Enzyme Catalysis

Chemical Essence of Enzymes

Enzymes are primarily protein-based biological catalysts that accelerate chemical reactions by lowering the activation energy required. This process increases reaction rates and is fundamental to cellular metabolism.

• Reduction of entropy of solution: Enzymes organize substrates, reducing randomness and facilitating productive collisions.

• Distortion of substrate (electron redistribution): Enzymes may induce strain or redistribute electrons in the substrate to favor reaction.

• Alignment of functional groups: Enzymes position reactive groups for optimal interaction.

• Breakdown of solvation shell: Enzymes may remove water molecules surrounding substrates, enhancing reactivity.

Enzyme Classification

Enzymes are classified based on the type of reaction they catalyze. The International Classification of Enzymes groups them into six major classes:

Enzyme Mechanisms: Models of Substrate Binding

Enzymes bind substrates through specific mechanisms that facilitate catalysis:

• No Enzyme: High activation energy; reaction proceeds slowly.

• Lock and Key Model: Enzyme active site is complementary to substrate shape; binding is highly specific.

• Induced Fit Model: Enzyme changes shape upon substrate binding, optimizing interaction with the transition state.

Example: The induced fit model explains how hexokinase undergoes conformational change upon glucose binding, enhancing catalytic efficiency.

Thermodynamics and Kinetics of Enzyme Reactions

Enzyme-catalyzed reactions are governed by thermodynamic and kinetic principles:

• Thermodynamics: Relates equilibrium constant () and standard free energy change (). Inverse relationship: when is negative, is positive (nonspontaneous).

• Kinetics: Measures how fast a reaction proceeds (rate constants , ) and how well a substrate binds to an enzyme (Michaelis constant ).

Acid-Base Catalysis

Enzymes may use acid-base catalysis to facilitate reactions:

• Specific Acid-Base Catalysis: Involves only H+ or OH- ions from water.

• General Acid-Base Catalysis: Involves weak acids or bases other than water, often amino acid side chains.

Key Amino Acids in General Acid/Base Catalysis:

• Glu, Asp: Acidic residues

• Lys, Arg, His: Basic residues

• Cys: Disulfide bonds

• Ser: Polar uncharged

• Tyr: Aromatic non-polar

Example: The catalytic triad in chymotrypsin (Ser, Asp, His) demonstrates general acid-base catalysis in peptide bond hydrolysis.

Covalent Catalysis

In covalent catalysis, a transient covalent bond forms between the enzyme and substrate, creating a new reaction pathway with lower activation energy.

• Mechanism: Enzyme forms a covalent intermediate with the substrate, which is then broken down to release the product and regenerate the enzyme.

• Example: Serine proteases form acyl-enzyme intermediates during peptide bond cleavage.

Michaelis-Menten Kinetics

The Michaelis-Menten model describes the rate of enzymatic reactions as a function of substrate concentration.

• Vmax: Maximum velocity of enzymatic reaction.

• kcat: Turnover number, the rate at which enzyme converts substrate to product.

• Km: Substrate concentration at which the reaction rate is half of Vmax; a measure of substrate affinity.

Michaelis-Menten Equation:

Key Rate Constants:

• : Rate constant for ES complex formation

• : Rate constant for ES complex dissociation

• : Rate constant for ES conversion to E + P (forward reaction)

Km vs Kd

• Kd (Dissociation constant): Measures how readily a molecule dissociates into smaller components, specifically the ES complex.

• Km: Often reflects substrate affinity, but can differ from Kd depending on reaction mechanism.

Lineweaver-Burk Plot

The Lineweaver-Burk plot is a double reciprocal graph used to determine Km and Vmax from experimental data.

Equation:

Application: Useful for distinguishing types of enzyme inhibition and calculating kinetic parameters.

Additional info: These notes expand on the provided material by including definitions, examples, and equations for clarity and completeness.