Enzyme Kinetics and Thermodynamics

Standard Free Energy Change (ΔG°')

The standard free energy change, denoted as ΔG°', is a thermodynamic quantity that predicts the spontaneity of a reaction under standardized conditions. It is a central concept in biochemistry for understanding metabolic pathways and enzyme-catalyzed reactions.

• Definition: ΔG°' is the change in free energy for a reaction under standard conditions.

• Standard Conditions: Typically, 1 M concentration of reactants and products, 1 atm pressure, 25°C (298 K), and pH 7.0 for biochemical reactions.

• Equation: where is the reaction quotient, is the gas constant, and is temperature in Kelvin.

• Interpretation: A negative ΔG°' indicates a spontaneous reaction under standard conditions.

Example: Hydrolysis of ATP to ADP and Pi has a large negative ΔG°', making it a key energy source in cells.

Enzyme Kinetics: Michaelis-Menten Constant (Km)

Definition and Significance of Km

The Michaelis constant (Km) is a key parameter in enzyme kinetics, reflecting the substrate concentration at which the reaction rate is half of its maximum (Vmax).

• Definition: Km is the substrate concentration at which the reaction velocity is half-maximal.

• Equation: where is the initial velocity, is substrate concentration, is maximum velocity, and is the Michaelis constant.

• Interpretation: Lower Km indicates higher affinity of enzyme for substrate; higher Km indicates lower affinity.

• Example: The brain enzyme hexokinase has a Km of 1.5 mM for D-fructose and 0.05 mM for D-glucose, indicating a much higher affinity for D-glucose. This reflects the enzyme's physiological role in glucose metabolism.

Additional info: The disparity in Km values for different substrates can reflect the enzyme's specificity and physiological function.

Enzyme Inhibition

Irreversible Inhibitors

Irreversible inhibitors bind covalently or very tightly to enzymes, permanently inactivating them. They are important tools in biochemistry and medicine.

• Purpose: To permanently block enzyme activity, often for regulatory or therapeutic reasons.

• Typical Uses: Used in drugs (e.g., penicillin), research (to study enzyme function), and as toxins.

• Mechanism: Usually form covalent bonds with active site residues, preventing substrate binding or catalysis.

Example: Aspirin irreversibly inhibits cyclooxygenase, reducing inflammation.

Enzyme Active Site and Catalysis

Features of Enzyme Active Sites

The active site of an enzyme is the region where substrate binding and catalysis occur. It is highly specific and often forms a unique microenvironment for the reaction.

• Key Features:

  • Specific amino acid residues for substrate binding and catalysis

  • Three-dimensional structure complementary to the substrate

  • May include cofactors or metal ions

• Function: Lowers activation energy, stabilizes transition state, and increases reaction rate.

Example: The serine residue in the active site of chymotrypsin acts as a nucleophile during peptide bond hydrolysis.

Enzyme Kinetics: Graphical Analysis

Michaelis-Menten and Inhibition Plots

Enzyme kinetics are often analyzed using graphical methods to distinguish between different types of inhibition and to determine kinetic parameters.

• Michaelis-Menten Plot: Plots reaction velocity (v) versus substrate concentration ([S]).

• Lineweaver-Burk Plot: Double reciprocal plot ( vs ) used to linearize data and distinguish inhibition types.

• Effect of Inhibitors: Competitive, noncompetitive, and uncompetitive inhibitors alter the shape and intercepts of these plots in characteristic ways.

Additional info: The provided graphs likely represent these relationships; for exam purposes, be able to interpret and sketch these plots.

Protein Structure and Function: Myosin and ATP

Conformational Changes in Myosin upon ATP Binding

Myosin is a motor protein that interacts with actin filaments to generate movement in muscle contraction. ATP binding induces significant conformational changes.

• Conformational Change: ATP binding causes the myosin head to detach from actin and undergo a structural change, "cocking" the head for the next power stroke.

• Effect on Structure: The overall structure of myosin shifts from a rigor (tightly bound) state to a relaxed state, allowing movement along actin filaments.

• Rate-Limiting Step: The release of inorganic phosphate (Pi) after ATP hydrolysis is often the rate-limiting step in the myosin ATPase cycle.

Example: Muscle contraction cycles depend on repeated ATP binding, hydrolysis, and product release by myosin.

Effects of Mutations on Enzyme Activity

Analysis of Holo-HasA Mutants

The provided bar graph compares the activity of wild-type and mutant forms of the enzyme holo-HasA. Mutations can significantly affect enzyme function.

• Interpretation: Mutations at H32 and Y75 reduce enzyme activity, with the double mutant and denatured enzyme showing the lowest activity. This suggests these residues are important for function.

Additional info: Such analyses help identify critical amino acids for enzyme activity and stability.