The mitochondrial respiratory chain and oxidative phosphorylation

Question 1

What is the role of the outer membrane of the mitochondrion?

Answer 1

The outer membrane of the mitochondrion is freely permeable to small molecules and ions, allowing their easy passage into and out of the mitochondrion.

Question 2

What is the role of the inner membrane of the mitochondrion?

Answer 2

The inner membrane of the mitochondrion is impermeable to small molecules and ions, including H+ (protons). It houses the electron transport chain and ATP synthase, which are crucial for oxidative phosphorylation and ATP production.

Question 3

What is the function of Complex I (NADH-Ubiquinone Oxidoreductase) in the electron transport chain?

Answer 3

Complex I is responsible for the transfer of electrons from NADH to ubiquinone (coenzyme Q) in the electron transport chain. The overall reaction is:
NADH + H+ + Q -> NAD+ + QH2
During this process, electrons from NADH are passed through a series of Fe-S (iron-sulfur) centers to ubiquinone, resulting in the formation of QH2.

Question 4

How are the electrons transduced into H+ pumping in Complex I?

Answer 4

The exact mechanism by which the flow of electrons in Complex I leads to H+ pumping is not yet fully understood.

Question 5

What happens to QH2 after it is formed in Complex I?

Answer 5

QH2, the reduced form of ubiquinone, diffuses into the lipid bilayer of the inner membrane.

Question 6

How do electrons pass through Complex I?

Answer 6

Electrons pass through Complex I one at a time, moving sequentially through a series of Fe-S (iron-sulfur) centers to reach ubiquinone.

Question 7

What is the function of Complex II (Succinate Dehydrogenase) in the electron transport chain?

Answer 7

Complex II, also known as Succinate Dehydrogenase, is a membrane-bound enzyme of the citric acid cycle. Unlike Complex I, it does not pump protons. It is responsible for transferring electrons from succinate to FAD (flavin adenine dinucleotide).

Question 8

How are electrons transferred through Complex II?

Answer 8

Electrons from succinate are transferred through a series of Fe-S (iron-sulfur) centers in Complex II to reach ubiquinone (coenzyme Q) and form reduced ubiquinol (QH2).

Question 9

What is the role of ubiquinone (Coenzyme Q) in the electron transport chain?

Answer 9

Ubiquinone, also known as Coenzyme Q, is a lipid-soluble molecule. It can accept one or two electrons, becoming ubiquinol (QH2). It is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and shuttles electrons between other less mobile electron carriers. It plays a central role in coupling electron flow.

Question 10

How does ubiquinol (QH2) serve as an entry point for electrons into the electron transport chain?

Answer 10

Ubiquinol (QH2) serves as an entry point for electrons into the electron transport chain from pathways other than Complex I and II. It acts as a shuttle, allowing electrons from other sources to enter the electron transport chain and contribute to ATP synthesis.

Question 11

What is the role of heme b in the electron transport chain?

Answer 11

Heme b is not part of the electron transport pathway but serves as a protective component. It prevents stray electrons from forming damaging reactive oxygen species.

Question 12

What is the function of Complex III (cytochrome bc1 complex) in the electron transport chain?

Answer 12

Complex III, also known as the cytochrome bc1 complex, is responsible for transferring electrons from ubiquinol (QH2) to cytochrome c in the electron transport chain.

Question 13

How does ubiquinone shuttle between two binding sites in Complex III?

Answer 13

Ubiquinone (Q) can shuttle between two binding sites in Complex III: QN (matrix side) and QP (intermembrane side). This allows for the transfer of protons and electrons during the electron transport process.

Question 14

What is the structure of Complex III?

Answer 14

Complex III is made up of two identical proteins, each consisting of 11 subunits.

Question 15

How many protons are transported from the matrix to the intermembrane space by Complex III?

Answer 15

In the process of transferring electrons from ubiquinol (QH2) to cytochrome c, Complex III transports four more protons (H+) from the matrix to the intermembrane space.

Question 16

What is the role of cytochrome c in the electron transport chain?

Answer 16

Cytochrome c is a soluble protein located in the intermembrane space. It contains a heme-C prosthetic group and can accept and donate electrons. It accepts an electron from Complex III and donates it to Complex IV.

Question 17

What is the function of the heme-C prosthetic group in cytochrome c?

Answer 17

The heme-C prosthetic group in cytochrome c can accept and donate one electron, as it undergoes a reversible change between the Fe2+ and Fe3+ states.

Question 18

What is the function of Complex IV (Cytochrome Oxidase) in the electron transport chain?

Answer 18

Complex IV, also known as Cytochrome Oxidase, is responsible for the final transfer of electrons to oxygen (O2) in the electron transport chain.

Question 19

What happens during the electron transfer process in Complex IV?

Answer 19

During the electron transfer process in Complex IV, oxygen (O2) binds to heme a3 and accepts donated electrons. The electrons flow through heme a to the Fe-Cu center (heme a3 and CuB). Two cytochrome c molecules each donate one electron to a copper center (CuA).

Question 20

What is the result of the electron transfer in Complex IV?

Answer 20

The delivery of two more electrons to Complex IV creates O22-, which combines with four H+ from the matrix to produce H2O (water).

Question 21

What is the summary of the flow of electrons through the respiratory chain?

Answer 21

For every 1 NADH molecule, 10 H+ are pumped across the inner mitochondrial membrane. The reduction potentials drive the transfer of electrons along the electron transport chain. The reduction potentials for various components involved in the electron transfer are as follows:

NAD+ + H+ + 2e- -> NADH (E’0 -0.320 V)
Ubiquinone + 2H+ + 2e- -> Ubiquinol + H2 (E’0 +0.045 V)
½ O2 + 2H+ + 2e- -> H2O (E’0 +0.816 V)
Complex IV (Cu2+) + e- -> Complex IV (Cu+) (E’0 +0.340 V)
Cytochrome c (Fe3+) + e- -> Cytochrome c (Fe2+) (E’0 +0.254 V)
Complex III (Fe3+) + e- -> Complex III (Fe2+) (E’0 +0.22 V)

Question 22

What is the purpose of the synthesis of ATP in the mitochondria?

Answer 22

The synthesis of ATP in the mitochondria is the ultimate purpose of the electron transport chain and oxidative phosphorylation. ATP is the primary energy currency of the cell and is required for various cellular processes and functions.

Question 23

What are the three specific systems in the inner mitochondrial membrane that play a role in ATP synthesis?

Answer 23

Transport systems for ADP and Pi: These systems facilitate the transport of ADP (adenosine diphosphate) and Pi (inorganic phosphate) into the mitochondrial matrix, where ATP synthesis occurs.
ATP synthase: ATP synthase is an enzyme complex located in the inner mitochondrial membrane that is responsible for the synthesis of ATP from ADP and Pi.
Transport system for ATP: This system allows for the transport of ATP out of the mitochondrial matrix and into the cytosol, where it can be utilized by the cell.

Question 24

What is the driving force for ATP synthesis in the mitochondria?

Answer 24

The driving force for ATP synthesis in the mitochondria is the proton motive force, which is created by the difference in H+ (proton) concentration across the inner mitochondrial membrane. This difference in concentration is generated by the proton pumping activity of the electron transport chain. The proton motive force includes both a chemical gradient (via the difference in proton concentration, or ΔpH) and an electrical gradient.

Question 25

How does the proton motive force drive the synthesis of ATP using ATP synthase?

Answer 25

he proton (H+) motive force, which includes both the chemical gradient (ΔpH) and the electrical gradient, provides the energy needed for ATP synthase to catalyze the synthesis of ATP from ADP and Pi. The flow of protons through ATP synthase drives the rotation of the enzyme complex, which in turn leads to the formation of ATP.

Question 26

What is the role of the β subunits in ATP synthase (F1)?

Answer 26

The β subunits in ATP synthase (F1) have catalytic sites for ATP synthesis. There are three β subunits in total.

Question 27

What are the different types of subunits in F1 of ATP synthase?

Answer 27

F1 comprises five different types of subunits: α3, β3, γ, δ, and ε. Together, they form a complex of nine subunits.

Question 28

How are the α and β subunits arranged in ATP synthase?

Answer 28

The α and β subunits are arranged alternately, like segments of an orange. They form a knob-like structure with the γ subunit running up the center. The δ subunit interacts with the two ‘b’ subunits of Fo and helps hold the α/β complex steady. The F1 subunits also interact with each other as a key part of the ATP synthesis process.

Question 29

What is the binding-change model for ATP synthesis in the β subunits of ATP synthase?

Answer 29

The binding-change model explains the mechanism of ATP synthesis in the β subunits of ATP synthase. Each β subunit takes turns catalyzing the synthesis of ATP. The β subunit starts in a conformation for binding ADP and Pi (β-ADP conformation), then changes conformation to tightly bind the product ATP (β-ATP conformation), and finally changes conformation again to have a low affinity for ATP (‘β-empty’ conformation), allowing ATP to be released.

Question 30

What is the rotational catalysis in ATP synthesis?

Answer 30

Rotational catalysis refers to the rotation of the shaft (g subunit) of ATP synthase driven by the proton-motive force. The rotation of the shaft causes conformational changes in the β subunits of ATP synthase, leading to the binding and release of ADP and ATP during ATP synthesis.

Question 31

How do the F1 subunits interact with each other in ATP synthesis?

Answer 31

The F1 subunits interact with each other in a coordinated manner. If one subunit adopts the β-empty conformation, its neighbor on one side must adopt the β-ADP conformation, while the other neighbor must adopt the β-ATP conformation. This coordination ensures the proper binding and release of ADP and ATP during the catalytic cycle.

Question 32

How much ATP is produced from the transport of protons (H+) in the electron transport chain?

Answer 32

For the full synthesis of 1 ATP, a total of 4H+ are used. This includes the 3H+ used in ATP synthase (as seen earlier in the binding-change model) and an additional 1H+ used in the transport of phosphate (Pi), ATP, and ADP.

Question 33

How is NADH transported into the mitochondria for oxidation?

Answer 33

NADH from glycolysis, which is in the cytosol, needs to be transported into the mitochondrial matrix for oxidation. This transport can occur via two mechanisms: the malate-aspartate shuttle and the glycerol-3-phosphate shuttle.

Question 34

What are the ATP yield and energy output from the complete oxidation of one glucose molecule?

Answer 34

The ATP yield from the complete oxidation of one glucose molecule can vary depending on the specific conditions and shuttle mechanisms involved. However, in general, it is estimated that the complete oxidation of one glucose molecule can yield up to 36-38 ATP molecules.

Question 35

What are the two shuttle mechanisms involved in the transport of NADH into the mitochondria?

Answer 35

The two shuttle mechanisms involved in the transport of NADH into the mitochondria are the Malate-Aspartate Shuttle and the Glycerol-3-Phosphate Shuttle.

Question 36

What is the purpose of uncoupling reagents?

Answer 36

Uncoupling reagents are substances that disrupt the coupling between electron flow and ATP synthesis in mitochondria. They dissipate the proton gradient by transporting H+ back into the mitochondrial matrix, bypassing ATP synthase. This severs the link between electron flow and ATP synthesis, with the released energy being dissipated as heat.

Question 37

Can uncoupling occur naturally in the body?

Answer 37

Yes, uncoupling can occur naturally in the body. For example, UCP1 (thermogenin) is found in brown adipose tissue and has a specific H+ channel through which the proton gradient can be dissipated. This natural uncoupling process releases energy as heat.

Question 38

What is the role of brown adipose tissue (BAT) in thermogenesis?

Answer 38

Brown adipose tissue is specialized for heat generation. It contains high numbers of mitochondria, which give it a brown appearance. The mitochondria in brown adipose tissue contain thermogenin (UCP-1), which allows for the dissipation of the proton gradient and the release of energy as heat. Brown adipose tissue and thermogenesis are particularly important in newborns.

Question 39

Where are the locations of brown adipose tissue in the body?

Answer 39

Brown adipose tissue is found in specific locations in the body, including between the shoulder blades, surrounding the kidneys, in the neck area, and along the spinal cord.

Question 40

What is an exogenous uncoupling agent?

Answer 40

An exogenous uncoupling agent is a substance that disrupts the coupling between electron transport and oxidative phosphorylation in mitochondria, resulting in the dissipation of the proton gradient. One example of an exogenous uncoupling agent is 2,4-dinitrophenol (DNP).

Question 41

What is the role of 2,4-dinitrophenol (DNP) as an exogenous uncoupling agent?

Answer 41

2,4-dinitrophenol (DNP) is a soluble weak acid that can carry H+ across the inner mitochondrial membrane, dissipating the proton gradient. This uncouples electron transport from oxidative phosphorylation and leads to an increase in metabolic rate.

Question 42

What are the potential dangers or side effects of using 2,4-dinitrophenol (DNP)?

Answer 42

The use of 2,4-dinitrophenol (DNP) as a diet drug or for increasing metabolic rate has been associated with serious risks. Overdosing on DNP has resulted in several deaths, and toxicity can lead to liver damage, respiratory acidosis, and hyperthermia.

Question 43

Can you provide a summary of 2,4-dinitrophenol (DNP) as an exogenous uncoupling agent?

Answer 43

2,4-dinitrophenol (DNP) is an exogenous uncoupling agent that disrupts the coupling between electron transport and oxidative phosphorylation. It carries H+ across the inner mitochondrial membrane, dissipating the proton gradient. However, its use as a diet drug or metabolic enhancer can be extremely dangerous and has resulted in fatalities and severe side effects.