Which of the following ratio is correct regarding dehydrogenation : decarboxylation : substrate - level phosphorylation steps in the TCA cycle ?

Options

A. `1:1:2`
B. `3:2:1`
C. `4:3:1`
D. `4:2:1`

Answer

To solve the question regarding the ratio of dehydrogenation, decarboxylation, and substrate-level phosphorylation steps in the TCA cycle, we can follow these steps:

### Step 1: Identify Dehydrogenation Steps
1. **Dehydrogenation** occurs in the TCA cycle at the following steps:
- Conversion of isocitrate to alpha-ketoglutarate (NAD+ is reduced to NADH)
- Conversion of alpha-ketoglutarate to succinyl-CoA (NAD+ is reduced to NADH)
- Conversion of succinate to fumarate (FAD is reduced to FADH2)
- Conversion of malate to oxaloacetate (NAD+ is reduced to NADH)

**Total Dehydrogenation Steps**: 4

### Step 2: Identify Decarboxylation Steps
2. **Decarboxylation** occurs in the TCA cycle at the following steps:
- Conversion of isocitrate to alpha-ketoglutarate (CO2 is released)
- Conversion of alpha-ketoglutarate to succinyl-CoA (CO2 is released)

**Total Decarboxylation Steps**: 2

### Step 3: Identify Substrate-Level Phosphorylation Steps
3. **Substrate-Level Phosphorylation** occurs in the TCA cycle at the following step:
- Conversion of succinyl-CoA to succinate (GTP is produced)

**Total Substrate-Level Phosphorylation Steps**: 1

### Step 4: Calculate the Ratios
4. Now we can establish the ratio of dehydrogenation : decarboxylation : substrate-level phosphorylation:
- Dehydrogenation: 4
- Decarboxylation: 2
- Substrate-Level Phosphorylation: 1

Therefore, the ratio is 4:2:1.

### Conclusion
The correct ratio regarding dehydrogenation : decarboxylation : substrate-level phosphorylation steps in the TCA cycle is **4:2:1**.

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