Electron transport chain chapter 6 (page 73) BCH 340 lecture 6

Electron transport chain chapter 6 (page 73) BCH 340 lecture 6

The Metabolic Pathway of Cellular Respiration All of the reactions involved in cellular respiration can be grouped into three main stages Glycolysis occurs in cytoplasm The Krebs cycle occurs in matrix of mitochondria Electron transport occurs across the mitochondrial membrane

Oxidative phosphorylation Energy-rich molecules, such as glucose, are metabolized by a series of oxidation reactions ultimately yielding CO2 and water The metabolic intermediates of these reactions donate electrons to specific coenzymes nicotinamide adenine dinucleotide (NAD+) and Flavin adenine dinucleotide (FAD) to form the energy-rich reduced coenzymes,nadh and FADH2.

Oxidative phosphorylation These reduced coenzymes can, in turn, each donate a pair of electrons to a specialized set of electron carriers, collectively called the electron transport chain As electrons are passed down the electron transport chain, they lose much of their free energy. Part of this energy can be captured and stored by the production of ATP from ADP and inorganic phosphate (Pi). The transfer of electrons down the electron transport chain is energetically favored because NADH is a strong electron donor and molecular oxygen is an avid electron acceptor. However, the flow of electrons from NADH to oxygen does not directly result in ATP synthesis.

Oxidative process Phosphorylation process inner membrane O 2 e - H 2 O H + ADP+ Pi H + ATP ATP Synthase outer membrane matrix intermembrane space Figure: Essential features of oxidative phosphorylation Redox reactions of respiratory chain use electrons to reduce oxygen to water Energy generated moves protons from matrix to intermembrane space Inward movement of protons recovers this energy to promote formation of ATP in the matrix.

1. Proton pump: Electron transport is coupled to the phosphorylation of ADP by the transport of protons (H+) across the inner mitochondrial membrane from the matrix to the intermembrane space This process creates an electrical gradient (with more positive charges on the outside of the membrane than on the inside) and a ph gradient (the outside of the membrane is at a lower ph than the inside The energy generated by this proton gradient is sufficient to drive ATP synthesis. 2. ATP synthase: The enzyme complex ATP synthase (Complex V, synthesizes ATP using the energy of the proton gradient generated by the electron transport chain.

ATP yield Only 4 of 38 ATP ultimately produced by respiration of glucose are derived from substrate-level phosphorylation (2 from glycolysis and 2 from TCA) The vast majority of the ATP (90%) comes from the energy in the electrons carried by NADH and FADH 2

Adding Up the ATP Cytosol High-energy electronscarried by NADH High-energy electrons carried mainly bynadh Mitochondrion Glycolysis 2 Glucose Pyruvic acid 2 Acetyl- CoA Krebs Cycle Electron Transport Maximum per glucose: by direct synthesis by direct synthesis by ATP synthase A Road Map for Cellular Respiration

The components of the electron transport chain are located in the inner membrane

Redox Reactions REDOX short for oxidation-reduction reactions Chemical reactions that transfer electrons from one substance to another are called oxidation-reduction reactions

REDOX FACTS A:H A Reductant Oxidant + e- B B:H Oxidant + e- Reductant (acceptor) (donor) Both oxidation and reduction must occur simultaneously The reductant of one pair donates electrons and the oxidant of the other pair accepts the electrons Red1 (AH) + Ox2 (B) Ox1(A) + Red2(BH)

Electrons can move through a chain of donors and acceptors In the electron transport chain, electrons flow down a gradient Electrons move from a carrier with low reduction potential (high tendency to donate electrons) toward carriers with higher reduction potential (high tendency to accept electrons)

Succinate E o = 0.03V E o = 0.07V II NADH I Coenzyme Q E o = -0.32V E o = 0.10V E o = 0.42V III E o = 0.19V electron flow Cytochrome C E o = 0.29V E o = 0.53V IV ½ O 2 E o = 0.82V The components of the RC are arranged in order of increasing redox potential The Eo values are the potential differences across the four complexes ( that span the mitochondrial inner membrane)

Potential (E O ): measure of the tendency of oxidant to gain electrons to become reduced. E O : Eo of the electron-accepting pair minus the Eo of the electron-donating pair

Succinate E o = 0.03V E o = 0.07V II NADH I Coenzyme Q E o = -0.32V E o = 0.10V E o = 0.42V III E o = 0.19V Cytochrome C E o = 0.29V E o = 0.53V IV ½ O 2 E o = 0.82V electron flow The overall voltage drop from NADH E 0 = -(-0.32 V) to O E 0 = +0.82 V is Eº = 1.14 V

RC exists as four large, multisubunit protein complexes The respiratory electron transport chain complex I is a NADHubiquinone reductase complex II is succinate dehydrogenase complex III is the ubiquinone -cytochrome c reductase complex IV is cytochrome oxidase

Figure: Complex I of the respiratory chain that links NADH and coenzyme Q. NADH Dehydrogenase (NADH-ubiquinone reductase) accepts 2e- from NADH and transfers them to ubiquinone (coenzyme Q), an electron carrier Uses two bound cofactors to accomplish this: FMN (Flavin mononucleotide) and 6 iron-sulfur (Fe-S) protein

Complex II: Succinate-CoQ reductase Prosthetic groups: FAD; Fe-S Succinate FAD SDH Fumarate FADH 2 CoQ SDH is succinate dehydrogenase an enzyme of the citric acid cycle (associated with membrane) 2 e- transferred from succinate to CoQ 1 mole FADH 2 produced

Electrons from complex I or II Complex III: cytochrome reductase Prosthetic groups: heme b; heme c 1 ; Fe-S CoQ cyt b/cyt c 1 cyt c Figure: Complex III of the respiratory chain linking CoQ and cytochrome C. Is composed of cytochome b, cytochrome C 1 and iron sulphur proteins Accepts e- from coenzyme Q and transfers e- to cytochrome c (Cytc is the only soluble cytochrome) coupled with the transfer of protons from the matrix to the intermembrane space

Figure: Complex IV -cytochrome oxidase- reducing oxygen to water Contains cytochromes a/a3 and 2 Cu ions involved in e- transfers Cytochrome oxidase passes electrons from cytochrome c through a series of heme groups and Cu ions to O 2, reducing it to H 2 O (end product)

ATP-synthase (complex V), present in the inner mitochondrial membrane, actually makes ATP from ADP and P i. ATPase used the energy of an existing proton gradient to power ATP synthesis. This proton gradient develops between the intermembrane space and the matrix. This concentration of H + is the proton-motive force.

The ATP synthase molecules are the only place that will allow H+ to diffuse back to the matrix This flow of H + is used by the enzyme to generate ATP a process called chemiosmosis (oxidative phosphorylation)

Properties of ATP Synthase Multisubunit transmembrane protein Molecular mass = ~450 kda Functional units F 0 : water-insoluble transmembrane protein (up to 8 different subunits) F 1 : water-soluble peripheral membrane protein (5 subunits),contains the catalytic site for ATP synthesis Flow of 3 protons through ATP synthase leads to phosphorylation of 1 ADP

Respiratory inhibitors These compounds prevent the passage of e- by binding a component of the ETC blocking the oxidation/reduction reaction