The respiratory chain and Oxidative phosphorylation By Dr. Manal Louis Assistant professor of Biochemistry

By the end of this lecture, the student should be able to: Recognize the meaning of oxidation reduction reactions. Determine the concept of redox potential.

3. Identify the components of the respiratory chain. 4. Explain the chemiosmotic theory.

What’s the point? The point is to make ATP! ATP

Diagram of a mitochondrion Double membrane outer membrane inner membrane highly folded cristae that increase the membrane area. enzymes & transport proteins. Inter membrane space fluid-filled space between membranes Mitochondria powerhouse of the cell.

Transport across mitochondrial membrane Small molecules as O2, CO2, NH3 and mono-carboxylic acids are freely permeable across the membrane. Di and tricarboxlycic acids need special transporters. ATP and ADP need special transporters to allow ADP in and ATP out of the mitochondria.

Oxidation Reduction Addition of oxygen or removal of hydrogen or an electron Reduction Addition of an electron or hydrogen or removal of oxygen

Oxidation and reduction reactions are always coupled

Redox Potential E₀ Is the tendency of the reactants in an oxidation–reduction reaction to donate electrons. The higher this tendency, the more is the ∆ G.

The more negative the standard reduction potential of a redox pair, the greater the tendency of that pair to lose electrons.

The more positive the Eo, the greater the tendency of that pair to accept electrons. Therefore, electrons flow from the pair with the more negative Eo to that with the more positive Eo.

Value of measurement of E₀ Prediction of the direction of flow of electrons from the electronegative couples to the electropositive couples. Prediction of the amount of free energy change ∆G by the movement of electrons: the more the difference in redox potential between two redox systems the more the energy is produced.

MCQ The more positive the Eº, the greater the tendency of that pair to: (A) Lose electrons (B) Gain electrons (C) Lose (or) gain electrons (D) Lose and gain electrons

Electron Transport Chain The respiratory chain is a sequence of enzymes and carriers which is located in the inner mitochondrial membrane . It is responsible for the transport of electrons from reducing equivalents (NADH and FADH) to O2 to produce H2O.

Movement of electrons occurs through series of oxidation-reduction reactions from the more electronegative component to the more electropositive ones until it reaches its final eֿ acceptor O2. This movement of electrons releases energy used in ATP synthesis So that’s the point!

Electrons flow downhill Electrons move in steps from carrier to carrier downhill to O2 Electrons move from molecule to molecule until they combine with O & H ions to form H2O It’s like pumping water behind a dam -- if released, it can do work electrons flow downhill to O2 2005-2006

Q: Why should electrons move downhill to reach its final acceptor? A:To allow energy to be released gradually in many steps. If the energy is produced and released in one step this will lead to losing of this energy in the form of heat and cell damage. Q: Why should electrons move downhill to reach its final acceptor?

Complex I It catalyzes oxidation of NADH+H, with reduction of coenzyme Q: NADH + H+ + Q  NAD+ + QH2 it is the most electronegative part of the chain. It is composed of: NADH dehydrogenase -prosthetic groups FMN & several Fe-S centers.

Complex I FMN accept 2 electrons from NADH + H to form FMNH2. Then FMNH2 donates 2 electrons to Fe-S centers. Electrons pass through a series of iron-sulfur centers, and are eventually transferred to coenzyme Q. Coenzyme Q accepts 2 e- and picks up 2 H+ to yield the fully reduced QH2 .

Complex I & Co Q I III Co-Q Cytochrome b c1 e- e- Fe-S FMN QH2 Fe-S NADH Dehydrogenase Fe-S III Cytochrome b c1 Co-Q QH2 e- e- H+ H+ H+

Complex II It catalyzes oxidation of FADH2, with reduction of coenzyme Q. It is composed of: - Succinate Dehydrogenase of the Krebs Cycle. - FAD is the initial e- acceptor. - Fe-S centers.

Complex II FAD is reduced to FADH2 during oxidation of succinate to fumarate. FADH2 is then re-oxidized by transfer of electrons through a series of iron-sulfur centers to CoQ, yielding QH2. The QH2 product may be re-oxidized via complex III.

Succinate Dehydrogenase Complex II & Co Q Fe-S FAD II Succinate Dehydrogenase Fe-S III Cytochrome b c1 Co-Q FADH2 QH2 H+ e-

Coenzyme Q Coenzyme Q works as a collecting point for hydrogen coming from complex 1 and II. Co Q acts as a mobile component of the respiratory chain (lipid soluble compound that can diffuse within the inner membrane). It is very similar in structure to vitamin K and E.

Electrons flow through cytochrome b,c1. Complex III -It accepts electrons from reduced Co Q. Electrons flow through cytochrome b,c1. (arranged in an order of increasing Redox Potential) Electrons are carried on Heme iron which oscillates between Fe+++ and Fe++.

IV III Cytochrome b c1 Cytochromr aa3 e- e- Cu++ Fe-S Cu+ Cytochrome C

Cytochrome C is the only mobile cytochrome Cytochrome C is the only mobile cytochrome. It carries one electron each time and transfers it to the final component of the chain Complex IV.

It is also called (Cytochrome C oxidase or Cytochrome aa3). Complex IV It is also called (Cytochrome C oxidase or Cytochrome aa3). It catalyzes the final reaction between 2H and ½ O2 to form H2O. (only hypothetical) It is the only irreversible reaction of the chain. It needs copper as a metal cofactor. O2 is most electropositive component Of the chain and is the final acceptor of electrons

IV O―• Cytochrome a a3 e- e- ½ O2 H2O Cu++ Cu+ Cu+ 2H+ Cytochrome C

The more negative standard reduction potential of a redox pair, the greater the tendency to: (A) To lose electrons (B) To gain electrons (c) To lose and gain electrons (d) To gain protons (e) To lose protons

http://www.youtube.com/watch?NR=1&v=_PgjsfY71AM

Overview of ETC http://www.wiley.com/college/boyer/0470003790/animations/electron_transport/electron_transport.swf http://www.youtube.com/watch?v=xbJ0nbzt5Kw