trypsin activates chymotrypsin).

Enzymes: Catalysts, usually proteins, that accelerate biochemical reactions. They possess specificity and regulate biological pathways.

Biocatalysis Advantages:

  • Greater specificity

  • Milder conditions

  • Higher rates

  • Capacity for regulation

Major Classes:

  • Oxidoreductases: Redox reactions

  • Transferases: Chemical group transfer

  • Hydrolases: Hydrolytic cleavage

  • Lyases: Non-hydrolytic bond cleavage

  • Isomerases: Molecular rearrangement

  • Ligases: Molecule joining

Enzyme Structure:

  • Simple: Only proteins

  • Conjugated: Apoenzyme + coenzymes

Enzyme-Substrate Complex:

  • Forms active holoenzyme. Specific binding energy lowers activation energy.

Kinetics:

  • Factors: Substrate & enzyme concentration, temperature, pH, inhibitors.

  • Michaelis Constant (Km): Reflects enzyme affinity for substrate.

Models:

  • Lock and Key: Perfect fit

  • Induced Fit: Shape change upon binding.

Inhibition:

  • Irreversible: Permanent shutdown

  • Reversible: Binds temporarily

Regulation:

  • Allosteric control and zymogen activation.

Medicinal applications: Many drugs target enzymes, affecting pathways like pain and cancer.