Excess of ATP inhibits respiration by inhibiting one of the following enzymes

Options

A. (a) phosphofructokinase
B. (b) pyruvic dehydrogenase
C. (c) isomerase
D. (d) acomitase

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks which enzyme is inhibited by an excess of ATP during respiration. This indicates that we need to identify the enzyme involved in glycolysis that is regulated by ATP levels.

2. **Glycolysis Overview**: Glycolysis is the metabolic pathway that converts glucose into pyruvate, producing ATP in the process. It consists of several steps, and the regulation of these steps is crucial for cellular energy management.

3. **Role of ATP**: ATP acts as a product of glycolysis. When ATP levels are high, it signals that the cell has enough energy, which leads to a decrease in the glycolytic pathway's activity. This is known as feedback inhibition.

4. **Identifying Key Enzymes**: In glycolysis, one of the key regulatory enzymes is phosphofructokinase (PFK). It catalyzes the conversion of fructose 6-phosphate to fructose 1,6-bisphosphate, which is a crucial step in the pathway.

5. **Effect of Excess ATP**: When there is an excess of ATP, it binds to phosphofructokinase allosterically, inhibiting its activity. This means that the glycolytic pathway slows down, as there is no need for more ATP when sufficient amounts are already present.

6. **Evaluating Other Options**:
- **Pyruvate Dehydrogenase**: This enzyme converts pyruvate to acetyl-CoA and is not directly regulated by ATP levels in glycolysis.
- **Isomerase**: This enzyme converts glucose 6-phosphate to fructose 6-phosphate and is not the main regulatory point.
- **Aconitase**: This enzyme is involved in the Krebs cycle, not glycolysis.

7. **Conclusion**: The enzyme that is inhibited by excess ATP during respiration is phosphofructokinase. Therefore, the correct answer is option A: phosphofructokinase.