Question

Which of the following depicts the sequence of cytochromes in ETS?

• A. Cyt a, cyt b, cyt c, cyt a${}_3$
• B. Cyt b, cyt c, cyt a, cyt a${}_3$
• C. Cyt b, cyt a, cyt a33, cyt c
• D. Cyt b, cyt c, cyt a33, cyt a

Answer 1

Answer: (B)

Cyt b, cyt c, cyt a, cyt a${}_3$

Electrons from NADH produced in the mitochondrial matrix during citric acid cycle are oxidised by an NADH dehydrogenase (complex I) and electrons are then transferred to ubiquinone located within the inner membrane.

Ubiquinone also receives reducing equivalents via $FAD{H}_2$ (complex II) that is generated during oxidation of succinate in the citric acid cycle.

The reduced ubiquinone (ubiquinol) is then oxidised with the transfer of electrons to cytochrome c via cytochrome bc1 complex (complex III). Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV. Complex IV refers to cytochrome c oxidase complex containing cytochromes a and a${}_3$ and two copper centres.

Thus, the sequence of cytochromes in respiratory electron transport system is b, c, a, a${}_3$.