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**: The Km value, also known as the Michaelis constant, is a key parameter in enzyme kinetics. It represents the substrate concentration at which the reaction velocity is half of its maximum velocity (Vmax).

2. **Definition of Vmax**: Vmax is the maximum rate of the reaction when the enzyme is saturated with substrate. At this point, increasing the substrate concentration further will not increase the reaction rate.

3. **Relation of Km to Reaction Velocity**: The Km value indicates how efficiently an enzyme converts a substrate into a product. A low Km value suggests high affinity between the enzyme and substrate, meaning that the enzyme can achieve half of its maximum velocity at a low substrate concentration.

4. **Analyzing the Options**:
- **Option 1**: "The substrate concentration at which the velocity of the chemical reaction is half its minimum value." - This is incorrect because Km is related to maximum velocity, not minimum.
- **Option 2**: "The substrate concentration at which the velocity of the chemical reaction is half its maximum value." - This is correct as it accurately describes the Km value.
- **Option 3**: "The velocity of the chemical reaction at which the substrate concentration is maximum." - This is incorrect as it does not relate to Km.
- **Option 4**: "The velocity of the chemical reaction at which the substrate concentration is half its maximum value." - This is also incorrect as it misrepresents the relationship.

5. **Conclusion**: Based on the analysis of the options, the correct answer is **Option 2**: "The substrate concentration at which the velocity of the chemical reaction is half its maximum value."