Enzyme Mechanisms and Regulation in Biochemistry

Irreversible Inhibition

Mechanisms of Irreversible Enzyme Inhibition

Irreversible inhibition refers to the permanent inactivation of enzyme activity, typically through covalent modification or destruction of essential functional groups at the active site.

• Kill an enzyme’s activity: Irreversible inhibitors permanently block enzyme function.

• Covalent bond formation: Inhibitors may form covalent bonds with active site residues, preventing catalysis.

• Functional group removal: Essential groups required for catalysis may be removed or modified.

• Suicide inhibitors: These mimic the substrate and undergo initial catalytic steps, forming a reactive intermediate that covalently modifies and destroys the enzyme.

Examples of Irreversible Inhibitors: Protein Toxins

Many protein toxins act as irreversible inhibitors, targeting key cellular enzymes.

• Cholera toxin: Adds ADP to Gs, disrupting chloride transport.

• Botulinum toxin (BOTOX): Protease that cleaves SNAP-25, inhibiting neurotransmitter release.

• Diphtheria toxin: Adds ADP ribosyl to EF-2, blocking mRNA translation.

• Ricin toxin: Protease that cleaves ribosomal proteins, halting protein synthesis.

• Anthrax toxin: Protease that cleaves MAPKK, inducing cell suicide.

Chymotrypsin and Serine Proteases

Chymotrypsin: Structure and Function

Chymotrypsin is a digestive protease that hydrolyzes peptide bonds adjacent to aromatic amino acids.

• Protease definition: Enzymes that digest proteins by cleaving peptide bonds.

• Location: Found in digestive tract and cytoplasmic compartments.

• Specificity: Recognizes specific amino acids (e.g., Phe, Trp, Tyr) and cleaves adjacent peptide bonds.

• Serine proteases: Chymotrypsin, trypsin, and elastase share a common catalytic mechanism involving a serine residue.

Chymotrypsin Active Site and Catalytic Triad

The catalytic activity of chymotrypsin depends on a network of critical amino acids forming the active site.

• Key residues: Ser195, Gly193, His57, Asp102.

• H-bonding network: Ser195, His57, and Asp102 form a catalytic triad, facilitating nucleophilic attack on the substrate’s carbonyl carbon.

• Mechanism: The triad alters the pKa of His57, enabling Ser195 to act as a nucleophile.

RNase A: Acid-Base Catalysis

Mechanism of RNase A

RNase A catalyzes the cleavage of RNA via acid-base catalysis in a two-step process without forming a covalent intermediate.

• Step 1: His12 extracts a proton from the 2’OH of RNA, promoting nucleophilic attack on the phosphorous atom. His119 protonates the leaving group.

• Step 2: His12 donates a proton to the cyclic intermediate, promoting bond scission. His119 extracts a proton from water, allowing the –OH to react with the second leaving group.

Enolase: Metal Ion Catalysis

Enolase Mechanism

Enolase catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate via dehydration, utilizing metal ion catalysis.

• Active site: Contains two Mg2+ ions.

• Role of Mg2+: Mg2+-O interactions acidify the C2 proton, facilitating its removal and completing the dehydration reaction.

Lysozyme: Glycosidic Bond Cleavage

Lysozyme Structure and Function

Lysozyme is a small enzyme found in egg white and tears, catalyzing the cleavage of peptidoglycan in bacterial cell walls.

• First enzyme structure solved by X-ray crystallography.

• Cleavage specificity: Breaks the glycosidic bond between GlcNAc and MurNAc.

• Mechanism: Two-step successive direct-displacement (Sn2) reactions.

Lysozyme Catalytic Mechanism

• Asp52: Attacks the C1 of MurNAc, forming a covalent intermediate.

• Glu35: Protonates the leaving GlcNAc group.

• Water: Attacks the C1 of MurNAc, displacing Asp52 and regenerating the active site.

Regulatory Enzymes

Role in Metabolic Pathways

Regulatory enzymes control the flow of metabolites through multi-step pathways by responding to substrate levels or environmental signals.

• Single reaction catalysis: Most enzymes catalyze one reaction in a pathway.

• Pathways: Series of steps converting one molecule to another.

• Regulatory enzymes: Act as checkpoints, often responding to feedback or allosteric regulation.

• Allosteric enzymes: Most regulatory enzymes are allosteric, meaning their activity is modulated by binding of effectors at sites other than the active site.

Feedback Inhibition

Feedback inhibition is a common regulatory mechanism in metabolic pathways.

• Mechanism: The final product of a pathway allosterically inhibits an enzyme early in the pathway, preventing overproduction.

Allosteric Regulatory Enzymes

Allosteric Regulation Mechanisms

Allosteric enzymes can be regulated positively or negatively through reversible interactions.

• Types of regulation: Binding/release events or covalent modifications (e.g., phosphorylation).

• Phosphorylation: Addition/removal of phosphate groups is the most common covalent modification.

PhoQP System in Bacteria

Magnesium Sensing and Allosteric Regulation

The PhoQP system senses Mg2+ levels outside the cell and uses reversible covalent allosteric molecules to signal low Mg2+.

• PhoQ: Allosteric kinase that responds to Mg2+ levels.

• PhoP: Allosteric DNA binder that regulates gene expression in response to Mg2+ signals.

Phosphorylation

Enzyme-Mediated Phosphorylation and Dephosphorylation

Phosphorylation is a reversible covalent modification that regulates protein function.

• Kinases: Enzymes that add phosphate groups, typically to Tyr, Ser, Thr, and in bacteria, Asp.

• Phosphatases: Enzymes that remove phosphate groups.

• Source of phosphate: Usually ATP.

• Effects: Addition of PO4- introduces a bulky negative charge, affecting structure and activity.

• Consensus sequences: Specific amino acid motifs recognized by kinases/phosphatases.

• Multiple sites: Proteins may have several phosphorylation sites, each affecting function or localization.

Proteolytic Regulation

Activation of Proenzymes (Zymogens)

Some enzymes are synthesized as inactive precursors (proenzymes or zymogens) and activated by proteolytic cleavage.

• Proenzyme: Inactive precursor form of an enzyme.

• Regulatory sequence: Segment of primary structure cleaved by a protease to activate the enzyme.

• Zymogens: Proteases synthesized as proenzymes.

Caspases and Apoptosis

Role of Caspases in Programmed Cell Death

Caspases are proteases that mediate apoptosis, a form of programmed cell death essential for development and homeostasis.

• Death receptors: Fas and TNF-R1 receive signals to initiate apoptosis.

• Effector proteins: Caspases execute cell death by cleaving specific substrates.

• Caspase cascade: Initiator caspases activate effector caspases, leading to DNA fragmentation and cell death.

Enzyme Mechanisms: Summary Table

Enzyme	Type of Catalysis	Key Residues/Features	Mechanism
Chymotrypsin	Serine protease	Ser195, His57, Asp102	Catalytic triad, nucleophilic attack on peptide bond
RNase A	Acid-base catalysis	His12, His119	Two-step, no covalent intermediate
Enolase	Metal ion catalysis	2 Mg2+	Mg2+ acidifies C2 proton, dehydration
Lysozyme	Glycosidic bond cleavage	Asp52, Glu35	Sn2 mechanism, covalent intermediate

Key Equations and Concepts

• General enzyme reaction: $E + S \rightarrow ES \rightarrow E + P$

• Phosphorylation reaction: $\text{Protein} + \text{ATP} \xrightarrow{\text{Kinase}} \text{Protein-PO}_4 + \text{ADP}$

• Feedback inhibition: $\text{Product} \xrightarrow{\text{inhibits}} \text{Enzyme (early in pathway)}$