Can someone explain this to me please?

. Cyanide can combine with and deactivate cytochrome a and cytochrome a3. In our bodies, however, cyanide tends to react first with the iron in hemoglobin to make it impossible for oxygen to bind to hemoglobin. Either way, cyanide poisoning has the same effect, it inhibits the synthesis of ATP. Explain why this is so using the information in the paragraph and what you have learned about cellular respiration.