Question

Cyanide is a potent poison because it irreversibly blocks cytochrome \(a_{3}\). What effect would its action have on the rest of the electron transport chain? What would be the redox state (reduced or oxidized) of ubiquinone in the presence of cyanide?

Short answer

Cyanide, by blocking cytochrome \(a_{3}\), would stop the electron flow in the ETC, preventing the transfer of electrons to the final acceptor - oxygen. This halt, further upstream, would result in ubiquinone staying in its reduced state - ubiquinol.

Step-by-step solution

Understand the Mechanism for Cytochrome \(a_{3}\)

Cytochrome \(a_{3}\) is a vital part of Electron Transport Chain (ETC) situated in the inner mitochondrial membrane and plays a crucial role in the final steps of electron transfer to oxygen, the final electron acceptor. It's an important component of Complex IV or Cytochrome c Oxidase.

Effect of Cyanide on Cytochrome \(a_{3}\) and Electron Transport Chain

Cyanide inhibits cytochrome \(a_{3}\) by binding to its active site and blocking the transfer of electrons to oxygen. As a result, the ETC gets backed up because the electrons cannot be transported further down the chain. Thus, the energy production through oxidative phosphorylation is halted, leading to cell death.

Understanding Ubiquinone and its Redox State

Ubiquinone (Coenzyme Q) is another component of the ETC, functioning largely in Complexes I and II, where it accepts electrons and is reduced to ubiquinol. From there, ubiquinol transfers its electrons to Complex III, which then transfers to cytochrome c, and then finally to Complex IV and Oxygen.

Redox State of Ubiquinone in Presence of Cyanide

Since cyanide blocks the electron flow at cytochrome \(a_{3}\), the entire ETC up to this point becomes backed up with electrons. As a result, ubiquinone, which acts just upstream, would also remain in its reduced state ubiquinol, as it cannot unload its electrons.

Key concepts

Electron Transport Chain

The electron transport chain (ETC) is a fundamental pathway in cellular respiration, critical for producing energy in the form of ATP. It is located within the inner mitochondrial membrane. The process involves transferring electrons through a series of complexes, namely Complex I, II, III, and IV, ultimately ending with the reduction of oxygen to water.

During the ETC, electrons are donated by NADH and FADH₂, which are products of other metabolic pathways like glycolysis and the Citric Acid Cycle. These electrons flow through the complexes via various carriers. As they flow, energy is released, which pumps protons across the mitochondrial membrane, creating a proton gradient.

This gradient powers ATP synthase to produce ATP, hence the name oxidative phosphorylation. However, if any part of the chain is interrupted, as in the case of cyanide binding to cytochrome a₃, electron flow halts. This leads to a reduction in ATP production, which can be fatal for cells.

Cytochrome c Oxidase

Cytochrome c oxidase, also known as Complex IV of the electron transport chain, is pivotal in the final electron transfer and is the last complex within the chain. It plays a direct role in converting oxygen into water, using the electrons transported by cytochrome c.

This complex consists of multiple subunits, including cytochrome a₃, which contains the active site. It is at this site that oxygen accepts the electrons to facilitate water formation. As electrons pass through, protons are also pumped across the membrane contributing to the gradient used for ATP production.

Cyanide poisoning becomes lethal because it binds to cytochrome a₃, blocking the active site, and stopping electron flow. This cessation means the inability to reduce oxygen into water, halting the entire ETC, leading to cellular energy failure.

Ubiquinone Redox State

Ubiquinone, also known as Coenzyme Q, serves a crucial intermediary role in the electron transport chain. It acts as a mobile electron carrier within the inner mitochondrial membrane, shuttling electrons between Complex I and II to Complex III. During its electron transport duties, ubiquinone is reduced to ubiquinol.

In a healthy functioning chain, ubiquinol then moves electrons downstream to Complex III where they continue towards oxygen at Complex IV. However, in the presence of inhibitors like cyanide, which block cytochrome a₃, electrons become backed up in the chain.

Consequently, ubiquinone remains in its reduced state (ubiquinol), unable to pass electrons downstream. This reduced state signifies a bottleneck effect as more electrons are unable to move forward, halting ATP production and energy generation.