In the entire process of glycolysis 4 phosphorylation occurs in two of the following steps :
I. 1-3 Diphosphoglyceric acid → 3-phosphoglyceric acid
II. Phosphoenol Pyruvic acid → Pyruvic acid
III. 2-phosphoglyceric acid → phosphoenol pyruvic acid
IV. 3-phosphoglyceric acid → 2-phosphoglyceric acid

Question

In the entire process of glycolysis 4 phosphorylation occurs in two of the following steps : 
I. 1-3 Diphosphoglyceric acid → 3-phosphoglyceric acid 
II. Phosphoenol Pyruvic acid → Pyruvic acid 
III. 2-phosphoglyceric acid → phosphoenol pyruvic acid 
IV. 3-phosphoglyceric acid → 2-phosphoglyceric acid

Accepted Answer

In the process of glycolysis, phosphorylation events are crucial for the transfer of phosphate groups, which are essential for the generation of ATP and the progression of the glycolytic pathway. Let's examine why Option 1, which includes steps I and II, is the correct answer.

1. **Explanation for Option 1 (Correct Answer):**

- **Step I: 1,3-Diphosphoglyceric acid → 3-Phosphoglyceric acid**
     - This step involves the conversion of 1,3-diphosphoglyceric acid to 3-phosphoglyceric acid. During this reaction, a phosphate group is transferred from 1,3-diphosphoglyceric acid to ADP, forming ATP. This is an example of substrate-level phosphorylation, where a high-energy phosphate group is directly transferred to ADP to form ATP.

- **Step II: Phosphoenol Pyruvic acid → Pyruvic acid**
     - In this step, phosphoenolpyruvate (PEP) is converted to pyruvic acid. This reaction also involves substrate-level phosphorylation, where the high-energy phosphate group from PEP is transferred to ADP, resulting in the formation of ATP. This is one of the key steps in glycolysis where ATP is generated.

2. **Explanation for why other options are incorrect:**

- **Option 2 (Only II and III):**
     - While step II involves phosphorylation, step III (2-phosphoglyceric acid → phosphoenol pyruvic acid) does not involve the transfer of a phosphate group to ADP to form ATP. Instead, it involves the dehydration of 2-phosphoglyceric acid to form phosphoenolpyruvate, which is a preparatory step for the subsequent phosphorylation in step II.

- **Option 3 (Only III and IV):**
     - Neither step III nor step IV (3-phosphoglyceric acid → 2-phosphoglyceric acid) involves phosphorylation that results in ATP formation. Step IV is an isomerization reaction where 3-phosphoglyceric acid is converted to 2-phosphoglyceric acid, and no phosphate transfer to ADP occurs.

- **Option 4 (Only I and IV):**
     - While step I involves phosphorylation and ATP formation, step IV does not involve any phosphorylation event that results in ATP production. As mentioned earlier, step IV is an isomerization reaction without ATP generation.

In conclusion, Option 1 is the correct answer because it includes the two steps in glycolysis where substrate-level phosphorylation occurs, resulting in the formation of ATP. Steps I and II are the only ones that involve the transfer of phosphate groups to ADP, leading to ATP synthesis, which is why Option 1 is the correct choice.