Chapter 19

Oxidative Phosphorylation

Electron transferring (flow ) through a chain of membrane bound carriers

(coupled redox reactions), generation of a transmembrane proton gradient, ATP biosynthesis (ADP phosphorylation).

For Biochemistry II lectures of Nov. 26 and Dec. 3, 2008 (Prof. Zengyi Chang)For Biochemistry II lectures of Nov. 26 and Dec. 3, 2008 (Prof. Zengyi Chang)

“Anyone who is not confused about oxidative phosphorylation just doesn’t understand the situation” Efraim Racker, 1970s

*all the electrons are transferred to O2;*ATP is made using a proton gradient.

Main events:

The three stages of biological oxidation

Coupling of energy-releasing & energy-requiring reactions.

Energy-requiring

Energy-releasing

History of understanding oxidative phosphoryation

• 1900s: vital role of phosphate in fermentation revealed.• 1910-1920S: role of iron in intracellular respiration realized. • 1920s: Cytochromes identified and the concept of the

respiratory chain (electron transport chain) formulated. • 1930s: pyruvate (product of glycolysis) known to be completely

oxidized to CO2 via the citric acid cycle (needing O2).• 1930s: NAD+ and FAD were found to be e- carriers between

metabolites and the respiratory chain.• 1930s: role of ATP and general importance of

phosphorylation in bioenergetics realized (Lipmann, 1939, “oxidative phosphorylation” introduced)

A historical perspective• Understanding the detail molecular process• 1940s: link between sugar oxidation and ATP synthesis

established; Role of NADH linking metabolic pathway and ATP synthesis proved.

• 1950s: isolated mitochondria found to effect the obligatory coupling of the phosphorylation of ADP and the e- transfer from NADH to O2.

• 1940s-1950s: “High energy intermediate” leading to ATP synthesis (chemical coupling hypothesis) was searched (but failed to be found.)

• 1960s: the chemiosmotic hypothesis proposed for linking the e- transfer and ADP phosphorylation (role of the membrane and the across-membrane proton gradient postulated, a new paradigm!).

• 1970s: the binding change model was proposed to explain how the proton gradient will be used to drives ADP phosphorylation.

• 1990s: structure of ATP synthase determined, supporting the binding change model.

Electrons of NADH and FADH2 are transferred to O2 via many intermediate electron carriers making up the respiratory chain.

History of understanding Photophosphoryation• 1790s: CO2 and H2O are taken up by green plants,

while O2 is released, all under the influence of light.

• 1930s: photosynthesis occur via light-dependent redox reaction; isolated chloroplasts made O2 but did not fix CO2; O2 is derived from H2O.

• 1950s: The path of carbon fixation revealed.

• 1960s: Existence of two photosystems elucidated.

NADH enters at NADH-CoQ oxidoreductase

Consisting ~34-46 polypeptide chains

The coupling mechanismbetween e- transferring and H+ pumping is still unknown!

~880 KDa~880 KDa （（ the largest the largest among fouramong four））

NADH: 2eFAD:1eFMN 1e or 2 e

At least eight different types of iron-sulfur centers (first revealed by Helmut Beinert) act in the respiratory chain: (in complex I, II and III) iron atoms cycle between Fe2+ (reduced)

and Fe3+ (oxidized).

2Fe-2S

4Fe-4S

Ubiquinone (or coenzyme Q)is the only e- carrier that is not bound to a protein and is able to diffuse freely in the lipid bilayer .

(or dihydroubiquinone)

The isoprenoid tail

Q10

FADH2 of flavoproteins also transfer their electrons to ubiquinone (Q), but with no H+ pumped.

Succinatedehydrogenase

Structure of mitochondrial complex II (porcine) was determined

Sun F et al. and Rao, Z, 2005, Cell, 121:1043-1057.

Proposed electron path

Succinate dehydrogenase

孙飞

The crystal structure of the cytochrome bc1 complex (Complex III) has been determined.

Structure ofthe threecore subunits out of the 11 subunits.

Spatial relationshipof the cofactors deduced

from the resolved structureof complex III.Science, 1997, 277:60-66.

Three types of heme groups are found in the cytochromes.

Bound to cytochromestightly and covalently

Also found in Hemoglobin & myoglobin

Bound to cytochromestightly but noncovalently

Bound to cytochromestightly but noncovalently

The iron interconverts between its reduced (Fe2+) and oxidized (Fe3+) froms, thus performing oxidation and reduction reactions

Reduced cytochromes has three absorptionbands in the visible wavelengths

The reduced (Fe2+)state of cytochromes a, b, and c has the longest wavelength band near 600, 560, and 550 nm respectively.

cb a

Cytochromes are classified on the basis of position of their lowest energy absorption band in the reduced state.

The e- transferring & H+ pumpingin Complex III occur via the Q cycle

For each 2 e- transferred, 4H+ are translocated.

the Q cycleFrom: http://en.wikipedia.org/wiki/Image:Theqcycle.gif

Cytochrome c oxidase (Complex IV ), contains 3 Cu and 2 heme A groups as electron carriers.

Heme a and heme a3 has identical structures but

different reduction potential.

The three critical subunits (out of 13) of complex IV.

a

a3

CuA -CuA

CuB

O2 acts as the final electron acceptor here. Science, 1996,272:1136-1144

Four electrons are transferredfrom 4 Cyt c to 1 O2 to make 2H2O in Complex IV,with 4H+ taken from the matrixto make 2H2O &4H+ pumped out.

~ 10 protons per NADH and ~6 protons per FADH2 oxidized are pumped across the inner membrane of mitochondria (or plasma membrane of bacteria)

The highly mobile Q and Cyt c molecules shuttle electronsfrom one large multiprotein complex to another.

A transmembrane H+ gradient is thus generated.

The electron motive force isconverted to an proton motive force.