Enzyme Specificity and Models of Enzyme Action

Lock and Key vs. Induced Fit Model

Enzymes exhibit specificity for their substrates, which is explained by two primary models: the lock and key model and the induced fit model.

• Lock and Key Model: Proposes that the enzyme's active site is a perfect fit for the substrate, much like a key fits into a lock. This model emphasizes structural complementarity.

• Induced Fit Model: Suggests that the enzyme's active site is flexible and molds itself around the substrate upon binding, enhancing the fit and facilitating catalysis.

• Biological Justification: The induced fit model is supported by evidence that enzymes undergo conformational changes upon substrate binding, which can increase catalytic efficiency and specificity.

• Example: Hexokinase undergoes a significant conformational change when binding glucose, supporting the induced fit model.

Reaction Coordinate Diagrams and Enzyme Effects

Parts of a Reaction Coordinate Diagram

A reaction coordinate diagram illustrates the energy changes during a chemical reaction.

• Reactants: Starting materials of the reaction.

• Products: Substances formed as a result of the reaction.

• Activation Energy (Ea): The energy barrier that must be overcome for the reaction to proceed.

• Transition State: The highest energy point along the reaction pathway.

• ΔG (Gibbs Free Energy Change): The difference in free energy between reactants and products.

Enzymes lower the activation energy (Ea) but do not change the overall ΔG of the reaction. The position of reactants and products remains the same, but the peak representing the transition state is lowered in the presence of an enzyme.

• Catalyzed vs. Uncatalyzed Reaction: The catalyzed reaction has a lower activation energy, resulting in a faster reaction rate.

Cofactors in Enzyme Activity

Definition and Examples

Cofactors are non-protein chemical compounds that are required for the biological activity of some enzymes.

• Types: Can be inorganic ions (e.g., Mg2+, Zn2+, Fe2+) or organic molecules (coenzymes, e.g., NAD+, FAD, coenzyme A).

• Examples: DNA polymerase requires Mg2+ as a cofactor; alcohol dehydrogenase uses Zn2+.

Enzyme Catalysis and the Activation Energy

Enzyme Mechanism and Transition State Stabilization

Enzymes accelerate biochemical reactions by lowering the activation energy required to reach the transition state.

• Enzyme-Substrate Complex: The enzyme binds the substrate, forming a transient complex that facilitates the reaction.

• Transition State Stabilization: Enzymes stabilize the transition state through specific interactions, reducing the energy barrier.

• Binding Energy: The energy derived from enzyme-substrate interactions is used to lower the activation energy.

Types of Enzyme Mechanisms

Acid-Base Catalysis, Covalent Catalysis, and Metal Ion Catalysis

Enzymes employ various catalytic strategies to accelerate reactions:

• Acid-Base Catalysis: Enzyme side chains donate or accept protons to stabilize the transition state (e.g., histidine in serine proteases).

• Covalent Catalysis: The enzyme forms a transient covalent bond with the substrate (e.g., serine in chymotrypsin forms an acyl-enzyme intermediate).

• Metal Ion Catalysis: Metal ions stabilize negative charges, orient substrates, or participate in redox reactions (e.g., Zn2+ in carbonic anhydrase).

Metal ions can stabilize the transition state by neutralizing charges or by acting as electrophilic catalysts.

Enzyme Kinetics: Reaction Velocity and Substrate Concentration

Michaelis-Menten Kinetics

The relationship between enzyme reaction velocity and substrate concentration is described by the Michaelis-Menten equation:

• Initial Velocity (V0): The rate of reaction when substrate concentration is much greater than enzyme concentration.

• Vmax: The maximum velocity achieved by the system, at saturating substrate concentration.

• Km (Michaelis Constant): The substrate concentration at which the reaction velocity is half of Vmax.

• Substrate Concentration ([S]): The amount of substrate available for the reaction.

The Michaelis-Menten equation is:

• Effect of Enzyme Concentration: Increasing enzyme concentration increases Vmax but does not affect Km.

Mathematical Analysis of Enzyme Kinetics

Lineweaver-Burk Plot and Kinetic Parameters

The Lineweaver-Burk plot is a double reciprocal plot used to determine kinetic parameters:

• Y-intercept:

• X-intercept:

This plot helps distinguish between different types of enzyme inhibition and calculate Vmax and Km.

Catalytic Constant (kcat) and Enzyme Efficiency

Definition and Significance

• kcat (Turnover Number): The number of substrate molecules converted to product per enzyme molecule per unit time at saturation.

• Enzyme Efficiency: Measured by the ratio , which reflects both the rate of catalysis and the enzyme's affinity for the substrate.

• Significance: High values indicate highly efficient enzymes.

Enzyme Inhibition

Types of Inhibition and Effects on Kinetic Parameters

Enzyme inhibitors reduce enzyme activity by interfering with substrate binding or catalysis. The main types are:

• Competitive Inhibition: Inhibitor resembles substrate and binds to the active site. Vmax is unchanged; Km increases.

• Uncompetitive Inhibition: Inhibitor binds only to the enzyme-substrate complex. Both Vmax and Km decrease.

• Non-competitive Inhibition: Inhibitor binds to an allosteric site, affecting both free enzyme and ES complex. Vmax decreases; Km remains unchanged.

In competitive inhibition, increasing substrate concentration can overcome inhibition. In uncompetitive and non-competitive inhibition, increasing substrate concentration does not restore full activity.

• Example: Methotrexate is a competitive inhibitor of dihydrofolate reductase.

Summary Table: Effects of Inhibitors on Enzyme Kinetics

Additional info: These notes expand upon the study guide questions by providing definitions, examples, and equations relevant to enzyme kinetics and catalysis, suitable for undergraduate biochemistry students.