Enzyme Structure and General Properties

Definition and Role of Enzymes

Enzymes are biological catalysts, primarily proteins, that accelerate chemical reactions without being consumed in the process. They act on specific substrates to produce products, often with remarkable specificity and efficiency.

- Catalyst: Increases reaction rate without being altered.

- Substrate: The molecule upon which an enzyme acts.

- Reaction specificity: Enzymes produce stereospecific products with minimal by-products.

- Regulation: Enzyme activity can be modulated by various mechanisms.

Energetics of Enzyme Catalysis

Enzymes lower the activation energy (ΔG‡) required for a reaction, thereby increasing the rate at which equilibrium is reached. However, they do not alter the overall change in Gibbs free energy (ΔG) or the equilibrium position.

- ΔG: Change in Gibbs free energy; determines spontaneity.

- ΔG‡: Activation energy; lowered by enzymes.

- Induced fit: Enzyme changes conformation to better bind substrate and stabilize the transition state.

Enzyme Mechanisms and Active Sites

Active Site and Substrate Binding

The active site of an enzyme is a specialized region where substrate binding and catalysis occur. Folding of the protein brings together key side chains and backbone groups to form this site.

- Proximity and orientation: Substrate is positioned optimally for reaction.

- Transition state stabilization: Enzyme binds tightly to the transition state, lowering activation energy.

- Acid-base catalysis: Amino acid side chains (e.g., His, Cys, Tyr, Asp, Glu, Lys) donate or accept protons.

- Covalent catalysis: Temporary covalent bond forms between enzyme and substrate (e.g., Ser, Tyr, Cys, Lys, His).

- Cofactors: Non-protein molecules (e.g., Zn²⁺) assist in catalysis.

Specificity and Recognition

Enzyme specificity is determined by the chemical properties and geometry of the substrate binding pocket.

- Substrate recognition: Enzyme site matches substrate shape and charge.

- Example: Chymotrypsin, trypsin, and elastase have similar structures but different substrate preferences due to their specificity pockets.

Chemical Strategies in Enzyme Catalysis

Nucleophilic and Electrophilic Catalysis

Enzymes utilize nucleophilic and electrophilic groups to facilitate bond formation and cleavage.

- Nucleophiles: Donate electron pairs (e.g., hydroxyl, sulfhydryl, amine, imidazole groups).

- Electrophiles: Accept electron pairs (e.g., protons, metal ions, carbonyl carbon, cationic imines).

Example: Aconitase Mechanism

Aconitase isomerizes citrate to isocitrate via dehydration and rehydration steps, using a [4Fe–4S] cluster as a cofactor.

- Dehydration: Removes H from C2 and OH from C3 to form cis-aconitate.

- Rehydration: Adds water across the double bond, moving OH to C2 and H to C3.

- Net effect: Converts a tertiary alcohol to a secondary alcohol.

Condensation and Hydrolysis Reactions

- Condensation: Two molecules combine, releasing water.

- Hydrolysis: Bond is broken by addition of water.

Proteolytic Enzymes: Serine Proteases

Chymotrypsin Mechanism and Specificity

Chymotrypsin is a serine protease that catalyzes the hydrolysis of peptide bonds, especially after aromatic residues (Phe, Trp, Tyr).

- Catalytic triad: Ser195, His57, Asp102 cooperate to activate Ser for nucleophilic attack.

- Specificity pocket: Determines substrate preference.

Enzyme Kinetics

Michaelis-Menten Model

The Michaelis-Menten equation describes the relationship between substrate concentration and reaction velocity for many enzymes. - Equation: - Km: Substrate concentration at which velocity is half-maximal; reflects enzyme affinity for substrate. - kcat: Turnover number; maximum number of substrate molecules converted per enzyme per second. - kcat/Km: Catalytic efficiency; higher values indicate more efficient enzymes.

Enzyme Inhibition

Competitive Inhibition

Competitive inhibitors bind to the active site, preventing substrate binding. - Effect: Apparent Km increases; Vmax remains unchanged. - Equation:

Non-Competitive Inhibition

Non-competitive inhibitors bind to a site other than the active site, affecting enzyme activity regardless of substrate binding. - Effect: Vmax decreases; Km remains unchanged.

Uncompetitive Inhibition

Uncompetitive inhibitors bind only to the enzyme-substrate complex, decreasing both Km and Vmax.

Mixed Inhibition

Mixed inhibitors bind to both the enzyme and the enzyme-substrate complex, affecting both Km and Vmax in variable ways.

Non Michaelis-Menten Kinetics and Allostery

Allosteric Enzymes and Regulation

Allosteric enzymes do not follow Michaelis-Menten kinetics and are regulated by effectors that bind at sites other than the active site. - Allosteric inhibition: Effector decreases enzyme activity. - Allosteric activation: Effector increases enzyme activity.

Metabolic Regulation of Enzymes

Mechanisms of Regulation

Enzyme activity is regulated at multiple levels, including synthesis, degradation, localization, allosteric regulation, covalent modification, and feedback inhibition. - Transcriptional regulation: Changes in enzyme gene expression. - Post-translational modification: Phosphorylation, acetylation, etc. - Feedback inhibition: End product inhibits pathway enzyme.

Summary Table: Types of Enzyme Inhibition

Key Equations

Michaelis-Menten Equation

Competitive Inhibition

Lineweaver-Burk Plot

Conclusion

These notes provide a comprehensive overview of enzyme structure, function, kinetics, inhibition, and regulation, with relevant examples and diagrams to reinforce key concepts. Enzymes are central to biochemistry, and understanding their mechanisms and regulation is essential for advanced study and practical applications in medicine and biotechnology.