Which one of the following enzymes is absent in electron transport system ?

Options

A. NADH dehydrogenase
B. Cytochrome C-oxidas
C. Succinate Q-reductase
D. `G_e` phosphate dehydrogenase

Answer

To determine which enzyme is absent in the electron transport system (ETS), we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Electron Transport System (ETS)**:
- The ETS is a series of complexes located in the inner mitochondrial membrane that facilitate the transfer of electrons from electron donors to electron acceptors via redox reactions.

2. **Identify the Enzyme Complexes Present in ETS**:
- The main enzyme complexes present in the ETS include:
- **NADH dehydrogenase (Complex I)**: Also known as NADH ubiquinone reductase.
- **Succinate ubiquinone reductase (Complex II)**: This complex is involved in the oxidation of succinate to fumarate.
- **Ubiquinone cytochrome c reductase (Complex III)**: This complex transfers electrons from ubiquinol to cytochrome c.
- **Cytochrome c oxidase (Complex IV)**: This complex transfers electrons from cytochrome c to oxygen.

3. **Examine the Options Given**:
- The options provided are:
- A) NADH dehydrogenase
- B) Cytochrome C oxidase
- C) Succinate ubiquinone reductase
- D) Glucose 6-phosphate dehydrogenase

4. **Evaluate Each Option**:
- **NADH dehydrogenase**: Present in ETS (Complex I).
- **Cytochrome C oxidase**: Present in ETS (Complex IV).
- **Succinate ubiquinone reductase**: Present in ETS (Complex II).
- **Glucose 6-phosphate dehydrogenase**: This enzyme is involved in the pentose phosphate pathway and is NOT part of the ETS.

5. **Conclusion**:
- The enzyme that is absent in the electron transport system is **Glucose 6-phosphate dehydrogenase**. Therefore, the correct answer is **Option D**.

### Final Answer:
**D) Glucose 6-phosphate dehydrogenase**