The rate of breakdown of glucose into pyruvic acid is under certain controlling steps. The major controlling step is :-

Options

A. Fructose 6–P `to` Fructose 1, 6 bisphosphate
B. PGAL `to` 1, 3 bisphosphoglyceric acid
C. Glucose 6-P `to` Fructose 6-P
D. 3PGA `to` 2PGA

Answer

To answer the question regarding the major controlling step in the breakdown of glucose into pyruvic acid, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Glycolysis**:
- Glycolysis is the metabolic pathway that converts glucose into pyruvic acid. This process involves a series of ten enzymatic reactions.

2. **Identifying the End Product**:
- The end product of glycolysis is two molecules of pyruvic acid (or pyruvate) from one molecule of glucose.

3. **Recognizing Controlling Steps**:
- Within glycolysis, certain steps are more critical than others in regulating the overall rate of the pathway. These are often referred to as "rate-limiting steps."

4. **Identifying the Major Controlling Step**:
- The major controlling step in glycolysis is catalyzed by the enzyme **phosphofructokinase** (PFK).

5. **Role of Phosphofructokinase**:
- Phosphofructokinase is activated by high levels of AMP (adenosine monophosphate) and ADP (adenosine diphosphate), which indicate a low energy state in the cell. This activation promotes glycolysis to produce more ATP.

6. **Inhibition by ATP**:
- Conversely, when ATP levels are high, it acts as an inhibitor for phosphofructokinase, slowing down glycolysis. This feedback mechanism helps to maintain energy balance in the cell.

7. **Conclusion**:
- Therefore, the major controlling step in the breakdown of glucose into pyruvic acid is the reaction catalyzed by phosphofructokinase, making it the rate-limiting step of glycolysis.

### Final Answer:
The major controlling step in the breakdown of glucose into pyruvic acid is the reaction catalyzed by **phosphofructokinase**.

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