Cyanide causes irreversible inhibition of cytochrome oxidase. This is because it
A) Combines with iron
B) Destroys tertiary structure
C) Attaches to copper
D)All of the above

Hint:Cytochrome oxidase is an enzyme which has a role in the Electron Transport System (ETS). It is generally found in the mitochondria of the eukaryotes.

 Complete answer:
Let us know a little about the electron transport system. The ETS is basically a cluster of proteins which is responsible for transferring the electrons from the membrane of mitochondria to the inside of mitochondria for producing ATP. It has 3 main steps,
        >Generate a proton motive force
         >ATP synthesis
        >Oxidative phosphorylation

Option a: In presence of cyanide, cytochrome oxidase undergoes irreversible inhibition. As cytochrome oxidase is inhibited it does not bind with iron inside the enzyme which leads to prevention of transport of electrons from cytochrome oxidase to oxygen. Therefore, this is the correct answer.
Option b: In presence of cyanide, there is no change made to the tertiary structure of cytochrome oxidase. Therefore, this is the incorrect option.
Option c: In presence of cyanide, cytochrome oxidase is inhibited therefore it cannot bind to copper. Therefore, this is the incorrect option.
Option d: As one of the options is correct, this is the incorrect option.

Thus, the correct option is (a) Combines with iron.

Note:Cytochrome oxidase catalyzes the transfer of electrons from ferricytochrome to oxygen, and the latter is converted to water.