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Chapter 15: Problem 31

Complex I, succinate dehydrogenase, acyl-CoA dehydrogenase, and glycerol-3-phosphate dehydrogenase (see Fig. 15.11) are all flavoproteins; that is, they contain an FMN or FAD prosthetic group. Explain the function of the flavin group in these enzymes. Why are the flavoproteins ideally suited to transfer electrons to ubiquinone?

Short Answer

Expert verified
Flavin groups in flavoproteins facilitate electron transport by undergoing redox reactions. They efficiently transfer electrons to ubiquinone due to their ability to change oxidation states.

Step by step solution

01

Understanding Flavoproteins

Flavoproteins are enzymes that have a flavin moiety as a prosthetic group, either flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD). These flavins are important for the redox reactions that occur in these enzymes.
02

Identifying the Function of Flavins

The flavin group in these enzymes acts primarily as an electron carrier. It undergoes reversible redox reactions, where it can accept two hydrogen atoms (equivalent to two electrons and two protons), getting reduced, and then donate these electrons and protons to other molecules.
03

Connecting Flavins to Electron Transfer

Flavoproteins like Complex I, succinate dehydrogenase, acyl-CoA dehydrogenase, and glycerol-3-phosphate dehydrogenase participate in the electron transport chain. The flavins' ability to exist in different oxidation states (oxidized, semiquinone, reduced) makes them efficient electron carriers in these processes.
04

Explain Electron Transfer to Ubiquinone

Flavoproteins are well-suited to transfer electrons to ubiquinone because their flavin groups can accept electrons and protons and can pass those electrons to the next carrier in the electron transport chain. Ubiquinone (coenzyme Q) is a small, lipid-soluble molecule in the mitochondrial membrane that acts as an electron shuttle, receiving electrons from these flavoproteins.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Electron Transport Chain
The electron transport chain (ETC) is a crucial part of cellular respiration, occurring within the inner mitochondrial membrane. Simply put, it's like a series of tiny conveyor belts, moving electrons down an energy gradient. As electrons are transferred through the chain, energy is released and used to pump protons across the membrane, creating a proton gradient. This gradient is then used by ATP synthase to produce ATP, the cell's main energy currency.
One of the unique features of the ETC is its role in oxidative phosphorylation, linking the oxidation of nutrients to the production of ATP. The chain comprises a series of protein complexes and small molecules, each with a higher affinity for electrons than the last, driving electrons forward from donors like NADH and FADH鈧� to oxygen, which forms water upon receiving these electrons. Without this chain, our cells would not be able to efficiently generate the energy needed for various functions.
FMN and FAD
FMN (flavin mononucleotide) and FAD (flavin adenine dinucleotide) are key players in the functioning of flavoproteins. These cofactors are derived from riboflavin (vitamin B2) and are vital in electron transport chains where they facilitate redox reactions.
  • FMN serves as the prosthetic group in some enzymes, like Complex I of the electron transport chain, helping shuttle electrons through the complex.
  • FAD, meanwhile, is integral to, for example, succinate dehydrogenase in the citric acid cycle and the electron transport chain.
FMN and FAD can accept two electrons and two protons, transitioning from their oxidized to reduced states. This ability to undergo structural changes allows them to function effectively as electron carriers, making them crucial for transferring electrons within cells.
Redox Reactions
Redox reactions, also known as oxidation-reduction reactions, are vital to the processes within flavoproteins. In these reactions, one substance loses electrons (oxidation) while another gains electrons (reduction). This electron exchange facilitates energy release, essential for cellular processes.
Flavoproteins, due to their FMN or FAD groups, are particularly efficient at mediating redox reactions. These cofactors can exist in multiple redox states:
  • Fully oxidized state
  • Semi-reduced (semiquinone) state
  • Fully reduced state
These states allow flavoproteins to transfer electrons step-wise, preventing unwanted reactions that could damage cells. This controlled electron transfer is crucial because it ensures that energy is released in manageable amounts, closely linked to ATP production.

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Most popular questions from this chapter

The sequence of events in electron transport was elucidated in part by the use of inhibitors that block electron transfer at specific points along the chain. For example, adding rotenone (a plant toxin) or amytal (a barbiturate) blocks electron transport in Complex I; antimycin A (an antibiotic) blocks electron transport in Complex III; and cyanide \(\left(\mathrm{CN}^{-}\right)\)blocks electron transport in Complex IV by binding to the \(\mathrm{Fe}^{2+}\) in the \(\mathrm{Fe}-\mathrm{Cu}\) binuclear center. a. What happens to oxygen consumption when these inhibitors are added to a suspension of respiring mitochondria? b. What is the redox state of the electron carriers in the electron transport chain when each of the inhibitors is added separately to the mitochondrial suspension?

At one time, it was believed that myoglobin functioned simply as an oxygen- storage protein. New evidence suggests that myoglobin plays a much more active role in the muscle cell. The phrase myoglobinfacilitated oxygen diffusion describes myoglobin's role in transporting oxygen from the muscle cell sarcolemma to the mitochondrial membrane surface. Mice in which the myoglobin gene was knocked out had higher tissue capillary density, elevated red blood cell counts, and increased coronary blood flow. Explain the reasons for these compensatory mechanisms in the knockout mice.

What size \(\mathrm{pH}\) gradient (the difference between \(\mathrm{pH}_{\text {matrix }}\) and \(\mathrm{pH}_{\text {cytasol }}\) ) would correspond to a free energy change of \(19.2 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}\) ? Assume that \(\Delta \Psi=170 \mathrm{mV}\) and \(T=37^{\circ} \mathrm{C}\).

Myoglobin is not confined to muscle cells. Tumor cells, which generally exist in hypoxic (low-oxygen) conditions because of limited blood flow, express myoglobin. How does this adaptation increase the chances of tumor cell survival?

In coastal marine environments, high concentrations of nutrients from terrestrial runoff may lead to algal blooms. When the nutrients are depleted, the algae die and sink and are degraded by other microorganisms. The algal die-off may be followed by a sharp drop in oxygen in the depths, which can kill fish and bottom-dwelling invertebrates. Why do these "dead zones" form?
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