Voet Biochemistry 3e © 2004 John Wiley & Sons, Inc.

Review of ETS and Oxidative Phosphorylation

There are organisms, lithotrophs, that metabolize outside the light cycle. For example, some anaerobic archaea are methanogens. They reduce CO2 with H2 from deep water vents.

Voet Biochemistry 3e © 2004 John Wiley & Sons, Inc.

4H2 + CO2 ÆCH4 +2 H2O ∆G0= -130 KJ/mol

Voet Biochemistry 3e © 2004 John Wiley & Sons, Inc.

Photosynthesis can use a number of reductants to 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.

Photosynthesis 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) 1017 kcal free energy/yr via solar energy - 10X all fossil fuel per yr (fossil fuels products of photosynthesis from millions yrs ago) Voet Biochemistry 3e © 2004 John Wiley & Sons, Inc.

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.

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Solar spectrum and light absorbing pigments.

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Some photo harvesting pigments

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Figure 24-3 Chlorophyll structures.

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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

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

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Figure 24-1

Chloroplast from corn.

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

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Harvesting Light

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

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Exciton transfer to a tripartite reaction center

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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.

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

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

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

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

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There are two main kinds of bacterial photosystems.

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“Simple”, one-center system

The parallel between cytbc1 and complex III of ETS is obvious

Photosynthetic electron-transport system of purple photosynthetic bacteria.

A more detailed view of the Z-diagram

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This is found in Higher plants and cyanobacteria

More later Photophosphorylation

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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 eflow from 2 H2O to 2 NADP+. Therefore, a total of 8 quanta must be absorbed, 4 by each photosystem, consistent with Z diagram.

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

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

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Figure 24-29 Segregation of PSI and PSII.

Coupling of Electron Transport with ATP Synthesis Chemiosmotic Hypothesis

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Proton Gradient

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

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

Another view of the overall organization of higher plant photo systems. In photophosporylation, electrons can dump from Fd to b6f and pump H+ for ATP synthesis, but no NADPH

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∆ pH ~3; 8 to 5

To generate one O2 we 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 giving 12 NADPH and ~21 ATP. We can follow those numbers later as we discuss the dark reactions.

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

Voet Biochemistry 3e © 2004 John Wiley & Sons, Inc.

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

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Fructose-6-P Glucose-6-P UDP-Glucose Glucose-1-P

Sucrose Starch Cellulose

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Figure 24-31 The Calvin cycle.

6-C5

12-C3 6-C3

2-C3

*

4-C5 Voet Biochemistry 3e © 2004 John Wiley & Sons, Inc.

2-C3

*

2-C3

2-C5

2-C6

2-C7 2-C4

1-C6

Regulation of the Calvin Cycle

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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.

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.

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

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

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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 Biochemistry 3e © 2004 John Wiley & Sons, Inc.

Bacteriorhodopsin from Halobacterium is a very simple light driven proton pump