Enzyme Kinetics and Thermodynamics: ΔG, Transition State, and Michaelis-Menten

ΔG (free energy)

Determines whether a reaction will occur spontaneously.

Exergonic reaction

ΔG < 0; spontaneous reaction.

Endergonic reaction

ΔG > 0; requires energy input.

Equilibrium

ΔG = 0; the reaction is at equilibrium.

Transition state (X‡)

A high-energy intermediate that a reaction goes through.

Activation energy (ΔG‡)

The energy input needed to attain the transition state.

Enzymes

Biological catalysts that stabilize the transition state.

Enzyme acceleration

Enzymes accelerate reactions by decreasing ΔG‡ and facilitating formation of the transition state.

Enzyme-substrate complex (ES)

Enzymes interact with substrate molecules and form enzyme-substrate complexes.

Active site

Has a nonaqueous environment, high specificity due to chemical complementarity, high binding affinity for substrate, and even higher affinity for the transition state.

Chemical complementarity

Enzyme active sites have chemical complementarity with substrates.

Lock & Key Hypothesis

Complementarity between the enzyme and substrate exists before substrate binding.

Induced Fit Hypothesis

Complementarity between the enzyme and substrate occurs after substrate binding.

Michaelis-Menten kinetics

Hyperbolic relationship between rate vs. [S].

Vmax

Rate when enzyme is saturated with substrate; Vmax is approached asymptotically.

Vmax equation

Vmax = kcat × [E]T.

[E]T

Total enzyme concentration: [E]T = [E] + [ES].

KM

Concentration of substrate at which enzyme works appreciably; KM is [S] when the velocity is Vmax/2.

kcat

Turnover number — the number of substrate molecules converted to product in a unit time when the enzyme is fully saturated by substrate.

kcat/KM

Measure of catalytic efficiency; it is a rate constant that can be used as a measure of catalytic efficiency.

Diffusion limit

kcat/KM cannot be faster than the diffusion-controlled encounter of enzyme and substrate; limit is approximately 10⁸-10⁹ s⁻¹ M⁻¹.