Question 61:
In a substrate-enzyme reaction, the term Kd denotes which one of the following?
The correct answer is (B) Dissociation constant.
In substrate-enzyme reactions, Kd denotes the dissociation constant, measuring the equilibrium tendency of the enzyme-substrate (ES) complex to dissociate into free enzyme (E) and substrate (S), where Kd = [E][S]/[ES].
Option Breakdown
(A) Equilibrium constant
An equilibrium constant (Keq) describes overall reaction equilibrium (products/reactants), not specifically ES binding dissociation like Kd.
(B) Dissociation constant
Correct; Kd quantifies binding affinity via Kd = k_off/k_on, where lower Kd indicates tighter binding. In rapid equilibrium kinetics, Km ≈ Kd.
(C) Association constant
Association constant (Ka = 1/Kd) measures binding strength, the inverse of dissociation; higher Ka means stronger affinity.
(D) Turnover rate
Turnover rate (kcat) measures catalytic events per enzyme per second after ES formation, unrelated to binding equilibrium.
Introduction to Substrate Enzyme Reaction Kd
Substrate enzyme reaction Kd represents the dissociation constant measuring ES complex stability, crucial for enzyme kinetics in GATE Life Sciences. Lower Kd signals higher substrate affinity.
Kd in Michaelis-Menten Kinetics
Kd = [E][S]/[ES] = k_{-1}/k_1; when kcat ≪ k_{-1}, Km ≈ Kd (rapid equilibrium). Steady-state Km = (k_{-1} + kcat)/k_1 ≥ Kd, reflecting both binding and catalysis.
Typical values: nanomolar for tight binders, micromolar for weaker.
Enzyme Kinetics Constants Table
| Term | Definition | Units | Relation to Kd |
|---|---|---|---|
| Equilibrium Constant | Overall Keq | Varies | Different purpose |
| Dissociation Constant | [Kd = k_off/k_on] | Molar | Binding equilibrium |
| Association Constant | Ka = 1/Kd | Molar⁻¹ | Inverse of Kd |
| Turnover Rate | kcat (catalytic cycles/sec) | s⁻¹ | Post-binding catalysis |
Clarifies substrate enzyme reaction Kd for exams.
GATE Exam Context
Links prior lipids (membrane fluidity affects enzyme activity) and liver function (enzyme synthesis); master Kd vs Km distinction for biochemistry kinetics PYQs.
