Enzymes: The Catalysts of Life

Introduction to Enzyme Function

Enzymes are biological catalysts that accelerate chemical reactions in cells, making life possible by lowering the activation energy required for reactions. Although many cellular reactions are exergonic and spontaneous, they do not occur instantly due to the presence of an activation energy barrier.

• Exergonic reactions are spontaneous but require overcoming an activation energy barrier.

• ΔG (Gibbs free energy change) indicates if a reaction can proceed, but not the rate at which it occurs.

Activation Energy and Reaction Rates

For a reaction to proceed, reactant molecules must possess enough energy to surpass the activation energy (EA), reaching the transition state.

• Activation energy (EA): Minimum energy required for reactants to form products.

• Only molecules with energy ≥ EA can react at a given time.

• Thermal activation increases the number of molecules able to react.

Equation:

Properties of Catalysts

Catalysts, including enzymes, possess distinct properties that enable them to accelerate reactions without being consumed.

• Increase reaction rates by lowering EA (often 106–1012 times faster).

• Form transient, reversible complexes with substrates.

• Alter the rate of equilibrium achievement, not the position of equilibrium.

Enzymes as Biological Catalysts

Most enzymes are proteins, though some RNA molecules (ribozymes) also exhibit catalytic activity.

• First enzymes studied in 1897 (ferments).

• First enzyme crystallized: urease (1926).

• Ribozymes discovered in the 1980s (Nobel Prize awarded to Altman and Cech in 1989).

Mechanism of Enzyme Action

Enzymes lower activation energy by stabilizing the transition state, making reactions more likely to occur.

• Enzyme active sites are complementary to the transition state, not just the substrate.

• “Stickase” model illustrates the importance of transition state stabilization.

Enzyme Structure and Specificity

All enzymes contain an active site where substrate binding and catalysis occur.

• Active sites confer remarkable specificity.

• Often contain cofactors (e.g., Mg2+) or coenzymes (e.g., NAD+).

Active Site Specificity Table

Optimal Conditions for Enzyme Activity

Enzymes function best at specific temperatures and pH values, which vary among organisms and enzymes.

• Enzyme activity decreases outside optimal temperature and pH ranges.

Substrate Binding and Activation

Substrate binding is usually reversible and involves hydrogen and/or ionic bonds. The induced fit model describes how enzyme conformation changes upon substrate binding, enhancing transition state stabilization.

• Substrate activation mechanisms include:

  • Bond distortion: Makes substrate more susceptible to attack.

  • Proton transfer: Increases substrate reactivity.

  • Electron transfer: Forms temporary covalent bonds.

Catalytic Cycle

The enzyme catalytic cycle involves substrate binding, transition state formation, product release, and enzyme regeneration.

• General reaction:

Enzyme Regulation

Enzyme activity is tightly regulated to maintain cellular homeostasis. Regulation occurs via inhibitors, feedback mechanisms, allosteric control, and covalent modification.

Types of Inhibition

• Competitive inhibition: Inhibitor binds active site, blocking substrate.

• Noncompetitive inhibition: Inhibitor binds elsewhere, altering enzyme function.

• Feedback inhibition: End product inhibits an earlier enzyme, often via allosteric regulation.

Allosteric Enzymes

• Multisubunit enzymes regulated by allosteric activators or inhibitors.

• Allosteric activation increases enzyme activity; inhibition decreases it.

Enzyme Cooperativity

• Binding of one substrate increases affinity of other subunits (e.g., hemoglobin).

Regulation by Covalent Modification

• Phosphorylation/dephosphorylation alters enzyme activity.

• Proteolytic cleavage activates zymogens (inactive precursors).

Proteolytic Cleavage Table

Major Classes of Enzymes

Enzyme Kinetics

Enzyme kinetics studies the rates of enzyme-catalyzed reactions, focusing on how substrate concentration affects reaction velocity.

• Initial reaction velocity (v): Rate of product formation per unit time, dependent on substrate concentration [S].

• At low [S], v increases proportionally with [S]; at high [S], v approaches a maximum (Vmax).

Michaelis-Menten Equation

• Km: Substrate concentration at which v = Vmax/2; indicates enzyme affinity for substrate.

• Lower Km means higher affinity and lower substrate needed for activity.

Lineweaver-Burk Equation (Double-Reciprocal Plot)

• Linearizes kinetic data for easier determination of Vmax and Km.

Effects of Enzyme Inhibitors Table

Example: Hexokinase Reaction

Glucose + ATP → Glucose-6-phosphate + ADP

• Used to determine Km and Vmax experimentally.

Additional info: These notes cover the essential concepts of enzyme structure, function, regulation, and kinetics, as outlined in Chapter 6 of a typical Cell Biology curriculum.