In an enzyme-mediated reaction, Km value is :

Options

A. The substrate concentration at which the velocity of the chemical reaction is half its minimum value
B. The substrate concentration at which the velocity of the chemical reaction is half its maximum value
C. The velocity of the chemical reaction at which the substrate concentration is maximum
D. The velocity of the chemical reaction at which the substrate concentration is half its maximum value

Answer

**Step-by-Step Solution:**

1. **Understanding Km Value**: Km, known as the Michaelis constant, is a crucial parameter in enzyme kinetics. It indicates the substrate concentration at which the reaction rate is half of its maximum velocity (Vmax).

2. **Definition of Vmax**: Vmax is the maximum rate of reaction that an enzyme can achieve when the substrate concentration is saturating. At this point, all active sites of the enzyme are occupied by the substrate.

3. **Half of Vmax**: The Km value specifically represents the substrate concentration at which the reaction rate is half of Vmax. This means that if you have a substrate concentration equal to Km, the reaction is proceeding at 50% of its maximum possible rate.

4. **Affinity of Enzyme for Substrate**: The Km value also provides insight into the affinity between the enzyme and the substrate. A low Km indicates high affinity (the enzyme can achieve half-maximal velocity at a low substrate concentration), while a high Km indicates low affinity (the enzyme requires a higher substrate concentration to reach half-maximal velocity).

5. **Evaluating the Options**:
- Option A states that Km is the substrate concentration at which the velocity is half its minimum value. This is incorrect because Km relates to half of the maximum value, not the minimum.
- Option B states that Km is the substrate concentration at which the velocity of the chemical reaction is half its maximum value. This is correct.
- Option C and D can be disregarded as they do not accurately describe the Km value.

6. **Conclusion**: Based on the definitions and explanations, the correct answer is that Km is the substrate concentration at which the velocity of the chemical reaction is half of its maximum value.

**Final Answer**: The correct answer is Option B: Substrate concentration at which the velocity of chemical reaction is half of its maximum value.

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