1. Assignment for next Friday, October 13th
Read for in-class discussion:
Photochem. Photobiol. Sci ,
DOI: /b506755c (

2. Distribution of Protein Complexes in Thylakoid Membrane
high [H+]

3. Overview of Oxygen-Generating Photosynthesis in Plants
 < 700 nm
 < 680 nm
Photosystem II
Photosystem I
Cytochrome bf
Two photosystems work together to generate a H+ gradient & NADPH in oxygenic photosynthesis

4. Light Harvesting Complex
Thylakoid
space
Stroma
membrane
Absorption spectra of chlorophyll a & b
Light
harvesting
complex
(from
photosynthetic
bacteria)
Chlorophyll a

5. Electron Flow in Oxygenic Photosynthesis
electron flow shown in red
Balanced equation for reaction occurring at PSII:
2 Q + 2 H2O  O2 + 2 QH2
Electrons used to reduce P960+ in PS II come from a Mn cluster that is capable of oxidizing H2O
(instead of a bound cytochrome as they do in photosynthetic bacteria)

6. Z Scheme in Oxygenic Photosynthesis:
A Linear Trace of Electron Flow from H2O to NADPH

7. Artificial Photosynthesis: ExampleMg NH
ATP
ADP + Pi
ATP Synthase H+
Light . - Q P Qs Q . P C + H+ C H+
Lipid Bilayer
~52Å
Key Features
Vectorial Electron Transport
Diffusionally-Coupled Donor and Acceptor Species
Ion Transport Coupled via Shuttle Quinone
Q-P-C:
Quinone-Porphyrin-Carotenoid
Triad
Moore, T. A et.al. Nature 1997, 385, 239

8. Combined Dark Reactions in Photosynthesis
Calvin Cycle

9. The Calvin Cycle: Synthesis of Hexoses from CO2 & H2O in the Stroma
Net Reaction: 6 CO ATP + 12 NADPH + 12 H2O  C6H12O ADP + 18 Pi + 12 NADP+ + 6 H+

10. Structure of Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase (Rubisco)
Role of Mg2+ in Rubisco
Active Site
2-Carboxy-3-keto-D-arabinitol
1,5-bisphosphate
16% of total chloroplast protein
8 large (L, 55 kD) subunits
8 small (S, 13kD) subunits
requires Mg2+ for activity

11. Carboxylase Reaction Sequence Catalyzed by Rubisco
rate
determining
step
(enzyme-bound
Intermediate)

12. Formation of Sucrose from 3-Phosphoglycerate in the Cytosol

13. Synthesis of Higher Carbohydrates from
Fructose 6-Phosphate

14. Rubisco Also Catalyzes a Wasteful Oxygenase Reaction
the enediolate intermediate can also react with O2 to give a less useful product, phosphoglycolate
the carboxylase reaction is four times faster, but the O2 concentration in the stroma is higher under normal atmospheric conditions (which leads to a competing oxygenase reaction)
a scavenge pathway exists within the peroxisome to recover a useful product (glycine)

15. The Activity of the Calvin Cycle Depends on Environmental Conditions
Rubisco is activated by light-driven changes in H+ and Mg2+ concentrations
the rate determining step of the carboxylase reaction of rubisco is favored by high Mg2+ concentrations and alkaline pH
Mg2+ rises in the stroma to compensate for the influx of H+ into the thylakoid due to light-driven ATP synthesis

16. The Activity of the Calvin Cycle Depends on Environmental Conditions
Thioredoxin is a 12 kD protein that contains neighboring cysteine residues that cycle between the reduced (bis)thiol form to the oxidized disulfide form
Thioredoxin regulates the activity of reduced ferredoxin (produced in a light-driven reduction reaction from H2O)
Thioredoxin also regulates other enzymes in the Calvin cycle by controlling their reduction state
In summary, the activities of the light and dark reactions of photosynthesis are coordinated through electron transfer from reduced ferredoxin to thioredoxin and then to the component enzymes of the Calvin cycle that are regulated by the presence of disulfide bonds
Calvin cycle
reactions

17. C4 Pathway: A Mechanism for Tropical Plants to Accelerate Photosynthesis
CO2 is concentrated in the chloroplasts of bundle-sheath cells (major site of photosynthesis) by shuttling a four-carbon (C4) compound, malate, from mesophyll cells that are in contact with air
Decarboxylation of the C4 compound in the bundle-sheath cell maintains a high local concentration of CO2 to support the Calvin cycle (this effectively acts as an ATP-driven “pumping” of CO2 into the bundle-sheath cells)
This system can increase the concentration of CO2 in bundle-sheath cells by a factor of 20, thus accelerating the carboxylase activity of rubisco & effectively outcompeting the oxygenase (photorespiration) reaction in tropical plants