An autopsy done on a woman who committed suicide reveals that she has high level

Question

An autopsy done on a woman who committed suicide reveals that she has high levels of hydrogen cyanide gas in her blood. The metabolic poison in hydrogen cyanide gas is cyanide, a toxin which is deadly within minutes of inhalation or ingestion. After doing further testing on the woman, the following information is revealed:

Blood and tissue oxygen levels are normal
Blood and tissue glucose levels are normal
Pyruvate levels are normal
Cytoplasmic NADH and NAD+ levels are normal
Cytoplasmic ATP levels are normal
Acetyl CoA levels are normal
Mitochondrial NADH and NAD+ levels are normal
Mitochondrial FADH2 and FADH levels are normal
Mitochondrial ATP levels are low

A hypothesis is developed: cyanide inhibits an enzyme in the Krebs cycle.

Based on the information given, is the hypothesis supported or unsupported? Thoroughly explain your answer using the specific information listed, and propose and explain an alternate hypothesis (if necessary).

Explanation / Answer

Answer: Unsupported. Why? Because mitochondrial NADH and FADH2 levels are both normal. The Krebs cycle uses acetyl-CoA (also normal levels) to produce NADH and FADH2 for the electron transport chain. Since these levels are normal, this cycle must be working fine. Also, cyanide is a poisonous inhibitor for specifically complex 4 of the electron transport chain. This prevents additional pumping of protons into the intermembrane space for a proton gradient generation. Less proton gradient means less ATP being generated.

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