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
Cyanide is toxic because it 
Select and explain. 
A. Inhibits cytochrome oxidase, B. Forms cyan meth Hb, C. Inhibits ATP carrier in mitochondria, D. Inhibits Na-K-ATP ase
A: Inhibits cytochrome oxidase
Answer: A: Inhibits cytochrome oxidase 
Explanation: Site-III (Complex IV) (CYTOCHROME OXIDASE ) * Cyanide Blocks electron transfer from cyt b to c1 Inhibits terminal * H2S * Azide transfer of electrons to molecular O2 * Co (Carbon monoxide): Inhibits Cyt. oxidase by combining with O2 binding site. It can be reversed by illumination with light.Ref: MN Chatterjea Textbook of Medical Biochemistry, 8th Edition