review of ets and oxidative phosphorylation -...

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Review of ETS and Oxidative Phosphorylation

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

There are organisms, lithotrophs, that metabolize outside the light cycle.

For example, some anaerobic archaeaare methanogens. They reduce CO2with H2 from deep water vents.4H2 + CO2 CH4 +2 H2O ∆G0= -130 KJ/mol

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Photosynthesis can use a number of reductantsto reduce CO2 to sugar.

Photosynthesis evolved from an anaerobic environment, and the simple bacterial systems preceded the modern oxygen releasing system. However, we will focus on the modern (higher plant) system.

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Aerobic, Heterotrophic organisms

C6H12O6 + 6 O2 6 CO2 + 6 H2O ∆Go’ = -2823 kJ/mol

Autotrophic (photosynthetic)

6 CO2 + 6 H2O C6H12O6 + 6 O2 ∆Go’ = +2823 kJ/mol

(ie. ~470 kJ/mol for each CO2 fixed and O2 released)

Photosynthesis

1017 kcal free energy/yr via solar energy - 10X all fossil fuel per yr (fossil fuels products of photosynthesis from millions yrs ago)

Photosynthetic plants trap solar energy, generate ATP and NADPH,which are used to drive the synthesis of glucose and other organic cell components from CO2 and water, while releasing O2 into atmosphere.

Photosynthesis is broken into light reactions, and “dark” reactions.

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Solar spectrum and light absorbing pigments.

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Some photo harvesting pigments

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Figure 24-3 Chlorophyll structures.

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Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

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Figure 24-4 Energy diagram indicating the electronic states of chlorophyll and their most important modes of interconversion.

Energy of photon: E = hc/λ ; h = 6.6 X 10-34 J•sec, c = vel of light (3 X 108 m/s)and λ = wavelength of light. e.g. red light at 700 nm = 170 kJ/einstein.

Note chlorophyll absorbs in blue and red, leaving green

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Figure 24-1 Chloroplast from corn.Pa

ge 8

72

Needless to say, photosynthetic bacteria do not have chloroplasts; the process occurs within the cells.

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Figure 24-7a Flow of energy through a photosynthetic antenna complex. (a) The excitation resulting from photon absorption randomly migrates by exciton transfer.

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Harvesting Light

Voet

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chem

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2004

Joh

n W

iley

& So

ns, I

nc.

Figure 24-7b Flow of energy through a photosynthetic antenna complex. (b) The excitation is trapped by the RC chlorophyll.

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877

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Exciton transfer to a tripartite reaction center

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

The light harvesting complex from the purple bacterium Rhodospirillium has been seen by X-ray crystallography.

The complex is of the form α8β8, with 24 chlorophylls and 8 lycopene carotenoids.

Voet

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chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Figure 24-9 Model of the light-absorbing antennasystem of purple photosynthetic bacteria.

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Voet

Bio

chem

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3e©

2004

Joh

n W

iley

& So

ns, I

nc.

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Figure 24-11 A ribbon diagram of the photosynthetic reaction center (RC) from Rb. sphaeroides.

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Figure 24-12b Sequence of excitations in the bacterial RC of Rps. viridis.

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881

Transfer of excited electron from special pair to Pheo leaves it with a + charge.

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

There are two main kinds of bacterial photosystems.

Voet

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chem

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3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Photosynthetic electron-transport system of purple photosynthetic bacteria.

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“Simple”, one-center system

The parallel between cytbc1 and complex III of ETS is obvious

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

A more detailed view of the Z-diagram

This is found in Higher plantsand cyanobacteria

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887

Photo-phosphorylation

More later

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Figure 24-6 The amount of O2 evolved by Chlorella algae versus the intensity of light flashes.

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For each electron flowing from H2O to NADP+, two light quanta are absorbed, one by each photosystem. To form one molecule of O2, 4 e-flow from 2 H2O to 2 NADP+. Therefore, a total of 8 quanta must be absorbed, 4 by each photosystem, consistent with Z diagram.

Voet

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3e©

2004

Joh

n W

iley

& So

ns, I

nc.

The O2 evolving complex (OEC) has a Mn cluster with 5 redox states.

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Figure 24-29 Segregation of PSI and PSII.

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Voet

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chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Coupling of Electron Transport with ATP Synthesis

Chemiosmotic Hypothesis

Proton Gradient

Voet

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3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Figure 24-17 Schematic representation of the thylakoid membrane showing the components of its electron-transport chain.

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This figure reflects the generation of 1 O2, movement of 4 e- via 8 photons.

Voet

Bio

chem

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3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Another view of the overall organization of higher plant photo systems.

To generate one O2we need to pump 4 e- (using 8 photons) to reduce 2 NADPs.This process pumps ~12 H+ into the thylakoid lumen; they can make 3-4 ATPs.In the stoichiometry of glucose synthesis, we would do this 6 times giving12 NADPH and ~21 ATP. We can follow those numbers later as we discuss the dark reactions.

∆ pH ~3; 8 to 5

In photophosporylation, electrons can dump from Fd to b6f and pump H+ for ATP synthesis, but no NADPH

Voet

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3e©

2004

Joh

n W

iley

& So

ns, I

nc.

The “Dark Reactions” use the NADPH and ATP generated by the light reactions to reduce 6 CO2s to glucose.

Voet

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2004

Joh

n W

iley

& So

ns, I

nc.

CO2 is fixed by the action of ribulose bisphosphatecarboxylase (Rubisco), the most common enzyme on earth.

Voet

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chem

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3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Glucose-1-PUDP-Glucose

SucroseStarchCelluloseGlucose-6-P

Fructose-6-P

Voet

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2004

Joh

n W

iley

& So

ns, I

nc.

2-C3

2-C32-C3

6-C3

2-C6

1-C6

12-C3

2-C4

2-C5

2-C7

4-C5

6-C5

*

*

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.Regulation of the Calvin Cycle

The Calvin cycle and glycolysis both occur in the stroma. Clearly they need to be coordinately regulated to prevent futile cycles of synthesis and destruction. The pathway has 3 irreversible steps,and all are regulated. The mechanisms are novel for us, involving pH and redox controls.

Voet

Bio

chem

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3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Control by pH

The light reactions pump H+ into the thylakoid lumen, leaving the stroma alkaline (pH ~8); Mg2+ is also pumped into the stroma.

Rubisco has a sharp pH optimum near 8 and is turned on in the light, as signaled by the alkaline pH.

FBPase and SBPase are also activated at alkaline pH and by Mg2+, but also by redox state.

Voet

Bio

chem

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3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Page

902

PS-I generates reduced ferrodoxin; it participates in coordinate regulation.

Reduced ferrodoxin (E0’< -400 mV), can ultimately reduce thioredoxin (E0’~ -230 mV), a widely used reductant for protein disulfides. Thioredoxin can reduce and activate FBPase and SBPase, turning on the Calvin cycle in the light. It also reduces and INACTIVATES PFK, the key regulator of an irreversible step in glycolysis.

Trx ~11kDa

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Bacteriorhodopsin from Halobacterium is a very simple light driven proton pump

review of ets and oxidative phosphorylation -...

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