Chemistry 2100 Chapter 23

Protein Functions + PL P L Binding Catalysis Structure

Why Enzymes? Higher reaction rates Greater reaction specificity Milder reaction conditions Capacity for regulation Metabolites have many potential pathways of decomposition Enzymes make the desired one most favorable

Specificity: Lock-and-Key Model Proteins typically have high specificity: only certain substrates bind High specificity can be explained by the complementary of the binding site and the ligand. Complementarity in –size, –shape, –charge, –or hydrophobic / hydrophilic character “Lock and Key” model by Emil Fisher (1894) assumes that complementary surfaces are preformed. +

Specificity: Induced Fit Conformational changes may occur upon ligand binding (Daniel Koshland in 1958). –This adaptation is called the induced fit. –Induced fit allows for tighter binding of the ligand –Induced fit can increase the affinity of the protein for a second ligand Both the ligand and the protein can change their conformations +

increase [reactant] increase temperature add catalyst Potential Energy Reaction Reactants Products Enzymatic Activity

increase [reactant] increase temperature add catalyst Potential Energy Reaction Reactants Products TS

increase [reactant] increase temperature add catalyst Potential Energy Reaction Reactants Products EaEa TS

increase [reactant] increase temperature add catalyst Potential Energy Reaction TS Reactants Products EaEa

increase [reactant] increase temperature add catalyst Potential Energy Reaction Reactants Products EaEa TS

increase [reactant] increase temperature add catalyst Potential Energy Reaction Reactants Products EaEa TS

EaEa ' increase [reactant] increase temperature add catalyst Reactants Products Potential Energy Reaction EaEa TS

How to Lower  G  ? Enzymes organizes reactive groups into proximity

How to Lower  G  ? Enzymes bind transition states best

Potential Energy Reaction H 2 O + CO 2 HOCO 2 – + H +

Potential Energy Reaction H 2 O + CO 2 HOCO 2 – + H +

Potential Energy Reaction H 2 O + CO 2 HOCO 2 – + H +

Potential Energy Reaction H 2 O + CO 2 HOCO 2 – + H +

Potential Energy Reaction H 2 O + CO 2 HOCO 2 – + H +

Potential Energy Reaction H 2 O + CO 2 HOCO 2 – + H + EaEa

Potential Energy Reaction EaEa EaEa '

How to Do Kinetic Measurements

Enzyme Activity Figure 23.3 The effect of enzyme concentration on the rate of an enzyme-catalyzed reaction. Substrate concentration, temperature, and pH are constant.

Enzyme Activity Figure 23.4 The effect of substrate concentration on the rate of an enzyme-catalyzed reaction. Enzyme concentration, temperature, and pH are constant.

Enzyme Activity Figure 23.5 The effect of temperature on the rate of an enzyme-catalyzed reaction. Substrate and enzyme concentrations and pH are constant.

Enzyme Activity Figure 23.6 The effect of pH on the rate of an enzyme-catalyzed reaction. Substrate and enzyme concentrations and temperature are constant.

What equation models this behavior? Michaelis-Menten Equation

Inhibitors Reversible inhibitors –Temporarily bind enzyme and prevent activity Irreversible inhibitors –Permanently bind or degrade enzyme

Reversible Inhibition

Mechanism of Action Enzyme kinetics in the presence and the absence of inhibitors. 72

Irreversible Inhibition

Acetylcholinesterase

F

lactase rennin papain high-fructose corn syrup pectinase clinical assays Commercial Enzymes

lactase rennin papain high-fructose corn syrup pectinase clinical assays

lactase rennin papain high-fructose corn syrup pectinase clinical assays

lactase rennin papain high-fructose corn syrup pectinase clinical assays

lactase rennin papain high-fructose corn syrup pectinase clinical assays

lactase rennin papain high-fructose corn syrup pectinase clinical assays

lactase rennin papain high-fructose corn syrup pectinase clinical assays