Lysozyme Catalytic Mechanisms and Reaction Steps

In lysozyme, this results in a covalent glycosyl-enzyme intermediate, where one part of the sugar is temporarily "stuck" to the enzyme before being released.

Acid-Base Catalysis

Enzymes also manipulate protons ($H^+$) to facilitate reactions. There are two main types:

1. Specific Acid-Base Catalysis: This involves the transfer of protons using only hydronium ($H\_3O^+$) or hydroxide ($OH^-$) ions from the surrounding water.
2. General Acid-Base Catalysis: This is much more efficient in enzymes. It involves the transfer of protons mediated by weak acids and bases (like the amino acid side chains within the enzyme's active site) rather than just water.

Exam Tip: If a question asks about the difference between specific and general acid-base catalysis, remember that general catalysis relies on the functional groups of the enzyme (like Glu or Asp) to move protons, whereas specific catalysis relies on the $pH$ of the solvent ($H_3O^+/OH^-$).

The Molecular Players: Glu35 and Asp52

The catalytic power of lysozyme resides in two specific amino acid residues located in its active site: Glutamic Acid 35 (Glu35) and Aspartic Acid 52 (Asp52).

• Asp52: Acts primarily as a nucleophile. It attacks the anomeric carbon of the sugar to form the covalent intermediate.
• Glu35: Acts as a versatile tool for proton transfer. It functions as a general acid in the first step and a general base in the second step.

The Reaction Mechanism: SN1 vs. SN2

For decades, scientists debated whether lysozyme followed an $S_N1$ or $S_N2$ mechanism.

• $S_N1$ mechanism: A multi-step reaction where the substrate first loses a leaving group to form a carbocation intermediate before the nucleophile attacks.
• $S_N2$ mechanism: A single-step reaction where the nucleophile attacks at the same time the leaving group departs, passing through a single transition state without a stable intermediate.

Current models suggest that lysozyme promotes two successive $S_N2$-type displacement reactions.

Step 1: Formation of the Intermediate

In the first displacement, Asp52 acts as a nucleophile and attacks the anomeric carbon of the sugar. Simultaneously, Glu35 acts as a general acid, donating a proton to the glycosidic oxygen (the leaving group) to facilitate its departure. This step results in the formation of the covalent glycosyl-enzyme intermediate.

Step 2: Hydrolysis of the Intermediate

In the second displacement, a water molecule enters the active site. Glu35 now switches roles, acting as a general base to deprotonate the water molecule. This makes the water oxygen a much stronger nucleophile, allowing it to attack the anomeric carbon, displace Asp52, and release the final products.

Summary of the Catalytic Cycle

To visualize the entire process, we can summarize the roles of the residues across the two steps:

Through this elegant "push-pull" mechanism, lysozyme lowers the activation energy required to break the incredibly stable glycosidic bonds of the bacterial cell wall.