Dehydrogenases and Redox Potential

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Questions and Answers

Which of the following statements about peroxidases is true?

They are used to form hydrogen peroxide from water.

Their main function is to increase the levels of superoxide.

They are found exclusively in plant cells.

They are formed when reduced flavins are reoxidized. (correct)

What role does superoxide dismutase serve in aerobic organisms?

It catalyzes the formation of hydrogen peroxide.

It directly reduces oxygen to water.

It produces reactive oxygen species.

It protects against oxidative stress by converting superoxide. (correct)

Where are the components of the electron transport chain located?

In the cytoplasm

In the inner mitochondrial membrane (correct)

Within the outer mitochondrial membrane

In the nuclear membrane

Which complex in the electron transport chain is not a proton pump?

Complex II Signup and view all the answers

What is the final electron acceptor in the electron transport chain?

Oxygen Signup and view all the answers

Which component of peroxidases uses hydrogen peroxide as both an electron donor and acceptor?

Catalase Signup and view all the answers

Glutathione peroxidase primarily protects what part of the cell?

RBC membrane Signup and view all the answers

What is the primary function of the electron transport chain?

Collect and transport reducing equivalents Signup and view all the answers

What is the primary function of oxygenases in biochemical reactions?

Catalyzing the incorporation of oxygen into the substrate molecule Signup and view all the answers

Which type of oxygenase incorporates both atoms of molecular oxygen into the substrate?

Dioxygenase Signup and view all the answers

What does a more negative Eo value indicate about a substance's ability to participate in redox reactions?

It is a better reductant. Signup and view all the answers

Which of the following statements accurately describes dehydrogenases?

They transfer hydrogen from one substrate to another. Signup and view all the answers

What type of oxygenase is also known as a mixed-function oxidase?

Monooxygenase Signup and view all the answers

In terms of redox potential, which of the following statements is true?

The more positive the Eo, the better the oxidant. Signup and view all the answers

What are the products formed by the action of monooxygenases?

Hydroxylated products Signup and view all the answers

Which of the following examples is a dioxygenase?

Homogentisate dioxygenase Signup and view all the answers

What type of coenzymes do pyridine-linked dehydrogenases utilize?

Derivatives of niacin. Signup and view all the answers

What is the primary function of dehydrogenases in cell metabolism?

To facilitate ATP generation via oxidation. Signup and view all the answers

Which of the following molecules is not considered an electron carrier in the inner mitochondrial membrane?

L-Tryptophan Signup and view all the answers

What distinguishes dioxygenases from monooxygenases?

The number of oxygen atoms incorporated into the substrate Signup and view all the answers

Which characteristic of a reductant is indicated by a more negative Eo?

It is better at donating electrons. Signup and view all the answers

What advantage do reducing equivalents provide in oxidative processes?

They allow oxidative processes to occur in the absence of oxygen. Signup and view all the answers

Which of these is not one of the electron carriers found in the electron transport assembly?

Hydroxyanthranilate Signup and view all the answers

What role do components of the electron transport chain (ETC) play concerning dehydrogenases?

They facilitate the transfer of reducing equivalents. Signup and view all the answers

Which complex in the electron transport chain is NOT considered a proton pump?

Complex II Signup and view all the answers

What is the role of CoQ in the electron transport chain?

It transfers electrons between complexes. Signup and view all the answers

Which of the following components serves as the prosthetic group in complex I?

FMN Signup and view all the answers

What type of proteins are found in complexes I, II, and III that are involved in electron transfer?

Iron-sulfur proteins Signup and view all the answers

During oxidative phosphorylation, which molecule is phosphorylated to form ATP?

ADP Signup and view all the answers

Which iron state is reduced to donate electrons to cytochrome c?

Ferrous iron (Fe2+) Signup and view all the answers

What is cytochrome b's role in complex III?

It transfers electrons from CoQH2. Signup and view all the answers

Which vitamin is a precursor for the flavoprotein FAD?

Vitamin B2 Signup and view all the answers

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

It links complex III to complex IV. Signup and view all the answers

Which type of iron is cytochrome b converted back to after being oxidized?

Fe3+ Signup and view all the answers

Which enzyme is NOT an example of a NAD-linked dehydrogenase?

Succinate dehydrogenase Signup and view all the answers

What does FADH2 carry compared to NADH?

Two electrons and two H+ ions Signup and view all the answers

Which of the following correctly characterizes Fe2+ in cytochrome c?

It is oxidized to Fe3+. Signup and view all the answers

What is a characteristic of complex III in the context of the electron transport chain?

It functions as a proton pump. Signup and view all the answers

What products does reduced NADH generate during metabolic pathways?

Electrons and protons Signup and view all the answers

Which enzyme is associated with the beta oxidation pathway?

Hydroxyacyl CoA dehydrogenase Signup and view all the answers

Study Notes

Dehydrogenase

Measures a biological system's tendency to release or accept electrons
Transfers hydrogen from one substrate to another, but not to oxygen
Expressed in volts (Eo)
- More negative Eo: better reductant (electron donor)
- More positive Eo: better oxidant (electron acceptor)

Redox Potential in Mammalian Oxidation Systems

Types of Dehydrogenases:
- Pyridine-linked dehydrogenases: use niacin derivatives as coenzymes
  - Nicotinamide adenine dinucleotide (NAD+)-linked dehydrogenases: involved in oxidative metabolic pathways generating ATP (e.g., peroxidases)
- Flavoprotein dehydrogenases: use flavins (FMN & FAD) as coenzymes
  - FAD-linked dehydrogenases: involved in the citric acid cycle and fatty acid metabolism.
  - FMN-linked dehydrogenases: involved in the electron transport chain.

Catalase

Uses hydrogen peroxide as both electron donor and acceptor to form water
Found in blood, bone marrow, mucous membranes, kidney, and liver
Destroys H2O2 formed by oxidases

Oxygenases

Catalyze the direct transfer and incorporation of oxygen into a substrate molecule
Two Kinds of Oxygenases:
- Dioxygenases: incorporate both atoms of molecular oxygen into the substrate (e.g. homogentisate, L-tryptophan, and hydroxyanthranilate dioxygenases)
- Monooxygenases (Mixed-function oxidases/Hydroxylases): incorporate only one atom of molecular oxygen into the substrate.

Superoxide Dismutase

Protects aerobic organisms against oxygen toxicity
Catalyzes the removal of superoxide by the following reaction: O2.- + O2.- + 2H+ → H2O2 + O2

Electron Transport Chain (Respiratory Chain)

Found in the inner mitochondrial membrane
Collects and transports reducing equivalents (electrons) and directs them to their final reaction with oxygen to form water
Final acceptor of electrons is oxygen
Components:
- Four large protein complexes (I, II, III, IV)
- Three complexes serve as proton pumps (I, III, IV)
- Complex II is not a proton pump
Coupled with oxidative phosphorylation
Components of ETC
- Flavoproteins:
  - Components of complexes I & II
  - FMN: Prosthetic group of NADH dehydrogenase in complex I
  - FAD: Prosthetic group of succinate dehydrogenase in complex II
- Iron-sulfur proteins (non-heme, Fe-S):
  - Found in complexes I, II, and III
  - Participate in single electron transfer reaction
Electron Carriers:
- NAD: Derived from niacin (vitamin B3), active portion: nicotinamide ring, carries 2 electrons but only one H+, carrier of reduced NADH
  - Reduced NADH derived from NAD-linked dehydrogenases:
    - Isocitrate, α-ketoglutarate, & malate dehydrogenases of the Krebs cycle
    - Pyruvate dehydrogenase (linking glycolysis to the TCA cycle)
    - Hydroxyacyl CoA dehydrogenase of the fatty acid beta-oxidation pathway.
- FAD: Derived from riboflavin (vitamin B2), active portion: isoalloxazine ring, carries 2 electrons and 2 H+, carrier of reduced FADH2
  - Reduced FADH2 derived from FAD-linked dehydrogenases:
    - Succinate dehydrogenase of the TCA cycle
    - Alpha-glycerophosphate shuttle dehydrogenase
    - Fatty acyl CoA dehydrogenase of beta oxidation

Complex III

Cytochrome b: Representative cytochrome in complex III. Other cytochromes can be used as carriers
CoQH2 (reduced substrate): Transfers its electron to the ferric iron (Fe3+) of cytochrome b. CoQH2 is oxidized to CoQ upon transferring electrons
Fe3+ is reduced to ferrous iron (Fe2+) which serves as the electron donor and transfers the electron to ferric iron (Fe3+) of cytochrome c.
Fe2+ of cytochrome b converts back to Fe3+ while Fe3+ of cytochrome c is reduced to Fe2+
Cytochrome c links complex III to complex IV
Complex III is a proton pump.

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Description

Explore the roles of dehydrogenases in biological systems, including their function in transferring hydrogen and their significance in oxidation-reduction processes. Learn about different types of dehydrogenases, such as pyridine-linked and flavoprotein dehydrogenases, and their involvement in metabolic pathways. Dive into the intricacies of redox potential measurements expressed in volts.