Q.35
The turnover numbers for the enzymes E1 and E2 are 150 s-1 and 15 s-1 respectively.
This means
Options:
(A) E1 binds to its substrate with higher affinity than E2
(B) The velocity of reactions catalyzed by E1 and E2 at their respective saturating substrate concentrations could be equal, if concentration of E2 used is 10 times that of E1
(C) The velocity of E1 catalyzed reaction is always greater than that of E2
(D) The velocity of E1 catalyzed reaction at a particular enzyme concentration and saturating substrate concentration is lower than that of E2 catalyzed reaction under the same conditions
Enzyme Turnover Numbers Explained: E1 vs E2 Analysis
Turnover number (kcat) measures catalytic speed per enzyme molecule, not substrate affinity. E1’s higher kcat means faster catalysis at saturation if enzyme amounts adjust properly.
Correct Answer
Option (B) is correct: Velocities at saturating substrate can equalize if [E2] is 10x [E1].
Vmax = kcat × [E], so E1’s 150 s⁻¹ requires 1/10th E2’s enzyme concentration (15 s⁻¹) for matching rates. This holds under Michaelis-Menten kinetics at [S] >> Km.
Core Concepts
Turnover number (kcat) defines maximum substrate molecules converted per enzyme per second at saturation.
It differs from Km, which reflects binding affinity—higher kcat signals efficiency, not tighter binding.
Velocity depends on both kcat and total enzyme concentration, enabling compensatory adjustments.
Option Breakdown
| Option | Statement | Correct? | Explanation |
|---|---|---|---|
| (A) | E1 binds substrate with higher affinity than E2 | No | kcat unrelated to affinity (Km metric); no Km data given |
| (B) | Vmax equal if [E2] = 10× [E1] at saturation | Yes | Vmax_E1 = 150[E1]; Vmax_E2 = 15[E2]; 150[E1] = 15(10[E1]) balances |
| (C) | E1 velocity always greater than E2 | No | Depends on [E]; equal possible with more E2 |
| (D) | E1 Vmax lower than E2 at same [E] and saturation | No | E1’s higher kcat yields 10× greater Vmax |
Exam Tips
For saturating [S], compare Vmax via kcat × [E] ratios—scale enzyme to match rates.
kcat suits efficiency ranking; pair with Km for full catalytic power (kcat/Km).
Ideal for biotech, enzymology papers targeting “enzyme kinetics MCQs”.
