Question

Complex I A. transfers electrons directly from NADH to ubiquinone. B. can transfer electrons from the \(\mathrm{FADH}_{2}\) of succinate dehydrogenase as well as from NADH. C. does not contain FeS centers. D. transfers electrons coupled to the transport of four protons across the membrane. E. is considered a mobile electron carrier because it can travel along the outer face of the inner membrane.

Short answer

A. Complex I transfers electrons directly from NADH to ubiquinone. B. Complex I can transfer electrons from the \(\mathrm{FADH}_{2}\) of succinate dehydrogenase as well as from NADH. C. Complex I does not contain FeS centers. D. Complex I transfers electrons coupled to the transport of four protons across the membrane. E. Complex I is considered a mobile electron carrier because it can travel along the outer face of the inner membrane. Answer: A. Complex I transfers electrons directly from NADH to ubiquinone.

Step-by-step solution

Statement A

Complex I transfers electrons directly from NADH to ubiquinone. This statement is true. Complex I, also known as NADH:ubiquinone oxidoreductase, transfers electrons from NADH to ubiquinone, a mobile electron carrier molecule. This process contributes to the generation of a proton gradient across the inner mitochondrial membrane, enabling the production of ATP. Since this statement is true, our search for the correct answer stops here. However, let's analyze the other statements to confirm that they are not accurate descriptions of Complex I.

Statement B

Complex I can transfer electrons from the \(\mathrm{FADH}_{2}\) of succinate dehydrogenase as well as from NADH. This statement is false. Complex I accepts electrons specifically from NADH molecules, not \(\mathrm{FADH}_{2}\). It is Complex II, also called succinate-ubiquinone oxidoreductase, that accepts electrons from \(\mathrm{FADH}_{2}\), which is generated by succinate dehydrogenase in the TCA (Krebs) cycle.

Statement C

Complex I does not contain FeS centers. This statement is also false. Complex I contains several iron-sulfur (FeS) centers that play an essential role in the transfer of electrons from NADH to ubiquinone. These FeS centers are essential for achieving the appropriate electron transfer within the molecule and maintaining the energy state necessary for proton transport across the membrane.

Statement D

Complex I transfers electrons coupled to the transport of four protons across the membrane. This statement is true. During the process of transferring electrons from NADH to ubiquinone, Complex I uses the energy released to transport four protons across the inner mitochondrial membrane. This proton transfer contributes to the generation of a proton gradient that is later utilized by ATP synthase to produce ATP.

Statement E

Complex I is considered a mobile electron carrier because it can travel along the outer face of the inner membrane. This statement is false. Complex I is actually a large integral membrane protein complex within the inner mitochondrial membrane. It is not mobile and does not travel along the membrane. The mobile electron carrier mentioned in the question is ubiquinone (CoQ), which accepts electrons from Complex I and transports them to Complex III. In conclusion, the correct answer is: A. Complex I transfers electrons directly from NADH to ubiquinone.

Key concepts

NADH to Ubiquinone Electron Transfer

The transfer of electrons from NADH to ubiquinone is a critical step in the body's production of cellular energy, and it occurs within a gargantuan protein complex called Complex I in the mitochondrial electron transport chain. This complex, known scientifically as NADH:ubiquinone oxidoreductase, is the gateway for electrons derived from NADH, which itself is a product of various metabolic processes like the Krebs cycle.

• Complex I captures high-energy electrons from NADH.
• It then passes these electrons to ubiquinone, also known as coenzyme Q10.
• Ubiquinone becomes reduced to ubiquinol post electron capture.
• This electron transfer is coupled with the transport of protons across the inner mitochondrial membrane.

The energy from the electrons is used to move protons from the mitochondrial matrix to the intermembrane space, creating a proton gradient essential for ATP synthesis. As a stationary complex, Complex I plays a foundational role in the electron transport chain by ensuring that the high-energy electrons from NADH are effectively handed over to ubiquinone, which can then mobilize them to the next complex.

Mitochondrial Electron Transport Chain

The mitochondrial electron transport chain is a series of complexes located within the inner membrane of mitochondria, and it's where the magic of cellular respiration happens. This chain is composed of multiple complexes, numbered I to IV, and two mobile electron carriers: ubiquinone and cytochrome c.

• Electrons enter the chain via NADH and FADH2 at Complex I and Complex II, respectively.
• These electrons are then methodically passed from one complex to the next, down an energy gradient.
• Each complex acts as an electron relay system, with the energy released during electron transfer utilized to power proton pumping across the membrane.
• The final electron acceptor is oxygen, which combines with electrons and protons to form water.

Through this intricate process, which involves carefully coordinated redox reactions, the chain ultimately converts the energy stored in electron carriers into a form that can be harnessed for ATP synthesis. Essential proteins like the FeS centers and cytochromes within these complexes facilitate the precise flow of electrons, ensuring the efficiency and safety of this vital cellular function.

Proton Gradient and ATP Synthesis

The ultimate goal of the mitochondrial electron transport chain is ATP synthesis, which is fueled by the proton gradient established by the complexes within the chain. This gradient is also known as the proton motive force and operates on a conceptually straightforward principle: it stores energy in the form of an electrochemical gradient.

• The gradient is established when protons are pumped from the mitochondrial matrix into the intermembrane space, creating a high concentration of protons on one side of the membrane.
• The difference in proton concentration creates a gradient with potential energy, similar to water behind a dam.
• This energy drives protons back into the matrix through ATP synthase, a complex enzyme that uses the flow of protons to synthesize ATP from ADP and inorganic phosphate.

This process, known as chemiosmosis, underscores the vital link between the energy from electrons and the phosphorylation of ADP. Without the establishment of this gradient, the energy from foodstuffs could not be transformed into the universal energy currency of the cell—ATP—making the function of proton gradients indispensable for life.