Section 6.1: An Introduction to Enzymes

Definition and Importance of Enzymes

Enzymes are biological catalysts that accelerate chemical reactions in living organisms, making life possible by increasing reaction rates under mild conditions.

• Enzyme activity: The rate at which an enzyme converts substrate to product.

• Active site: The region of the enzyme where substrate binding and catalysis occur.

• Substrate: The molecule upon which an enzyme acts.

• Cofactor: A non-protein chemical compound required for enzyme activity (e.g., metal ions).

• Coenzyme: An organic cofactor, often derived from vitamins (e.g., NAD+).

• Prosthetic group: A tightly bound cofactor or coenzyme essential for enzyme function.

• Holoenzyme: The complete, active enzyme with its cofactors/coenzymes.

• Apoenzyme: The protein part of an enzyme, without its cofactors/coenzymes.

Section 6.2: How Enzymes Work

Enzyme Catalysis and Reaction Energy

Enzymes lower the activation energy of reactions, allowing them to proceed faster and more efficiently.

• Free energy diagrams: Visual representations showing the energy changes during uncatalyzed and enzyme-catalyzed reactions. Enzymes lower the activation energy () but do not change the overall free energy change () of the reaction.

• Enzyme mechanisms: Include substrate binding, transition state stabilization, acid-base catalysis, and metal-ion catalysis.

• Specificity: Enzymes exhibit high specificity for their substrates due to precise active site interactions.

• Induced fit model: The enzyme changes shape upon substrate binding to facilitate catalysis.

Example: Hexokinase catalyzes the phosphorylation of glucose, demonstrating substrate specificity and induced fit.

Section 6.3: Enzyme Kinetics as an Approach to Understanding Mechanism

Michaelis-Menten Kinetics

Michaelis-Menten kinetics describes the rate of enzyme-catalyzed reactions using a simple model involving substrate binding and conversion to product.

• Two-step mechanism:

• Rate constant: (formation of ES), (dissociation of ES), (formation of product).

• Michaelis-Menten equation:

• Steady-state assumption: The concentration of the ES complex remains constant during the reaction.

• Initial rate (): The rate measured at the beginning of the reaction before significant product accumulates.

• Maximum velocity (): The rate when the enzyme is saturated with substrate.

• Michaelis constant (): The substrate concentration at which the reaction rate is half of .

• Catalytic efficiency: , where is the turnover number (number of substrate molecules converted per enzyme per unit time).

Example: The enzyme carbonic anhydrase has a high and low , making it highly efficient.

Graphical Analysis

• Lineweaver-Burk plot: Double reciprocal plot of vs to linearize the Michaelis-Menten equation.

• Plot interpretation: Slope = , y-intercept = .

Section 6.4: Examples of Enzymatic Reactions

Proteases and Inhibitors

Proteases are enzymes that catalyze the hydrolysis of peptide bonds in proteins. Their activity can be regulated by inhibitors.

• Serine proteases: Enzymes like trypsin and chymotrypsin use a serine residue for catalysis.

• Inhibitors: Molecules that decrease enzyme activity. Types include competitive, uncompetitive, and mixed inhibitors.

• Irreversible inhibitors: Bind covalently to the enzyme, permanently inactivating it (e.g., suicide inhibitors).

• HIV protease: An aspartyl protease, mechanism differs from serine proteases.

Example: Penicillin acts as a suicide inhibitor of bacterial transpeptidase.

Enzyme Activity and pH

• pH dependence: Enzyme activity varies with pH due to ionization of active site residues. Each enzyme has an optimum pH.

Section 6.5: Regulatory Enzymes

Regulation of Enzyme Activity

Enzyme activity can be regulated by various mechanisms, including allosteric regulation, covalent modification, and feedback inhibition.

• Allosteric enzymes: Enzymes whose activity is modulated by binding of effectors at sites other than the active site.

• Positive/negative modulators: Increase or decrease enzyme activity, respectively.

• Covalent modification: Addition or removal of chemical groups (e.g., phosphorylation) alters enzyme activity.

• Feedback inhibition: End product of a pathway inhibits an earlier step.

• Phosphorylase regulation: Example of enzyme regulated by phosphorylation/dephosphorylation.

Example: Glycogen phosphorylase is activated by phosphorylation in response to hormonal signals.

Allosteric Kinetics

• Sigmoidal kinetics: Allosteric enzymes often show sigmoidal (S-shaped) velocity vs substrate concentration plots.

• : Substrate concentration at half-maximal velocity for allosteric enzymes (analogous to for Michaelis-Menten enzymes).

Comparison Table: Michaelis-Menten vs Allosteric Enzymes

Additional info: These notes expand on the original question prompts by providing definitions, explanations, and examples for each concept, ensuring a self-contained study guide suitable for biochemistry students.