Enzyme Action Models Quiz

Lecture Outline

• Enzymes are specialized, catalytically active biological macromolecules.
• They act as specific, efficient catalysts for chemical reactions in aqueous solution.
• Most enzymes are globular proteins; some are RNA (e.g., ribozymes and ribosomal RNA).

Enzyme Naming and Classification

• Enzymes are named by adding the suffix "-ase" to the name of their substrate or a description of their catalytic action.
• Classified based on the type of reaction catalyzed.
  • Oxidoreductases: Oxidation-reduction reactions.
  • Transferases: Transfer of functional groups.
  • Hydrolases: Hydrolysis reactions.
  • Lyases: Group elimination to form double bonds.
  • Isomerases: Isomerization.
  • Ligases: Bond formation coupled with ATP hydrolysis.

International Classification of Enzymes

• Each enzyme is assigned a four-part classification number and a systematic name.
• The systematic name identifies the reaction it catalyzes.
• (Each class has subclasses further classifying the type of reaction.)

Naming of Enzymes

• Each enzyme has a formal name, often including the substrate and type of reaction.
• Enzymes also have a systematic (EC) number, uniquely identifying the specific enzyme.

Key Structure-Function Features of Enzymes

• Enzymes are proteins.
• They have a globular shape and complex 3D structure.
• They have an active site with a specific shape and chemical environment to bind substrates.
• Some enzymes require cofactors (metal ions or organic/metallo-organic molecules) to function properly.

Enzyme Cofactors

• Cofactors are helper molecules that increase enzyme function.
• Metallic ions like Mg²⁺, Zn²⁺, Fe³⁺ assist in catalysis.
• Coenzymes are organic cofactors readily released.
• Prosthetic groups are tightly bound organic cofactors.

Enzyme Cofactors (Metal Ion Cofactors)

• Small inorganic ions (Mg²⁺, K⁺, Ca²⁺, Zn²⁺, Cu²⁺, Co, Fe) may be free or held in complexes within enzyme proteins.
• Help with catalysis.

Enzyme Cofactors (Organic/Metallo-organic Cofactors)

• Coenzymes are organic molecules, easily released.
• Prosthetic groups are organic molecules, tightly bound.
• Many are derived from vitamins.
• Examples include NAD (niacin, B3), FAD (riboflavin, B2), Coenzyme A.
• Some coenzymes participate as co-substrates or as transient carriers (for functional groups).
• Coenzymes may be helpful for transferring or acting on specific parts of a reactant molecule.

Enzyme & Cofactors

• Holoenzyme: The complete, catalytically active enzyme with bound cofactors and/or metal ions.
• Apoenzyme/apoprotein: The protein portion of the enzyme (before the cofactor is added).

Why Are Enzymes Important?

• Enzymes catalyze biochemical reactions in the body by speeding them up.
• Most biochemical and physiological reactions proceed at very slow paces without enzymes.

Enzymes & Cellular Metabolism

• Anabolism is the building of complex molecules from simpler ones via bond formation.
• Catabolism is the breakdown of complex molecules into simpler ones via bond breaking.
• Both anabolic and catabolic reactions require energy expenditure/release, and are tightly regulated by enzymes.

Enzymes & Cellular Metabolism(Anabolism)

• Enzymes create complex molecules through biosynthetic reactions, involving dehydration synthesis.
• Anabolic reactions generally consume more energy than they release.

Enzymes & Cellular Metabolism(Catabolism)

• Enzymes break down complex molecules into simpler ones, requiring hydrolytic reactions (use of water).
• Catabolic reactions generally produce more energy than they consume.

Cellular metabolism

• Catabolic reactions transform complex molecules to simpler ones, converting energy into usable forms like ATP.
• Anabolic reactions take small molecules and build them into large ones, while consuming energy.

Enzyme-catalyzed Metabolic Reactions

• Enzymes speed up metabolic reactions by lowering activation energy (EA).
• Enzymes provide an alternative pathway for reactions to occur.
• Enzymes transiently bind to a substrate, and then releases it as products once the reaction is complete.

How Do Enzymes Work?

• Enzymes act as catalysts to speed up reactions without being consumed or chemically altered.
• Enzymes accelerate the reaction rate.
• The shape of the active site slightly changes to bind the substrate, (induced fit).

Enzyme Action (Lock and Key/Induced Fit)

• The Lock and Key model is a simpler representation.
• The induced fit model is more accurate, as the active site changes shape to accommodate the substrate, forming a more precise, tight fit.

Enzyme-catalyzed Metabolic Reactions

• Graphs illustrate enzyme activity (rate or velocity) in the presence of different substrate concentrations.
• Maximum reaction velocity(V_max) is reached when all active sites are occupied, and addition substrate doesn't lead to a higher reaction rate.
• The Michaelis constant (K_M)—the substrate concentration when velocity is half of V_max—indicates an enzyme's affinity for its substrate; lower K_M = higher affinity.

Factors Affecting Enzyme Function

• Enzyme concentration, substrate concentration, temperature, pH, and salinity influence the rate of enzyme-catalyzed reactions.

Factors Affecting Enzyme Activity (Enzyme Concentration)

• Increasing enzyme concentration leads to a faster reaction rate, up to a certain point where all active sites for the reaction are bound.

Factors Affecting Enzyme Activity (Substrate Concentration)

• Increasing substrate concentration increases reaction rate until all active sites are occupied, resulting in the maximum sustainable rate, V_max.

Factors Affecting Enzyme Activity (Temperature)

• Increasing temperature initially enhances the reaction rate due to more collisions but excessively high temperatures lead to enzyme denaturation which decreases activity.

Factors Affecting Enzyme Activity (Optimum Temperature)

• The optimal temperature for enzyme function where it is most effective at catalyzing reactions, due to the maximum possible amount of collisions between the enzyme and substrate.

Factors Affecting Enzyme Activity (pH)

• Each enzyme has an optimal pH range; extremes can negatively affect enzyme function by denaturation, or disrupting the charge of amino acid residues within the active site.

Factors Affecting Enzyme Activity (Salinity)

• Changes in salinity, causing an increase or decrease in ions affect enzyme function. Extreme salinity disruptions between charged amino acids within the enzyme and affects its 3D structure leading to denaturation.

Enzyme Kinetics

• Enzyme kinetics studies the rates of enzyme-catalyzed reactions to understand mechanisms through which activity is controlled.
• Michaelis-Menten kinetics explain relationships between substrate concentration, reaction velocity, and enzyme activity.

Michaelis-Menten Kinetics

• All enzymes display a saturation effect with their substrate(s).
• At low substrate concentrations, velocity (V) is proportional to substrate concentration.
• At high substrate concentrations, velocity plateaus and becomes independent of substrate concentration.

Michaelis-Menten Kinetics

• Initial reaction velocities(V₀) against substrate concentration([S]) display a rectangular hyperbola shape.
• Derived equations use kinetics models and explain the saturation effect for enzyme activity.

Michaelis-Menten Kinetics

• Michaelis-Menten model and scheme postulates how enzymes bind substrates, and undergoes catalytic reactions and/or reverses back to its original form.

Graphical Determination of the Inhibitor Constant (K_i)

Significance of K_M (Michaelis Constant)

• Lower K_M values, the greater the affinity of an enzyme for its substrate, as evident through faster and more efficient reactions, with lower substrate concentrations.

Significance of V_max

• V_max indicates the maximum catalytic rate of an enzyme under optimal conditions.
• Higher V_max correlates to a greater reaction rate; or efficiency at which the substrate is converted into products.

Enzyme Inhibition

• The activity of enzymes (or their efficiency in catalyzing reactions) must be regulated.
• Enzyme inhibition is a mechanism for regulating and controlling the activity.

Types of Enzyme Inhibition

• Two types are irreversible and reversible.
• Reversible inhibition can be competitive, non-competitive, or uncompetitive in nature.

Types of Enzyme Inhibition-Competitive

• Competitive inhibition happens when an inhibitor competes with the substrate for binding to the active site on the enzyme.
• The presence of excess substrate can shift the balance in favor of substrate binding to the active site, effectively reversing the inhibition.

Types of Enzyme Inhibition- Non-Competitive

• Non-competitive inhibition occurs when the inhibitor binds to a separate site on the enzyme other than the enzyme's active site, which alters the active site structure that affects or preventing the binding of substrate(s).
• Substrate cannot bind to the altered active site.

Types of Enzyme Inhibition- Uncompetitive

• Uncompetitive inhibition takes place when the inhibitor binds only to the enzyme-substrate complex, preventing the formation of the product.
• The binding of the inhibitor decreases both apparent K_M and V_max values.

Summary of the Effect of Enzyme Inhibition on the K_M and V_max

• Each type of enzyme inhibition results in distinct effects on apparent K_M and/or V_max, providing a mechanism for regulating enzyme activity.

Inhibitors Constant (K_i)

• K_i is a measure of how tightly an inhibitor binds to the enzyme, enabling comparison and classification.

Enzymes as Important Drug Targets

• Enzymes are potential targets for drugs; inhibitors can be used to treat various diseases.

Test your knowledge on the 'Lock and Key' and 'Induced Fit' models of enzyme action. This quiz covers important concepts such as coenzymes, enzyme-substrate interactions, and the significance of enzymes in biochemical reactions. Perfect for biochemistry students seeking to deepen their understanding of enzyme function.