1. Why study photosynthesis?
Largest biochemical process on earth.
Source of earth’s atmospheric oxygen (began billion years ago).
Source of nearly all biologically useful energy.
Crystal structures known for each membrane protein complex (several Nobel Prizes).
Light is the substrate and ultra-fast kinetic events can be studied under extremely short time periods.
Intellectually challenging and complex (biophysics, biochemistry, molecular biology, ecology).

2. 6C(H2O) + 6O2 + 18 ADP + 18Pi + 12 NADP + 6H2O
The Dark Reactions
6CO2 + 12H2O + 18ATP + 12NADPH 
6C(H2O) + 6O ADP + 18Pi + 12 NADP + 6H2O
The Light Reactions

3. Two Types of Reaction Centers

4. Michel, Deisenhoffer and Huber - Nobel Prize in Chemistry, 1989
Rhodobacter viridis Photosynthetic Bacterial Reaction Center Non-oxygenic Photosynthesis
First membrane protein structure resolved at atomic levels of resolution by X-ray diffraction.
Michel, Deisenhoffer and Huber - Nobel Prize in Chemistry, 1989

5. The R. viridis BRC is a type-II reaction center
L subunit- Binds chlorophyll, pheophytin and quinone cofactors involved in electron transfer
M subunit – Binds chlorophyll, pheophytin and quinone cofactors involved in electron transfer
H subunit – Stabilizes complex
Cyt C – extrinsic protein binds four hemes involved in electron transfer
Cofactors: 4 hemes, Chlsp, 2 Chlm,
2 Pheophytin, 2 quinones, 1 non-heme
Fe.

6. Pigments in photosynthesis
Beta carotene

7. Quinones in photosynthesis mobile and fixed, electron and proton donors and acceptors

8. Cofactor orientation and energetics in the BRC
Energy
Level
Chlsp
+0.45 eV
eV =
0.7 Qa
-0.2 eV

9. Protein-cofactor interactions
Cofactor Ligands
Chlsp L-H173, M-H200
Chlm L-H153, M-H180
Fe L-H190, L-H230, M-H217, M-H264
Carotenoid No residues involved in coordination
Cyt C hemes 17 amino acid helix followed by a turn and Cys-X-Y-Cys-His. Hemes bound by thioether bond to Cys.

10. Function of the BRC in Photosynthesis
A light driven proton pump working in concert with the cytochrome bc1 complex to generate a proton gradient and ATP.
Menaquinone in the QA site is singly reduced by an electron initially derived from the ChlSP (primary electron donor).
Ubiquinone in the QB site is then sequentially reduced (2 e-) and protonated (2H+ ) via QA forming UQH2.
UQH2 then exits from the QB binding pocket as a mobile 2 electron and 2 proton carrier and is oxidized by the cytochrome bc1 complex.
The result is the transfer of protons across membrane to establish a pH gradient that drives ATP synthesis. 

11. Bacterial Photosynthesis

12. Oxygenic Photosynthesis

13. Linear photosynthetic electron transfer chain of oxygenic photosynthesis

14. Lateral heterogeneity of membrane protein complexes

15. Mobile electron carriers
Plastocyanin
Cu +2

16. Mobile electron carriers - ferredoxin

17. Photosystem II Model

18. D1 - D2 Proteins: cofactors and amino acid ligands
Fe – D1-H215, D1-H272
D2-H214, D2-H268
ChlZ – D1-H118
ChlZ – D2-H117
YZ – D1-Y161
YD – D2-Y160
ChlSP – D1-H198, D2-H197

19. Organization of cofactors in the PSII RC psuedo-C2 symmetry
Ferreira et al. (2004) Science 303: 1831

20. Relationship between the PSII RC and the proximal antennae complexes

21.  eV
ChlZ cycle
Charge Stabilization

22. P680+ is a very strong oxidant

23. Period 4 oscillation of oxygen evolution following single-turnover flashes
2H2O + 8hv  4H+ + 4e- + O2
Pierre Joliot

24. Kok’s clock, S-state transitionsH+ H+

25. Metallo-radical model for water oxidation

26. PSI Structure

27. The PSI RC polypeptides also function as proximal antennae complexes
psaA and psaB RC proteins are large - 81 kD.
N-terminal six transmembrane spans bind the proximal antennae Chls analogous to the PSII CP43 and CP47 proteins (43-47 kD).
Five, C-terminal transmembrane spans bind the reaction center cofactors analogous to the PSII D1 and D2 proteins (32 kD).
Since PSII is very sensitive to photodamage and proteolytic turnover, unlike PSI, it is more efficient to repair only the damaged protein (D1) than the D1 and CP-43 protein. As a corollary, since photodamage is rare in PSI. its unnecessary for split proteins to facilitate repair

28. Photosystem I RC cofactors

29. PSI redox potential (energy level) and kinetics

30. Cytochrome b6f complex Cyt f = 32 kD, heme
Cyt b6 = 24 kD, 2 hemes with different Em
Rieske Fe-S = 19 kD, nuclear gene, Fe-His ligands shift Em of 2Fe-2S (+)
Subunit IV = 17 kD, analogous to C-terminus of mitochondrial Cyt b
PetG, PetL and PetM = 3-4 kD, ?

31. Chlamydomonas Cytochrome b6f structure
Stroebel et al., (2003) Nature 426: 413

32. Chlamydomonas cytochrome b6f complex contains an unexpected c-type heme near the high-potential heme
The novel heme may control access to the Qi site or participate in cyclic electron transfer between photosystem I

33. FeS head group moves between Qo site and heme c1,of analogous Cytochrome bc ComplexQi bH Qo bL
FeS c1

34. Q-Cycle oxidant-induced reduction
PQH2 = 0.0 eV
PQ- = eV
(PQ- more negative than PQH2 !!!)
Qn heme = eV
Qp heme = eV
Rieske 2Fe-2S = eV
Cyt f heme = eV
PC Cu+2 = eV
Rate PQH2 ox at Qp = ms
(rate limiting step in Ps)
Rate PQ red at Qn = 0.1 ms