This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory chain, Subject name: Biochemistry
Which complex of ETC reacts directly with O2? 
Select and explain. 
A. I, B. II, C. III, D. IV
D: IV
Answer: D: IV 
Explanation: Complex IV: Cyt-c Oxidase The system functions: * As proton pump * Catalyses transfer of electrons to molecular O2 to form H2O. This is the terminal component of ETC. It catalyses the transfer of electrons from Cyt-c to molecular O2 Cyt-a, Cu++ ions and Cyt-a3.Ref: MN Chatterjea Textbook of Medical Biochemistry, 8th edition, page no: 142