Which of the following ratio is correct regarding dehydrogenation : decarboxylation : substrate - level phosphorylation steps in the TCA cycle ?
1 : 1 : 2
3 : 2 : 1
4 : 3 : 1
4 : 2 : 1

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 will analyze each process step by step.

### Step 1: Identify the processes in the TCA cycle
The TCA cycle, also known as the citric acid cycle or Krebs cycle, involves several key biochemical reactions, including dehydrogenation, decarboxylation, and substrate-level phosphorylation.

### Step 2: Count the dehydrogenation steps
1. **Isocitrate to Alpha-Ketoglutarate**: This step involves dehydrogenation.
2. **Alpha-Ketoglutarate to Succinyl CoA**: This step also involves dehydrogenation.
3. **Succinate to Fumarate**: This is another dehydrogenation step.
4. **Malate to Oxaloacetate**: This step is catalyzed by malate dehydrogenase and involves dehydrogenation.

**Total dehydrogenation steps = 4**

### Step 3: Count the decarboxylation steps
1. **Isocitrate to Alpha-Ketoglutarate**: This step involves decarboxylation.
2. **Alpha-Ketoglutarate to Succinyl CoA**: This step also involves decarboxylation.

**Total decarboxylation steps = 2**

### Step 4: Count the substrate-level phosphorylation steps
1. **Succinyl CoA to Succinate**: This step involves substrate-level phosphorylation.

**Total substrate-level phosphorylation steps = 1**

### Step 5: Compile the ratios
Now that we have the counts:
- Dehydrogenation: 4
- Decarboxylation: 2
- Substrate-level phosphorylation: 1

The ratio of dehydrogenation : decarboxylation : substrate-level phosphorylation is:
**4 : 2 : 1**

### Conclusion
The correct answer is **4 : 2 : 1**.