6_Enzymes_
Enzymes Overview
Definition: Proteins or molecules acting as catalysts in biological reactions.
Function of Catalysts: Increase reaction rates without changing in the process; lower activation energy.
Role in Equilibrium: Accelerate approach to equilibrium without altering thermodynamic favorability.
Key Features of Enzymes
Biological Mediators: All bodily reactions are mediated by enzymes.
Protein Catalysts: Enzymes are not changed during reactions, maintaining their structure.
Denaturation: Loss of activity occurs if enzymes are denatured.
Types: Can be simple proteins or include cofactors (metal ions, organic vitamins).
Nomenclature of Enzymes
Name Structure: Two names per enzyme; short recommended and systemic name.
Suffix: Ends in –ase, e.g., sucrase (reacts with sucrose).
Common Names: Digestion enzymes often retain historical names, e.g., pepsin, trypsin.
Enzyme Classification by IUBMB
Categories: Six groups based on reactions catalyzed:
Oxidoreductases: Catalyze oxidation-reduction reactions.
Transferases: Transfer functional groups between molecules.
Hydrolases: Catalyze hydrolysis reactions (water addition).
Lyases: Add/remove atoms to/from double bonds.
Isomerases: Rearrange molecular structure.
Ligases: Bond molecules using ATP.
Enzyme Action Models
Lock and Key Model: The active site of an enzyme is complementary in shape to the substrate.
Induced Fit Model: Enzymes adjust shape upon substrate binding to enhance specificity and catalysis.
Cofactors and Coenzymes
Definition: Non-protein molecules required for enzymatic activity—metal ions (e.g., Zn, Fe) or organic molecules (e.g., NAD+).
Holoenzyme vs. Apoenzyme: Holoenzyme includes its cofactor; apoenzyme is the protein portion without activity.
Turnover Number and Efficiency
Turnover Number (kcat): Molecules converted into product per enzyme per second, indicating enzyme efficiency.
Specificity: Enzymes are specific, often for one or a few substrates; enantiomers are also specific.
Factors Affecting Enzyme Action
Concentration Factors: Substrate concentration, enzyme concentration, temperature, pH.
Activity Loss: Enzymes lose activity at extreme pH or high temperatures (denaturation).
Michaelis-Menten Kinetics
Assumptions: Formation of enzyme-substrate complex (ES) and constancy of substrate concentration.
Equations: Describes reaction velocity as a function of substrate concentration—hyperbolic response.
Km Characteristic: Reflects enzyme affinity for a substrate (low Km = high affinity).
Enzyme Inhibition Types
Competitive Inhibitors: Compete for active site; reversed by increasing substrate concentration.
Noncompetitive Inhibitors: Bind at a different site; decrease Vmax without affecting Km.
Uncompetitive Inhibitors: Bind only to ES complex; lower both Vmax and Km.
Allosteric Regulation
Definition: Regulation by effectors that bind non-covalently at sites other than the active site.
Types of Effectors: Positive (increase activity) and negative (decrease activity).
Feedback Inhibition: Final product inhibits an earlier enzyme in the pathway to regulate processes.
Summary of Key Points
Enzymes increase rates of reactions through various mechanisms and maintain specificity.
Their activity can be regulated by environmental factors, inhibitors, and cofactors.
Michaelis-Menten kinetics provide a framework for understanding enzyme behavior regarding substrate concentration.