Stories of Enzyme Mechanisms

Classic Mechanisms of Enzyme Catalysis

Enzymes are biological catalysts that accelerate chemical reactions in living organisms. Several classic mechanisms are studied in biochemistry, with serine proteases serving as a key example for understanding hydrolytic cleavage of peptide bonds.

• Serine Proteases: Enzymes that hydrolyze amide or ester bonds, such as chymotrypsin.

• Lysozyme: Hydrolyzes glycosidic bonds (not covered in detail here).

• Hexokinase: Induced fit and active site closure (not covered in detail here).

Serine Proteases: Focus on Chymotrypsin

Proteases in General

Proteases, also known as peptidases, are enzymes that catalyze the cleavage of peptide bonds in proteins. They play essential roles in various biological processes.

• Digestion: Breakdown of dietary proteins.

• Blood Clotting: Activation of clotting factors.

• Immune System: Processing of immune proteins.

• Apoptosis: Programmed cell death.

• Protein Turnover: Degradation and recycling of cellular proteins.

The scissile bond is the specific peptide bond targeted for cleavage, typically between the carbonyl of the N-terminal product peptide and the amine of the C-terminal product peptide.

Classes of Proteases

Proteases are classified based on their mechanism and the location of cleavage:

• Endoproteases: Cleave internal peptide bonds.

• Exoproteases: Cleave peptide bonds at the terminal amino acids.

Endoproteases are further classified by the residue at their active site:

• Serine Proteases: Use serine as the nucleophile (e.g., chymotrypsin, trypsin, elastase).

• Cysteine Proteases: Use cysteine as the nucleophile (e.g., papain, caspases).

• Aspartyl Proteases: Use aspartate residues (e.g., cathepsins, HIV protease).

• Metalloproteases: Use a metal ion (usually Zn2+) to activate water as the nucleophile.

Example: The serine protease subtilisin is used in laundry detergents for protein stain removal.

Zymogens

Activation of Digestive Proteases

Digestive proteases are synthesized in the pancreas as inactive precursors called zymogens. Activation involves proteolytic cleavage:

• Enteropeptidase (enterokinase) cleaves the first six residues of trypsinogen to activate trypsin.

• Trypsin then activates other protease zymogens, such as chymotrypsinogen and proelastase.

This cascade ensures that proteases are only active in the appropriate location (e.g., the digestive tract).

Serine Proteases: Key Concepts

S1 Recognition Pocket and Specificity

The specificity of serine proteases is determined by the S1 binding pocket, which selects for the residue N-terminal to the scissile bond. The size and composition of this pocket dictate substrate specificity:

• Trypsin: Prefers basic residues (Arg, Lys).

• Chymotrypsin: Prefers aromatic residues (Phe, Tyr, Trp).

• Elastase: Prefers small, neutral residues (Ala, Gly, Val).

Example: The deep, narrow pocket of trypsin accommodates long, basic side chains, while the shallow pocket of elastase fits small residues.

Catalytic Triad

Serine proteases utilize a catalytic triad at their active site:

• Ser195: Acts as the nucleophile.

• His57: Functions as a base catalyst.

• Asp102: Modulates the protonation state of His57, shifting its pKa and facilitating catalysis.

These residues work together to enable efficient peptide bond hydrolysis.

Oxy-Anion Hole

The oxy-anion hole is a structural feature that stabilizes the negative charge on the tetrahedral intermediate formed during catalysis. This stabilization is crucial for lowering the activation energy of the reaction.

Mechanism of Serine Protease Catalysis

Stepwise Reaction Mechanism

The catalytic mechanism of serine proteases involves several key steps:

1. Substrate Binding: The substrate binds to the active site, positioning the scissile bond near the catalytic triad.

2. Nucleophilic Attack: Ser195 attacks the carbonyl carbon of the peptide bond, forming a tetrahedral intermediate.

3. Tetrahedral Intermediate Stabilization: The oxy-anion hole stabilizes the negative charge on the intermediate.

4. Acyl-Enzyme Formation: The intermediate collapses, releasing the C-terminal fragment and forming an acyl-enzyme.

5. Deacylation: Water, activated by His57, attacks the acyl-enzyme, forming a second tetrahedral intermediate.

6. Product Release: The intermediate collapses, releasing the N-terminal fragment and regenerating the free enzyme.

Equation:

Irreversible Inhibition of Serine Proteases

Chemical Modification

Irreversible inhibitors target particularly reactive residues in the active site, such as Ser195 and His57. These inhibitors are useful for studying enzyme mechanisms and for therapeutic applications.

• TPCK (tosyl-L-Phe-chloromethyl ketone): Inhibits chymotrypsin by modifying His57.

• DFP (diisopropyl fluorophosphate): Inhibits serine proteases by modifying Ser195.

These inhibitors help identify catalytically important residues and can be detected by proteolysis and mass spectrometry.

Summary Table: Classes of Proteases

Additional info: The notes are based on figures and explanations from "Lehninger Principles of Biochemistry" and provide foundational knowledge for understanding enzyme mechanisms, especially serine proteases. The catalytic triad and substrate specificity are central to the function and inhibition of these enzymes.