Question

A researcher is interested in isolating fully reduced cytochrome a. All of the following inhibitors can be used for this purpose EXCEPT A. cyanide. B. sodium azide. C. rotenone. D. carbon monoxide.

Short answer

C. Rotenone.

Step-by-step solution

- Understand Cytochrome a

Cytochrome a is a component of the electron transport chain in mitochondria. It is involved in the final steps of electron transfer and is fully reduced when it gains electrons completely.

- Identify Inhibitors Affecting Cytochrome a

Inhibitors can block the electron transport chain, causing cytochrome a to remain in its reduced state. Inhibitors of Complex IV can fully reduce cytochrome a.

- Determine the Effect of Each Inhibitor

A. Cyanide inhibits Complex IV, fully reducing cytochrome a. B. Sodium azide also inhibits Complex IV, fully reducing cytochrome a. C. Rotenone inhibits Complex I, which does not directly affect the state of cytochrome a. D. Carbon monoxide inhibits Complex IV, fully reducing cytochrome a.

- Identify the Exception

Rotenone inhibits Complex I, not Complex IV. Therefore, it does not isolate fully reduced cytochrome a.

Key concepts

electron transport chain

The electron transport chain (ETC) is a series of protein complexes located in the inner mitochondrial membrane. It plays a crucial role in cellular respiration by transferring electrons from electron donors to electron acceptors through redox reactions.
Eventually, this process leads to the formation of water by combining oxygen and electrons.
The ETC is composed of several complexes: Complex I (NADH: ubiquinone oxidoreductase), Complex II (succinate dehydrogenase), Complex III (cytochrome bc1 complex), and Complex IV (cytochrome c oxidase).
Electrons move down this chain and release energy which is used to pump protons across the membrane, creating a proton gradient.
This gradient is then used by ATP synthase to produce ATP, the energy currency of the cell.
Cytochrome a is part of Complex IV, playing a critical role in the final stages of the chain.

mitochondrial complex inhibitors

Mitochondrial complex inhibitors are substances that disrupt the normal function of the mitochondrial electron transport chain.
By blocking specific complexes, they can halt the flow of electrons, affecting cellular respiration and ATP synthesis.
Different inhibitors target different complexes:

• Rotenone inhibits Complex I.
• Antimycin A inhibits Complex III.
• Cyanide, sodium azide, and carbon monoxide inhibit Complex IV.

These inhibitors can be used experimentally to study the function of the electron transport chain or to isolate specific biochemical states, like the fully reduced state of cytochrome a.

reduction of cytochrome a

Reduction of cytochrome a occurs when it accepts electrons and goes into a fully reduced state.
Cytochrome a is part of Complex IV in the electron transport chain and participates in the final electron transfer steps.
When one of the Complex IV inhibitors is introduced, the electron flow is blocked,
leading to the accumulation of fully reduced cytochrome a.
Fully reduced means that the cytochrome a has received all the electrons it can accept.
Substances like cyanide, sodium azide, and carbon monoxide can effectively reduce cytochrome a by inhibiting Complex IV.

Complex IV inhibitors

Complex IV inhibitors block the function of the last complex in the electron transport chain, preventing the transfer of electrons to oxygen.
This action stops the formation of water and the consequent proton pumping required for ATP synthesis.
Common Complex IV inhibitors include cyanide, sodium azide, and carbon monoxide.
These inhibitors bind to the active site of cytochrome c oxidase, the main enzyme in Complex IV.
This binding makes it impossible for oxygen to accept electrons, effectively halting the entire ETC.

• Cyanide effectively stops electron transfer by binding to the ferric form of cytochrome c oxidase.
• Sodium azide operates similarly to cyanide.
• Carbon monoxide competes with oxygen for binding sites, also stopping the electron flow.

Understanding these inhibitors is vital in biochemistry, as it highlights how the disruption of one complex can affect cellular respiration and energy production.