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Why study photosynthesis?
• Largest biochemical process on
earth.
• Source of earth’s atmospheric
oxygen (began 1.5-2.0 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).
The Dark Reactions
6CO2 + 12H2O + 18ATP + 12NADPH 
6C(H2O) + 6O2 + 18 ADP + 18Pi + 12 NADP + 6H2O
The Light Reactions
Two Types of Reaction Centers
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
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.
Pigments in photosynthesis
Beta carotene
Quinones in photosynthesis
mobile and fixed, electron and proton donors and acceptors
Cofactor orientation and energetics in the
BRC
Energy
Level
Chlsp
+0.45 eV
eV =
0.7
Qa
-0.2 eV
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.
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.
Bacterial Photosynthesis
Oxygenic Photosynthesis
Linear photosynthetic electron
transfer chain of oxygenic
photosynthesis
Lateral heterogeneity of membrane protein
complexes
Mobile electron carriers
Plastocyanin
Cu +2
Mobile electron carriers - ferredoxin
Photosystem II Model
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
Organization of cofactors in the PSII RC
psuedo-C2 symmetry
Ferreira et al. (2004) Science 303: 1831
Relationship between the PSII RC and the proximal
antennae complexes

eV
ChlZ cycle
Charge Stabilization
P680+ is a very strong oxidant
Relative Midpoint Potentials
1.4
P680 PSII
1.2
Histidine
Tyrosine
eV
1
0.8
BChl a
0.6
0.4
0.2
0
Water
Chl a
P870
P840
P700 PSI
Period 4 oscillation of oxygen evolution
following single-turnover flashes
2H2O + 8hv  4H+ + 4e- + O2
Pierre Joliot
Kok’s clock, S-state transitions
H+
H+
Metallo-radical model for water
oxidation
PSI Structure
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
Photosystem I RC cofactors
PSI redox potential (energy level) and
kinetics
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, ?
Chlamydomonas Cytochrome b6f structure
Stroebel et al., (2003)
Nature 426: 413
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
FeS head group moves between Qo site and heme c1,of analogous
Cytochrome bc Complex
bH
Qi
Q
o
b
L
FeS
c1
Q-Cycle oxidant-induced
reduction
PQH2 = 0.0 eV
PQ- = - 0.2 eV
(PQ- more negative than PQH2 !!!)
Qn heme = - 0.05 eV
Qp heme = - 0.15 eV
Rieske 2Fe-2S = + 0.3 eV
Cyt f heme = + 0.34 eV
PC Cu+2 = + 0.365 eV
Rate PQH2 ox at Qp =
10 - 20 ms
(rate limiting step in Ps)
Rate PQ red at Qn = 0.1 ms