As a forensic pathologist, you have just completed an autopsy of a poisoning victim. After a thorough examination, you conclude that the victim died of cyanide poisoning. You know that cyanide binds to the cytochrome oxidase complex, and therefore list the official cause of death as suffocation due to cyanide exposure. However, if you wanted to provide a more technical explanation as to the cause of death, which process was specifically inhibited directly by cyanide?

Question

A.  The reduction of NAD+
B.  The oxidation of FADH2
C.  All proton pumping into the intermembrane space
D.  The formation of water from oxygen
E.  ATP synthesis

Clarify Question
What is the key concept addressed by the question?
What type of thinking is required?

  Gather Content
What do you already know about cellular respiration and cytochrome oxidase? What other information is related to the question?
   Choose Answer
  Do you have all the information needed to determine which specific process is directly inhibited by cyanide?

  Reflect on Process
  Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

D.  The formation of water from oxygen

Clarify Question
What is the key concept addressed by the question?
        · The question asks you to provide a technical explanation as to which cellular respiration process facilitated by cytochrome oxidase would be inhibited by cyanide poisoning.
What type of thinking is required?
        · You are being asked to break down, or analyze, the processes associated with cytochrome oxidase that would be inhibited by cyanide poisoning.

  Gather Content
What do you already know about cellular respiration and cytochrome oxidase? What other information is related to the question?
        · You know that the stages of cellular respiration include glycolysis, pyruvate oxidation, the Krebs cycle, and electron transport. Cytochromes, particularly cytochrome oxidase, are found in the electron transport chain. Essentially what this means is that cyanide works to inhibit the electron transport chain, but how? What specific processes in the electron transport chain would be most affected by inhibiting cytochrome oxidase?
   Choose Answer
  Do you have all the information needed to determine which specific process is directly inhibited by cyanide?
        · Determining the right answer for this question is fairly easy if you know the right clues to look for. As before, let’s see if we can eliminate some of the incorrect options and increase our chances of getting the correct answer.
        · FADH2 does in fact become oxidized, and specifically at complex II of the electron transport chain. However this option is not connected to cytochrome oxidase. Nor is the reduction of NAD+ related to cytochrome oxidase. We know that proton pumping occurs using multiple complexes in the electron transport chain, so that seems an unlikely option also. ATP synthesis occurs via ATP synthase, and as long as there is a proton gradient, that enzyme will continue to make ATP. Probably not a suitable choice either.
        · So what exactly does cytochrome oxidase do? In fact, that complex takes the reduced-energy electrons that have been passed through the electron transport chain via the redox reactions and reduces atmospheric oxygen to form water. There’s your correct answer.

  Reflect on Process
  Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        · Answering this question correctly depended on your ability to analyze the specific processes associated with the electron transport chain to determine which is involved with cytochrome oxidase and which would be affected by cyanide poisoning.         · If you got the correct answer, well done! If you got an incorrect answer, were you able to see that the available answers involved something having to do with the electron transport chain? Were you able to associate cytochrome oxidase with the end of the electron transport chain? Were you able to infer that cyanide poisoning would affect the conversion of atmospheric oxygen into water?