Substrate level phosphorylation occurs during

Options

A. Citric acid `rarr` alpha - ketoglutaric acid
B. Malic acid `rarr` oxalo - acetic acid
C. alpha - ketoglutaric acid `rarr` Succinctly Co A
D. Succinyl-co A `rarr` Succinic acid

Answer

To solve the question regarding substrate level phosphorylation, we need to identify the specific steps in the tricarboxylic acid (TCA) cycle where this process occurs. Here’s a step-by-step breakdown:

### Step 1: Understanding Substrate Level Phosphorylation
Substrate level phosphorylation is a process where a phosphate group is directly transferred from a substrate molecule to ADP to form ATP (or GTP). This process occurs in specific reactions during cellular respiration.

**Hint:** Remember that substrate level phosphorylation is different from oxidative phosphorylation, which occurs in the electron transport chain.

### Step 2: Identifying the TCA Cycle
The TCA cycle, also known as the citric acid cycle or Krebs cycle, includes several key steps where various substrates are converted into products. The cycle begins with the combination of acetyl-CoA and oxaloacetate to form citrate.

**Hint:** Familiarize yourself with the main substrates and products of the TCA cycle to identify where phosphorylation might occur.

### Step 3: Analyzing the Given Steps
The question provides several steps in the TCA cycle:
1. Citrate to α-ketoglutarate
2. α-ketoglutarate to succinyl-CoA
3. Succinyl-CoA to succinate
4. Malate to oxaloacetate

We need to determine which of these steps involves substrate level phosphorylation.

**Hint:** Look for steps that involve the conversion of a high-energy substrate that can directly transfer a phosphate group.

### Step 4: Identifying the Correct Step
Among the steps listed:
- The conversion of **succinyl-CoA to succinate** is where substrate level phosphorylation occurs. During this reaction, a molecule of GTP (or ATP) is synthesized directly from the energy released in the reaction.

**Hint:** Focus on the step where a high-energy thioester bond in succinyl-CoA is broken, leading to the formation of GTP or ATP.

### Step 5: Conclusion
The correct answer to the question is that substrate level phosphorylation occurs during the conversion of **succinyl-CoA to succinate**.

**Final Answer:** Substrate level phosphorylation occurs during the conversion of succinyl-CoA to succinate in the TCA cycle.