Chapter 18- Oxidative Phosphorylation

Cellular respiration

Drives ATP formation by transferring electrons to molecular oxygen

Respiratory chain (electron transport chain)

Four large protein complexes that are embedded in the inner mitochondrial membrane

Oxidative phosphorylation

Set of electron-transfer reactions that captures the energy of high-energy electrons from NADH and FADH₂

• takes place in the electron transport chain

• ultimately generates ATP and reduces oxygen to water

• cellular respiration

cellular respiration

The generation of high-transfer potential electrons by the citric acid cycle, their flow through the respiratory chain, and the accompanying synthesis of ATP

Coupling of electron carrier oxidation and ADP phosphorylation

The flow of electrons from reduced carriers such as NADH is highly exergonic

• NADH + 1/2O₂ + H⁺ → H₂O + NAD⁺

  • Favorable

• complexes of the electron-transport chain use released energy to pump protons out of the mitochondrial matrix

  • generates a pH gradient and a transmembrane electron potential that creates a proton-motive force that is used to power the synthesis of ATP

    • ADP + P_i + H⁺ → ATP + H₂O

      • unfavorable

Mitochondria structure

• The citric acid cycle, the electron-transport chain, and ATP synthesis occur in the mitochondria.

• Mitochondria have two membranes (which creates two distinct internal compartments)

  • an outer membrane with porins

  • an extensive, highly folded inner membrane

• intermembrane space (IM space) = compartment between the outer and inner membranes

• matrix = compartment bounded by the inner membrane

Where do most citric acid and fatty acid oxidation reactions take place?

Mitochondrial matrix

Where does Oxidative phosphorylation take place?

Inner mitochondrial membrane

What does the respiratory chain consist of?

Four complexes: three proton pumps and a physical link to the citric acid cycle

• electrons flow from NADH to O₂ through three protein complexes embedded in the inner mitochondria membrane

• electron flow through complexes I, III, and IV is highly exergonic and power generation of a proton gradient

• Complex I, III, and IV are proton pumps

Complex II

contains succinate dehydrogenase from the citric acid cycle

• electrons from this FADH₂ enter the electron-transport chain at Q-cytochrome c oxidoreductase

• It does not pump protons

Pathway of electrons through the complexes

Complexes I, III, and IV appear to be associated in a supramolecular complex

• facilitates the rapid transfer of substrate

• Prevents the release of reaction intermediates

• KNOW GRAPH

cytochromes

electron-transferring proteins that contain a heme prosthetic group

Electron transfer through NADH-Q Oxidoreductase is coupled to…

Proton transfer reactions

What is the entry point for electrons from FADH₂ of flavoproteins

Ubiquinol

Complex II-

cytochrome c oxidase (Complex IV)

Catalyzes the transfer of four electrons from four reduced molecules of cytochrome c to O₂

does cytochrome c oxidase catalyze the reduction of molecular oxygen to water?

Yes

Two components of proton transport by cytochrome c oxidase

• Four chemical protons reduce O2 to two H2O.

• Cytochrome c oxidase uses free energy from this reduction to pump 4 H+ from the matrix into the intermembrane space

Electrons flow via two pathways through the electron-transport chain

How are toxic reactive oxygen species limited in the mitochondria?

• scavenged by protective enzymes

  • partial reduction of O₂ generates highly reactive oxygen derivatives called reactive oxygen species (ROS)

  • ROS are implicated in aging and a growing list of diseases

  • ROS include superoxide ion, peroxide ion, and hydroxyl radical

  • cytochome c oxidase does not release ROS by holding O₂ tightly between Fe and Cu ions

What powers the synthesis of ATP?

A proton gradient

• flow of NADH to O₂ is an exergonic process

• Synthesis of ATP is an endergonic process

ATP synthase (Complex V)

A molecular assembly in the inner mitochondrial membrane that carries out the synthesis of ATP

Chemiosmotic hypothesis

Proposes that electron transport and ATP synthesis are coupled by a proton gradient across the inner mitochondrial membrane

• suggested that ATP formation is powered by a proton gradient

Proton-Motive Force

The energy-rich unequal distribution of protons across a membrane

• consists of a chemical gradient and a change gradient

• powers the synthesis of ATP

• proton-motive force (delta p) = chemical gradient (delta pH) + charge gradient

How does ATP Synthase assist in the formation of cristae?

Cristae formation allows proton pumps to localize the proton gradient in the vicinity of the synthases, which are located at the tips of the cristae

• enhances efficiency of ATP synthesis

Oxidative phosphorylation overview

ATP-ADP translocase (adenine nucleotide translocase, ANT)

Specific transport protein that enables the exchange of cytoplasmic ADP for mitochondrial ATP

• constitutes 15% of the protein of the inner mitochondrial membrane

ATP and ADP bind to ANT without Mg²⁺

Inhibition of ANT leads to the inhibition of cellular respiration

Entry of ADP into mitochondria is coupled to the exit of ATP by ATP-ADP translocase

ATP-ADP translocase catalyzes the exchange of entry of ADP and ATP

• the translocase contains a single nucleotide-binding site that alternately faces the matrix and the cytoplasmic sides of the membrane

Regulation of cellular respiration

• The ATP needs of the cell determine the rate of the respiratory pathways and their components

• molecules of ATP formed when glucose is completely oxidized to CO₂

• Electrons do not flow through the electron-transport
  chain unless ADP is available to be converted into ATP

• The regulation of the rate of oxidative phosphorylation by ADP level is called respiratory (or acceptor) control.

• At low ADP levels:

  • NADH and FADH2 are not consumed by the electron- transport chain.

  • the citric acid cycle slows because there is less NAD+ and FAD to feed the cycle.

Regulated uncoupling

Leads to the generation of heat

• nonshivering thermogenesis = the ability to generate heat without using shivering by uncoupling oxidative phosphorylation from ATP synthesis

  • occurs in mitochondria-rich brown adipose tissue in animals

  • activated in response to a drop in the core body temperature

• uncoupling protein 1 (UCP-1; also called thermogenin) = an inner mitochondria membrane protein that transports protons from the intermembrane space to the matrix with the assistance of fatty acids

  • generates heat by transporting protons without the synthesis of ATP

    • energy of the proton gradient, normally captured as ATP is released as heat as the protons flow through UCP-1 to the mitochondrial matrix