5-1_-_Enzymes

Topic 5: Energetics & Metabolism

5-1: Enzymes Lecture Overview

• Overview of enzyme basics

• Role in assisting metabolic reactions

• Mechanisms for regulating enzymatic reactions

• Reference: Textbook Chapter 3.5

Importance of Enzymes

• Reaction example: H2 + ½ O2 → H2O (ΔGo’ = -237 kJ)

  • Reaction is exergonic, indicating favorable energetics

  • Without a catalyst, the reaction has a negligible rate

  • Key Concept: Activation Energy

    • Bonds need to be broken to initiate the reaction

    • Without a catalyst, significant energy is needed to break bonds

Enzyme Functionality

• Enzymes as catalysts

  • Lower the activation energy of reactions

  • Do not alter the energetics (ΔG) or equilibrium of the reaction

  • Increase reaction rates significantly (by many orders of magnitude)

  • Reference: Textbook Figure 3.9 (essential to understand)

Mechanisms of Enzyme Action

• How enzymes increase reaction rates:

  • Local concentrations of substrates increased at the enzyme's active site

  • Proper orientation for optimal reaction

  • Alteration of electronic distribution of substrates enhancing reactivity

  • Stabilization of transition states during reactions

  • Use of coenzymes or prosthetic groups (e.g., Fe, Mg, Zn, Co) to aid reactions

Example of Enzyme-Catalyzed Reaction

• Lysozyme cleavage of the β-1,4 glycosidic bond in peptidoglycan

  • Visual reference: Textbook Figure 3.10

Enzymes in Metabolism

• Lack of enzyme leads to negligible rates for most biological reactions

• Enzymes catalyze specific reactions for product accumulation

• In metabolic pathways, product from one reaction serves as reactant for the next

  • Prevents product accumulation which could promote reverse reactions

• Control of enzyme activity impacts which metabolic reactions occur in cells

Control of Enzyme Activity

Competitive Inhibition

• Competitive inhibitors mimic substrates and fit into the enzyme's active site

  • Inhibit substrate binding and reaction rate

  • E.g., sulfa drug as an antibiotic prohibiting folate biosynthesis.

Allosteric Regulation

• Involves molecules (effectors) that bind at locations other than the active site

  • Can either activate or inhibit enzyme activity

  • Visual: shows allosteric inhibitors activating enzymes by altering conformations

Feedback Inhibition

• Prevents overproduction of metabolic products

  • End product acts as an inhibitor of an early step in the metabolic pathway

  • Typically affects the first committed step

  • Binding affinity evolves to bind only when end product levels are high

  • Once the product is consumed, the inhibition ceases, and metabolic pathway resumes.