The value of `K_(m)` (Michaelis-Meten constant) varies form `10^(6)` to `10^(-6)` M but for allosteric enzyme, there is no constant `k_(m)` value. This `K_(m)` is .

Options

A. substrate concentration at which the enzymatic reaction attains half its maximum vleocity `((1)/(2)V_("max"))`
B. enzyme concentration at which the reaction attaines `(1)/(2)V_("max")`.
C. end prouduct concentration at which reaction attains `(1)/(2)V_("max")`.
D. none of the above statements is correct.

Answer

### Step-by-Step Solution:

1. **Understanding Km (Michaelis-Menten Constant)**:
- The Michaelis-Menten constant (Km) is a crucial parameter in enzyme kinetics. It represents the substrate concentration at which the reaction velocity is half of the maximum velocity (Vmax) of the enzyme-catalyzed reaction.

2. **Variation of Km**:
- The value of Km can vary widely, ranging from \(10^{-6}\) M to \(10^{6}\) M. This variation indicates how different enzymes interact with their substrates. A low Km value suggests high affinity between the enzyme and substrate, while a high Km value indicates low affinity.

3. **Allosteric Enzymes**:
- Allosteric enzymes differ from Michaelis-Menten enzymes in that they do not follow the simple hyperbolic relationship described by the Michaelis-Menten equation. Instead, they exhibit a sigmoidal (S-shaped) curve when plotting reaction velocity against substrate concentration.

4. **Absence of a Constant Km**:
- For allosteric enzymes, there is no single Km value because their activity can be influenced by the binding of effector molecules, which can enhance or inhibit enzyme activity. This means that the relationship between substrate concentration and reaction velocity is more complex and cannot be captured by a single constant.

5. **Conclusion**:
- Therefore, while Km is a valuable parameter for many enzymes, for allosteric enzymes, it is more accurate to describe their kinetics without a fixed Km value due to their unique regulatory mechanisms.