Lysozyme: Structure, Function, and Mechanism

Introduction to Lysozyme

Lysozyme is a natural antibacterial enzyme found in tears and egg whites. It plays a crucial role in the innate immune system by hydrolyzing the polysaccharide chains in bacterial cell walls, leading to cell lysis.

• Definition: Lysozyme is an enzyme that catalyzes the hydrolysis of β(1→4) glycosidic bonds between N-acetylmuramic acid (MurNAc) and N-acetylglucosamine (GlcNAc) in peptidoglycan.

• Biological Role: Provides defense against bacterial infection by breaking down cell wall peptidoglycan.

• Location: Found in tears, saliva, and egg whites.

• Molecular Weight: Hen egg white lysozyme has a molecular weight of approximately 14,300 Da.

Mechanism of Action

Lysozyme acts by cleaving the glycosidic bond in the peptidoglycan layer of bacterial cell walls, leading to cell lysis.

• Substrate: Peptidoglycan, composed of alternating units of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc).

• Active Site Residues: Key amino acids involved include Glu35 and Asp52.

• Mechanism:

  1. Lysozyme binds to the peptidoglycan substrate, positioning the glycosidic bond for cleavage.

  2. Glu35 acts as a general acid, donating a proton to facilitate bond cleavage.

  3. Asp52 stabilizes the transition state and assists in hydrolysis.

  4. The result is the breaking of the β(1→4) glycosidic bond, leading to cell wall degradation.

• Example: The hydrolysis of the peptidoglycan in Staphylococcus aureus cell walls by lysozyme.

Structure of Peptidoglycan and Lysozyme Interaction

Peptidoglycan is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of most bacteria.

• Repeating Units: Alternating GlcNAc and MurNAc residues linked by β(1→4) bonds.

• Lysozyme Binding: Lysozyme binds to six sugar residues in the peptidoglycan, distorting the substrate to facilitate catalysis.

• Structural Representation: The enzyme-substrate complex can be visualized with the active site accommodating the polysaccharide chain.

Enolase: Mechanism and Catalytic Function

Introduction to Enolase

Enolase is a metalloenzyme that catalyzes the conversion of 2-phosphoglycerate (2-PGA) to phosphoenolpyruvate (PEP) in glycolysis. This reaction is essential for energy production in cells.

• Definition: Enolase is an enzyme involved in the glycolytic pathway, facilitating the dehydration of 2-PGA to PEP.

• Biological Role: Critical for ATP generation during glycolysis.

• Active Site: Contains two magnesium ions (Mg2+) that stabilize the substrate and reaction intermediates.

Mechanism of Enolase-Catalyzed Reaction

The reaction catalyzed by enolase involves two main steps: formation of an enolate intermediate and elimination of water to produce phosphoenolpyruvate.

• Step 1: Formation of Enolate Intermediate

  • 2-PGA binds to the active site, coordinated by Mg2+ ions.

  • Lys345 and Glu211 are key residues involved in proton abstraction and stabilization.

  • Glu211 facilitates elimination of the –OH group by general acid catalysis.

• Step 2: Elimination of Water

  • Water is eliminated, resulting in the formation of phosphoenolpyruvate (PEP).

  • The overall reaction can be summarized as:

• Example: The conversion of 2-PGA to PEP in muscle cells during glycolysis.

Enolase Active Site and Catalytic Residues

• Magnesium Ions: Two Mg2+ ions are essential for substrate binding and stabilization of the enolate intermediate.

• Key Residues: Lys345 and Glu211 play critical roles in catalysis.

• Enolate Intermediate: The intermediate is stabilized by interactions with the active site residues and Mg2+ ions.

Comparison Table: Lysozyme vs. Enolase

Additional info: The notes infer the detailed mechanism of lysozyme and enolase based on standard biochemistry knowledge, as some handwritten and image content was fragmented or unclear. The comparison table is constructed to summarize the main differences and similarities between the two enzymes for exam preparation.