Oxidative Phosphorylation Overview

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Questions and Answers

How many protons are pumped by Complex III for every two electrons transferred?

8 protons

4 protons (correct)

2 protons

6 protons

Which of the following statements about cytochromes is incorrect?

Cytochromes can be membrane-bound or diffusible.

Heme is a type of protein. (correct)

Cytochromes contain a heme group.

Cytochromes can function as single electron carriers.

What is the primary function of cytochrome oxidase (Complex IV)?

Transfer electrons from NADH to oxygen

Pump protons into the intermembrane space

Transfer electrons from cytochrome c to oxygen (correct)

Reduce oxygen to hydrogen peroxide

Which of the following best describes the electron transfer process in cytochrome oxidase?

Electrons are transferred one at a time from cytochrome c to oxygen. Signup and view all the answers

Which of the following intermediates may remain tightly bound during the incomplete reduction in Complex IV?

Hydrogen peroxide Signup and view all the answers

What is the primary role of the proton gradient established across the inner membrane of the mitochondria?

To drive the synthesis of ATP from ADP and Pi Signup and view all the answers

How many ATP molecules are synthesized for every two electrons donated by NADH?

2.5 Signup and view all the answers

Which shuttle system is utilized for NADH generated in the cytoplasm specifically by skeletal muscle and brain tissues?

Glycerol 3-P shuttle Signup and view all the answers

What is the net yield of ATP from the complete oxidation of one glucose molecule, considering all pathways?

30 or 32 ATP Signup and view all the answers

Which of the following complexes does not directly transport protons during electron transport?

Complex II Signup and view all the answers

What components contribute to the electrochemical energy stored in the proton-motive force?

Both chemical and electrical potential energy Signup and view all the answers

Which of the following statements about FADH2 is correct in the context of ATP synthesis?

It generates 1.5 ATP when donating electrons Signup and view all the answers

In which location within a mitochondrion does ATP synthesis occur through ATP synthase?

Mitochondrial matrix Signup and view all the answers

What is the primary function of the electron transport chain (ETC) in mitochondria?

To transfer electrons and pump protons to generate ATP Signup and view all the answers

Which complex in the electron transport chain directly accepts NADH?

Complex I Signup and view all the answers

How do FADH2 and NADH differ in their role in the electron transport chain?

FADH2 donates electrons to Complex II, while NADH donates to Complex I Signup and view all the answers

What is the result of the transfer of electrons through the electron transport chain?

Pumping of protons into the intermembrane space Signup and view all the answers

What is the role of ATP synthase in oxidative phosphorylation?

To generate ATP using the energy from proton flow Signup and view all the answers

Which of the following statements about redox potentials in the electron transport chain is correct?

Complex IV has the highest reduction potential among the complexes Signup and view all the answers

Which substance serves as the final electron acceptor in the electron transport chain?

O2 Signup and view all the answers

What is the approximate energy stored in the reduced cofactors after the oxidation of 1 mole of glucose?

$2564 ext{ kJ/mol}$ Signup and view all the answers

Study Notes

Oxidative Phosphorylation Learning Objectives

Explain the significance of oxidative phosphorylation
Understand electron flow and proton pumping in the electron transport chain (ETC) complexes
Recognize initial and final electron donors/acceptors in each ETC complex
Explain ATP formation by ATP synthase
Recognize two different shuttle systems for cytosolic NADH to enter the ETC

Oxidative Phosphorylation Details

Glycolysis and the TCA cycle produce very little ATP directly (substrate-level phosphorylation)
Glycolysis: 2 ATPs; TCA cycle: 2 ATPs per glucose
Remaining energy stored in reduced cofactors (10 NADH and 2 FADH2 per glucose)
Overall reaction: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (ΔG°' = -2870 kcal/mol)
Oxidation of reduced co-factors releases energy:
- NADH + H⁺ + 1/2 O₂ → NAD⁺ + H₂O (ΔG°' = -220 kcal/mol)
- FADH₂ + 1/2 O₂ → FAD + H₂O (ΔG°' = -182 kcal/mol)
Oxidation of one glucose mole stores 2564 kJ of energy (about 90% of total under standard conditions)

NAD+/NADH and NADP+/NADPH

Diffusible 2-electron carriers
Accept or donate one hydride ion (H⁻) = (1 proton + 2 electrons)

FAD/FADH₂ and FMN/FMNH₂

Bound to enzymes as prosthetic groups
Accept or donate one H⁺ + one electron at a time

Oxidation of Fatty Acids and Amino Acids

Oxidation of fatty acids and some amino acids produce acetyl-CoA and NADH/FADH₂

The Mitochondrion

Aerobic oxidation of biomolecules occurs within the mitochondrion
Structure includes outer membrane, inner membrane, cristae, matrix, and intermembrane space

Electron Transport Chain (ETC) in Mitochondria

Electron flow is favorable from donors with lower reduction potential (more negative E values) to acceptors with higher reduction potential.
ETC consists of four integral membrane protein complexes (I-IV)

Summary of Electron Flow in Inner Membrane

Diagram showing the flow of electrons from NADH to oxygen through complexes I, II, III, and IV. The table shows the standard reduction potentials for each complex's substrates.

Standard Reduction Potentials

Table of standard reduction potentials for various ETC components and complexes

NADH Dehydrogenase (Complex I)

Cofactors: FMN, iron-sulfur clusters
Transfers electrons from NADH to ubiquinone (Q)
Ubiquinol (QH₂) carries electrons through the inner membrane to Complex III
Overall reaction: NADH + H⁺ + Q → NAD⁺ + QH₂
Proton pumping occurs, moving protons from the matrix to the intermembrane space

FMN

Protein-bound co-factor
Accepts or donates one H⁺ + one electron at a time
Exists in oxidized (FMN), radical (FMNH•), reduced (FMNH₂) forms

Iron-Sulfur (Fe-S) Clusters

Single electron carriers
Each iron (Fe) is coordinated by four sulfur atoms (S)
Four cysteine residues from a protein contribute to the cluster
Can have a single Fe coordinated by four cysteine residues

Q (or CoQ)/QH₂

Diffusible through inner membrane
Accepts or donates one electron and one proton at a time

Succinate Dehydrogenase (Complex II)

Only membrane-bound enzyme in the TCA cycle
Contains internal electron transfer cofactors (FAD and Fe-S centers)
QH₂ carries electrons through inner membrane to Complex III
No proton pumping occurs

Cytochrome bc₁ Complex (Complex III)

Electron carriers: Fe-S centers, cytochrome b and cytochrome c₁
Transfers electrons from ubiquinol (QH₂) to cytochrome c
Complex III pumps four protons per two electrons transferred

Cytochromes

Proteins containing a heme group (which coordinates an Fe(II/III) atom)
- Heme a: within complex IV
- Heme b: within complex III
- Heme c: within complexes III and IV

The Q Cycle in Complex III

Diagram and description of the Q cycle, including rounds 1 and 2.
The cycle transports electrons and pumps protons

Cytochrome Oxidase (Complex IV)

Electron carriers include two copper ions and two heme A groups (cytochrome a proteins).
Transfers electrons from cytochrome c to oxygen.
Four cytochromes c (reduced) + O₂ 4 cytochrome c (oxidized) + 2 H₂O
Incompletely reduced intermediates (e.g. peroxide, hydroxyl radicals) remain bound until complete reduction to water

Electron Transport and Proton Pumps

Overall schematic showing electron flow through complexes I, II, III, and IV, alongside proton pumping.

Proton Gradient

Protein gradient across the inner membrane
Energy released when protons flow down gradient drives ATP synthesis

ATP Synthesis by ATP Synthase

ATP synthase (FoF₁ complex) is in inner membrane.
Protons flow through Fo unit down the gradient, driving ATP synthesis by the F₁ unit (ATPase) from ADP and Pi
For every 2 electrons donated by NADH, 2.5 ATPs are synthesized. For every 2 electrons donated by FADH₂, 1.5 ATPs are synthesized.

Shuttle Systems for NADH Generated in Cytoplasm

Inner mitochondrial membrane is impermeable to NADH.
- Glycerol 3-phosphate shuttle: prevalent in skeletal muscle and brain
- Malate-aspartate shuttle: prevalent in liver, kidney, and heart
These shuttles ferry reducing equivalents (NADH equivalents) across the membrane.

Net Profit of Aerobic Metabolism

Summary of ATP yield from complete glucose oxidation (30-32 ATPs) - accounting for different NADH shuttle systems.

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Description

This quiz explores the significance and mechanisms of oxidative phosphorylation. It covers the electron transport chain, ATP formation by ATP synthase, and the role of various electron donors and acceptors. Dive into the details of energy production from glucose and learn about the shuttle systems for NADH.