Cyanide binds to the iron in cytochrome c oxidase (Complex IV) and prevents this enzyme from transferring electrons to oxygen. Describe how this would change

Question

a.     How oxygen is used

b.    The proton gradient

c) ATP levels

Answer 1

Let's take a step back for a moment and look at the two big picture processes associated with Cytochrome c oxidase: The electron transport chain and oxidative phosphorylation.

Cytochrome C oxidase functions in the last step of the electron transport chain. The purpose of the electron transport chain is to pump protons from the inner mitochondrial space to the intermembrane space to generate a proton gradient. The energy needed to pump these protons comes sequential oxidation-reduction reactions, starting with NADH. Each reaction releases a little bit of energy that is used to pump protons across the membrane. Complex IV is unique in that it's redox reaction does not involve transferring electrons to another electron carrier but rather to oxygen in a reaction that can be summarized as: cytochrome c oxidase + 4e- + 4H+ + O₂ -> cytochrome c oxidase + 2H₂O.

In this process, an addition 4 protons are translocated across the membrane as well, sustaining the proton gradient. The proton gradient is needed for synthesis of ATP by ATP synthase. ATP synthase is like a watermill: the kinetic energy of water rushing past the water wheel is harnessed to produce constructive work. Likewise, ATP synthase captures the energy released as protons flow down their electrochemical gradient, similar to a watermill. This captured energy is used to make ATP.

Blockade of this reaction would completely halt the ETC and stop O₂ from being reduced to H₂O, and would lead to the build up of O₂. This would also lead to the dissolution of the proton gradient and would lower cellular ATP levels (due to stopping the flow of protons across ATP synthase).