Question

What is the approximate \(P / O\) ratio that can be expected if intact mitochondria are incubated in the presence of oxygen, along with added succinate?

Short answer

The approximate P/O ratio for succinate is 1.5.

Step-by-step solution

Identify the Substrate

Succinate is the substrate in the given problem. It is part of the Citric Acid Cycle and donates electrons via FADH2.

Determine the Electron Transport Chain Entry Point

Succinate donates electrons to Complex II in the electron transport chain, which then transfers them to Complex III via Coenzyme Q.

Assess Proton Pumping

Electrons from FADH2 bypass Complex I and enter the electron transport chain at Complex II. Complex II does not pump protons, but Complex III and Complex IV do. Thus, fewer protons are pumped across the mitochondrial membrane compared to substrates that start with Complex I.

Calculate ATP Yield per FADH2

Each FADH2 results in approximately 1.5 ATP in oxidative phosphorylation because the proton pumping leads to fewer protons being translocated compared to NADH, which yields approximately 2.5 ATP.

Establish the P/O Ratio

The P/O ratio, which is the number of ATP (P) molecules generated per oxygen (O) molecule reduced, is therefore approximately 1.5 for succinate because it enters electron transport chain as FADH2.

Key concepts

electron transport chain

The electron transport chain (ETC) is a series of protein complexes located in the inner mitochondrial membrane. Its main function is to transfer electrons from electron donors to electron acceptors through redox reactions. This process releases energy, which is used to pump protons across the mitochondrial membrane, creating a proton gradient.

The complexes involved are:

• Complex I: Also known as NADH-ubiquinone oxidoreductase, it accepts electrons from NADH and pumps protons into the intermembrane space.
• Complex II: Also known as succinate-ubiquinone oxidoreductase, it accepts electrons from FADH2 generated in the Citric Acid Cycle. It is unique because it does not pump protons.
• Complex III: Also known as cytochrome bc1 complex, it transfers electrons from ubiquinol to cytochrome c and pumps protons.
• Complex IV: Also known as cytochrome c oxidase, it transfers electrons to oxygen, the final electron acceptor, forming water. It also pumps protons.

The electron transport chain is essential in the process of oxidative phosphorylation, where the energy from electrons is used to generate ATP.

FADH2

FADH2 (flavin adenine dinucleotide) is a redox coenzyme that plays a vital role in cellular respiration, specifically within the electron transport chain. It is generated in the Citric Acid Cycle during the conversion of succinate to fumarate by the enzyme succinate dehydrogenase.

FADH2 differs from NADH in several ways:

• Entry Point: FADH2 donates electrons directly to Complex II, bypassing Complex I. This means it enters the electron transport chain downstream of NADH.
• Proton Pumping: Because Complex II does not pump protons, FADH2 contributes to a smaller proton gradient compared to NADH. This results in fewer ATP molecules being produced.
• ATP Yield: Each FADH2 molecule typically yields about 1.5 ATP molecules, whereas NADH yields approximately 2.5 ATP molecules.

The role of FADH2 is crucial in the overall efficiency of oxidative phosphorylation, even though its ATP yield is comparatively lower.

ATP yield

ATP yield refers to the number of ATP molecules generated from the electron transport chain and oxidative phosphorylation. The yield depends on the type of electron donor and the efficiency of the proton gradient established during the process.

Key points to understand ATP yield:

• NADH vs. FADH2: NADH, which donates electrons to Complex I, leads to more proton pumping and ultimately more ATP generation per molecule (approximately 2.5 ATP). In contrast, FADH2 leads to less proton pumping and yields around 1.5 ATP per molecule due to its entry at Complex II.
• Proton Gradient: The proton gradient generated by the electron transport chain is used by ATP synthase to produce ATP. The more protons pumped across the membrane, the higher the ATP yield.
• P/O Ratio: This ratio indicates the number of ATP molecules produced per oxygen molecule reduced in the electron transport chain. For FADH2, the P/O ratio is approximately 1.5, due to its reduced contribution to the proton gradient.

Understanding ATP yield helps in grasping the efficiency of cellular respiration and the important roles that different cofactors and electron donors play in energy production.