9. Iron-sulphur clusters (Fe-S) are the key prosthetic groups that carry electrons in of the below EXCEPT.
(1) NADH – CoQ reductase
(2) Succinate – CoQ reductase
(3) Cytochrome c oxidase
(4) COQH2 – Cytochrome C reductase

Iron-sulfur (Fe-S) clusters are essential prosthetic groups that play a critical role in electron transfer within the mitochondrial electron transport chain (ETC). These clusters consist of iron and inorganic sulfur atoms coordinated by cysteine residues in proteins and facilitate rapid electron transfer through redox reactions.

Role of Iron-Sulfur Clusters in the Electron Transport Chain

Fe-S clusters are integral components of several ETC complexes:

• Complex I (NADH–CoQ reductase): Contains multiple Fe-S clusters that transfer electrons from NADH to ubiquinone (CoQ).

• Complex II (Succinate–CoQ reductase): Contains Fe-S clusters that facilitate electron transfer from succinate to CoQ.

• Complex III (CoQH2–Cytochrome c reductase): Contains the Rieske iron-sulfur protein, which has a [2Fe-2S] cluster critical for electron transfer to cytochrome c.

These clusters enable efficient electron flow through the complexes by cycling between reduced and oxidized states.

Exception: Cytochrome c Oxidase (Complex IV)

• Complex IV (Cytochrome c oxidase) is the terminal enzyme of the ETC, transferring electrons from cytochrome c to molecular oxygen, forming water.

• Unlike Complexes I, II, and III, Complex IV does not contain iron-sulfur clusters. Instead, it contains heme groups and copper centers as its redox-active prosthetic groups.

Summary Table

Conclusion

Iron-sulfur clusters are key electron carriers in Complexes I, II, and III of the mitochondrial electron transport chain. The exception is Cytochrome c oxidase (Complex IV), which does not contain Fe-S clusters but relies on heme and copper centers for electron transfer.

Final Answer:
(3) Cytochrome c oxidase