Enzymology and Catalytic Mechanisms
Principles of Catalysis
Lowering activation barriers
Enzymes accelerate reactions by stabilizing the transition state relative to the ground state. They accomplish this through proximity and orientation effects, general acid-base catalysis, covalent catalysis, metal ion catalysis, electrostatic preorganization, and desolvation. Catalysis changes reaction rate, not reaction equilibrium; the free-energy difference between reactants and products remains unchanged.
What does a lower typically suggest, with appropriate caveats?
is often interpreted as an inverse proxy for apparent affinity, though it is a kinetic parameter rather than a pure binding constant.
Correct answer: Higher apparent affinity or altered kinetics
Why is transition-state stabilization central to enzyme catalysis?
The transition state is the highest-energy point along the reaction coordinate; stabilizing it accelerates catalysis.
Correct answer: Because enzymes preferentially bind and stabilize the transition state, lowering activation free energy and increasing reaction rate.
Kinetic Models and Inhibition
Michaelis-Menten and beyond
For many enzymes, initial-rate kinetics can be approximated by the Michaelis-Menten equation Yet modern enzymology often requires more elaborate models: burst kinetics, allosteric cooperativity, multisubstrate mechanisms, substrate inhibition, and time-dependent conformational selection. In living cells, reaction kinetics are shaped by compartmentation and metabolite channeling, so in vitro parameters must be interpreted cautiously.
A competitive inhibitor typically does which of the following?
Competitive inhibitors compete with substrate for binding, raising apparent while leaving maximal velocity unchanged at saturating substrate.
Correct answer: Increases apparent while leaving unchanged
Give one reason enzyme activity can differ dramatically in vivo versus in vitro.
Cellular context strongly influences catalysis beyond purified assay conditions.
Correct answer: Because intracellular crowding, compartmentation, post-translational modifications, and metabolite concentrations alter enzyme behavior.