Question

Oxidation-Reduction Reactions Complex I, the NADH dehydrogenase complex of the mitochondrial respiratory chain, promotes the following series of oxidation- reduction reactions, in which \(\mathrm{Fe}^{3+}\) and \(\mathrm{Fe}^{2+}\) represent the iron in iron-sulfur centers, \(\mathrm{Q}\) is ubiquinone, \(\mathrm{QH}_{2}\) is ubiquinol, and \(\mathrm{E}\) is the enzyme: 1\. \(\mathrm{NADH}+\mathrm{H}^{+}+\mathrm{E}-\mathrm{FMN} \rightarrow \mathrm{NAD}^{+}+\mathrm{E}-\mathrm{FMNH}_{2}\) 2\. \(\mathrm{E}-\mathrm{FMNH}_{2}+2 \mathrm{Fe}^{3+} \rightarrow \mathrm{E}-\mathrm{FMN}+2 \mathrm{Fe}^{2+}+2 \mathrm{H}^{+}\) 3\. \(2 \mathrm{Fe}^{2+}+2 \mathrm{H}^{+}+\mathrm{Q} \rightarrow 2 \mathrm{Fe}^{3+}+\mathrm{QH}_{2}\) Sum: \(\mathrm{NADH}+\mathrm{H}^{+}+\mathrm{Q} \rightarrow \mathrm{NAD}^{+}+\mathrm{QH}_{2}\) For each of the three reactions catalyzed by Complex I, identify (a) the electron donor, (b) the electron acceptor, (c) the conjugate redox pair, (d) the reducing agent, and (e) the oxidizing agent.

Short answer

In Reaction 1, NADH is the donor and \(\mathrm{E}-\mathrm{FMN}\) the acceptor. In Reaction 2, \(\mathrm{E}-\mathrm{FMNH}_{2}\) is the donor and \(2 \mathrm{Fe}^{3+}\) the acceptor. In Reaction 3, \(2\mathrm{Fe}^{2+}\) is the donor and Q the acceptor.

Step-by-step solution

Identify Components of Reaction 1

In the reaction \( \mathrm{NADH}+\mathrm{H}^{+}+\mathrm{E}-\mathrm{FMN} \rightarrow \mathrm{NAD}^{+}+\mathrm{E}-\mathrm{FMNH}_{2} \), the electron donor is \( \mathrm{NADH} \) because it donates electrons to \( \mathrm{E}-\mathrm{FMN} \). The electron acceptor is \( \mathrm{E}-\mathrm{FMN} \) as it receives electrons. The conjugate redox pair involves \( \mathrm{NADH}/\mathrm{NAD}^{+} \). Here, \( \mathrm{NADH} \) acts as a reducing agent, and \( \mathrm{E}-\mathrm{FMN} \) is the oxidizing agent.

Identify Components of Reaction 2

In the reaction \( \mathrm{E}-\mathrm{FMNH}_{2}+2 \mathrm{Fe}^{3+} \rightarrow \mathrm{E}-\mathrm{FMN}+2 \mathrm{Fe}^{2+}+2 \mathrm{H}^{+} \), the electron donor is \( \mathrm{E}-\mathrm{FMNH}_{2} \), donating electrons to \( 2 \mathrm{Fe}^{3+} \). The electron acceptor is \( 2 \mathrm{Fe}^{3+} \). The conjugate redox pair is \( \mathrm{FMNH}_{2}/\mathrm{FMN} \). \( \mathrm{E}-\mathrm{FMNH}_{2} \) acts as the reducing agent, and \( 2 \mathrm{Fe}^{3+} \) is the oxidizing agent.

Identify Components of Reaction 3

In the reaction \( 2 \mathrm{Fe}^{2+}+2 \mathrm{H}^{+}+\mathrm{Q} \rightarrow 2 \mathrm{Fe}^{3+}+\mathrm{QH}_{2} \), the electron donor is \( 2 \mathrm{Fe}^{2+} \), donating electrons to \( \mathrm{Q} \). The electron acceptor is \( \mathrm{Q} \) as it receives electrons. The conjugate redox pair is \( \mathrm{Fe}^{2+}/\mathrm{Fe}^{3+} \). \( 2 \mathrm{Fe}^{2+} \) is the reducing agent, and \( \mathrm{Q} \) is the oxidizing agent.

Key concepts

Complex I

Complex I, also known as NADH dehydrogenase, is the largest complex in the mitochondrial electron transport chain. This complex plays a critical role in cellular respiration. It is responsible for transferring electrons from NADH, a product of metabolic pathways, to ubiquinone (Coenzyme Q).
This transfer enables the movement of protons across the mitochondrial membrane. The movement of protons is crucial since it creates an electrochemical gradient, which is later used by other complexes to produce ATP, the cell's energy currency.

• Complex I acts as an entry point for electrons into the electron transport chain.
• It's composed of multiple subunits, each playing a role in the electron transfer process.
• Through a series of redox reactions, this complex kicks off the cellular process to generate energy efficiently.

NADH dehydrogenase

NADH dehydrogenase serves a specific function by catalyzing oxidation-reduction reactions involving NADH.
This enzyme helps in extracting electrons from NADH and transferring them to the electron transport chain. As it performs this task, it catalyzes the conversion of NADH to NAD+.

• As electrons are removed from NADH, energy is released, which is instrumental in driving additional biochemical reactions.
• This process increases the efficiency of ATP production.
• The action of NADH dehydrogenase is essential for maintaining a continuous flow of electrons, which is necessary for ongoing cellular energy production.

Electron Donor

An electron donor is a chemical entity that donates electrons to another compound in a redox reaction. In the context of Complex I, NADH is a prime example acting as an electron donor as it loses electrons to the enzyme's flavoprotein component, FMN.
Electron donors are often referred to as reducing agents because they reduce other molecules by donating electrons to them.

• NADH is an electron-rich molecule crucial for metabolic processes, acting as a primary electron donor in pathways like glycolysis and the citric acid cycle.
• By donating electrons, NADH is converted to its oxidized form, NAD+.
• This donation process is essential for the continuous cycle of energy production.

Electron Acceptor

In redox reactions, the electron acceptor is a substance that acquires electrons from another species. In the reactions involving Complex I, FMN acts as an electron acceptor, receiving electrons from NADH.
Electron acceptors are usually oxidizing agents, as they gain electrons and help oxidize the donor molecule.

• The electron acceptor, FMN, is a critical component in the electron transport chain as it passes the electrons further down the chain.
• Through its ability to accept electrons, FMN becomes reduced, which subsequently allows the redox process to continue efficiently.
• Effective electron acceptation is central to maintaining the function of the mitochondrial respiratory chain.

Conjugate Redox Pair

A conjugate redox pair is formed by two species that can undergo interconversion by the gain or loss of electrons. In biological systems, these pairs are integral to biochemical energy transfer processes. For example, in Complex I, the conjugate redox pair NADH/NAD+ is pivotal.
This pair represents a reversible process where NADH donates electrons and becomes NAD+, and vice versa, during cellular respiration.

• The concept of conjugate pairs helps in understanding the reversible nature of redox reactions.
• The transition between the pairs facilitates the transfer of electrons and energy, underlying many metabolic processes.
• These pairs ensure continuity in redox processes, which is crucial for cellular metabolism and energy efficiency.