CHE 296 Lecture 6 Terms

Ribozymes

RNA molecules that act as enzymes.

Transition-State Hydrogen Bonding

A single hydrogen bond to the transition state can increase reaction rate by more than 100-fold.

E, S, and P Notation

E = enzyme, S = substrate, P = product.

Arrhenius Equation

k = A e^(-Ea/RT)

Pre-Exponential Factor (A)

Constant in the Arrhenius equation related to collision frequency and orientation.

Ea in Arrhenius Equation

Activation energy.

R in Arrhenius Equation

Gas constant.

T in Arrhenius Equation

Absolute temperature in kelvin.

Lowering Ea and Rate

Lowering activation energy increases reaction rate.

Key Questions in Process Chemistry

How much product can be made and how quickly it can be made.

Enzyme Activity Units

Usually reported as U/mL.

Definition of One Enzyme Unit

Amount of enzyme that converts 1 μmol of substrate per minute under specified conditions.

Factors Affecting Enzyme Units

Temperature, pH, substrate concentration, and presence of inhibitors or activators.

How Enzymes Are Made

Often produced biologically using recombinant DNA technology in microbes or cell cultures.

Enzyme Purity

Enzymes may be purified or used in mixtures depending on production method.

Enzyme Performance Optimization

Optimized by temperature, pH, and substrate concentration.

Temperature Effect on Enzymes

Increasing temperature generally increases activity until denaturation becomes significant.

pH and Temperature Activity Curve

Typically bell-shaped with an optimum.

Why Enzymes Have Different Optima

Their amino acid composition and structure affect stability and activity.

High-Temperature Denaturation

Elevated temperatures can denature enzymes and destroy catalytic activity.

Competing Rate for Catalysis (k2)

Rate constant describing product-forming step.

Competing Rate for Denaturation (kd)

Rate constant describing enzyme deactivation or denaturation.

Why k2 and kd Are Not Directly Comparable

They may have different units.

Temperature Effect on kd

kd often increases sharply with temperature because denaturation has a high activation barrier.

Long Exposure at High Temperature

Prolonged exposure can significantly denature proteins.

Rate of Product Formation

Depends on concentration of the enzyme-substrate complex.

Units of Reaction Rate

Amount per time, often mol/time.

Overall Reaction Speed

Determined by k2 and the concentration of ES complex.

How to Increase Reaction Speed

Increase k2 or increase concentration of ES complex.

Vmax

Maximum reaction rate when substrate is not limiting.

Vmax Equation

Vmax = kcat[E]total

Fast ES Formation Assumption

Formation of the ES complex is often much faster than product formation.

Negligible Reverse Reaction Assumption

At initial times, reverse reaction is often negligible because product is low.

Binding Constant

Measure of enzyme-substrate binding strength.

Substrate Affinity

Describes how strongly a substrate binds to an enzyme.

Rapid Equilibrium Assumption

Assumes ES forms quickly and approximates binding equilibrium.

Enzyme Saturation

Condition where most or all active sites are occupied by substrate.

Briggs-Haldane Assumption

Quasi-steady-state assumption used to analyze enzyme kinetics.

Initial-Rate Experiments

Measure rate early in the reaction before much product accumulates.

Lineweaver-Burk Plot

Plot of 1/v versus 1/[S] used to estimate Vmax and Km.

Lineweaver-Burk Axes

x-axis = 1/[S], y-axis = 1/v.

Vmax from Lineweaver-Burk Plot

Determined from the y-intercept, 1/Vmax.

Km from Lineweaver-Burk Plot

Determined from the x-intercept, -1/Km.

Double-Reciprocal Transformation

Linearizes the Michaelis-Menten equation.

Eadie-Hofstee Equation

Alternative linear transformation used in enzyme kinetics.

Hanes-Woolf Equation

Alternative linear transformation used in enzyme kinetics.

Noncompetitive Inhibition

Inhibitor binds at a site other than the active site and lowers activity.

Effect of Competitive Inhibition on Kinetics

Increases apparent Km but does not change Vmax.

Effect of Noncompetitive Inhibition on Kinetics

Decreases Vmax without changing Km in the ideal case.

Competitive Inhibitors

Often resemble the substrate and compete for the active site.

Altering a Reaction Rate Down

Reaction rate can be decreased by inhibitors or unfavorable conditions.

Bimolecular Reaction

Reaction involving two reactants, such as A + B → P.

Bimolecular Rate Law

v = k[A][B]

Pseudo First-Order Reaction

Bimolecular reaction simplified to first-order when one reactant is in excess.