AIIMS 2005 Zoology Enzyme Kinetics MCQ Question

To answer the question regarding the effect of substrate concentration on the rate of reaction for the enzyme green-gram-phosphatase, we need to consider the typical behavior of enzyme kinetics, particularly as described by the Michaelis-Menten model.

Explanation of the Correct Answer (B)

Correct Answer: B) Presence of an enzyme inhibitor in the reaction mixture.

1. Understanding the Graph: In enzyme kinetics, the graph typically illustrates how the rate of reaction (velocity, $V$) varies with increasing substrate concentration ($[S]$). Initially, as the substrate concentration increases, the rate of reaction also increases. However, there comes a point where the rate begins to plateau, indicating that the enzyme is saturated with substrate.

2. Interpreting the Plateau: If the graph shows that the reaction rate does not increase with further increases in substrate concentration, this could indicate the presence of an enzyme inhibitor. Inhibitors can reduce the enzyme's effectiveness by binding to the enzyme or the enzyme-substrate complex, preventing the reaction from proceeding efficiently. The saturation effect combined with a reduced maximum velocity suggests that an inhibitor might be present, which aligns with the observation that the rate does not increase despite high substrate levels.

Clarification of Incorrect Options

A) The rate of enzyme reaction is directly proportional to the substrate concentration.

• This statement is only true at low substrate concentrations. According to the Michaelis-Menten equation:

$$V = \frac{V_{max} [S]}{K_m + [S]}$$

At low $[S]$, the reaction rate increases linearly with substrate concentration. However, as $[S]$ increases and approaches $K_m$ (the Michaelis constant), the rate begins to level off, which is contrary to the statement.

C) Formation of an enzyme-substrate complex.

• While the formation of an enzyme-substrate complex is indeed a fundamental step in enzyme kinetics, this option does not explain the observed behavior in the graph. All enzymatic reactions involve forming this complex; however, the graph's specific details, such as the plateau indicating saturation and potential inhibition, point to other factors at play, such as inhibitor presence rather than just complex formation.

D) At higher substrate concentration the pH increases.

• This statement is not typically associated with enzyme kinetics. The pH can affect enzyme activity, but it is not a direct consequence of increasing substrate concentration. The relationship between substrate concentration and pH is not a standard aspect of enzyme kinetics and would require specific experimental data to assess.

Relevant Concepts

• Michaelis-Menten Kinetics: The relationship between enzyme activity and substrate concentration is described by the Michaelis-Menten equation. As mentioned earlier, at low substrate concentrations, the reaction rate increases linearly. At high concentrations, the enzyme becomes saturated, and the rate approaches $V_{max}$.

• Inhibition: There are different types of enzyme inhibitors:

  • Competitive Inhibitors: Compete with the substrate for the active site.
  • Non-competitive Inhibitors: Bind to the enzyme at a different site, reducing its activity regardless of substrate concentration.

Conclusion

In summary, the graph indicating a plateau in the reaction rate at high substrate concentrations suggests the presence of an enzyme inhibitor, making option B the correct choice. The other options either misinterpret the behavior of enzyme kinetics or introduce unrelated concepts. Understanding enzyme kinetics principles is crucial for interpreting such data accurately in biological systems.