Question

A reaction mixture containing optimum condition of reaction for enzymes, E1, E2 and E3 react with a particular substrate S. Km of E1, E2 and E3 for that particular substrate S are $5\times10^{-12}$, $5\times10^{-10}$ and $5\times10^{-15}$ respectively then
(A) E1 is more specific for that substrate than E2
(B) E2 is more specific for that substrate than E3
(C) E3 is more specific for that substrate than E1
(D) E3 is more specific for that substrate than E2
Choose the correct answer from the options given below:

• A. (A), (C) and (D) only.
• B. (A), (B) and (C) only.
• C. (A), (B), (C) and (D).
• D. (B), (C) and (D) only.

Hint:

Always remember:
The smaller the $K_m$ value, the tighter the enzyme-substrate binding, and the higher the affinity and specificity for that substrate.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The Michaelis constant ($K_m$) is a fundamental parameter in enzyme kinetics defined as the substrate concentration at which the reaction velocity is half of its maximal value ($V_{\text{max}}/2$).
In many systems, $K_m$ is an indicator of the affinity or specificity of an enzyme for a particular substrate: a lower $K_m$ value indicates higher affinity/specificity, while a higher $K_m$ value indicates lower affinity/specificity.
Detailed Explanation:
Let us compare the given $K_m$ values for the three enzymes:
- $K_m$ of E1 = $5\times10^{-12}$
- $K_m$ of E2 = $5\times10^{-10}$
- $K_m$ of E3 = $5\times10^{-15}$
Arranging these $K_m$ values in increasing order:
\[ 5\times10^{-15}\text{ (E3)} < 5\times10^{-12}\text{ (E1)} < 5\times10^{-10}\text{ (E2)} \] Since $K_m$ is inversely related to substrate affinity and specificity:
- E3 has the lowest $K_m$, so it has the highest specificity/affinity.
- E2 has the highest $K_m$, so it has the lowest specificity/affinity.
- The order of specificity is: $\text{E3} > \text{E1} > \text{E2}$.
Now let us evaluate the given statements:
- (A) E1 is more specific than E2: True, because $K_m$ of E1 ($5\times10^{-12}$) $<$ $K_m$ of E2 ($5\times10^{-10}$).
- (B) E2 is more specific than E3: False, because $K_m$ of E2 ($5\times10^{-10}$) $>$ $K_m$ of E3 ($5\times10^{-15}$).
- (C) E3 is more specific than E1: True, because $K_m$ of E3 ($5\times10^{-15}$) $<$ $K_m$ of E1 ($5\times10^{-12}$).
- (D) E3 is more specific than E2: True, because $K_m$ of E3 ($5\times10^{-15}$) $<$ $K_m$ of E2 ($5\times10^{-10}$).
Therefore, statements (A), (C), and (D) are correct.

Step 2: Final Answer:
The correct option is (A).