Photosynthetic Electron Transport
hν
½ O2 + 2 H +
H 2O
cytb6f
2H
e–
PC
hν
–
+
e
lumen
–
e
–
e
e–
PQ
PQ
PSI
PSII
2H
+
H 2O
OEC
–
–
Chla
–
P680
(Chla2)
e
pheo
e
–
O
O
PQ (bound)
Fe
O
O
–
–
e
e
+
4H + O2
2 H2O
S0
S1
S2
S3
S4
3,3,3,4
3,3,4,4
3,4,4,4
4,4,4,4
5,4,4,4
Sn
Sn+1
NADP+ + H+
NADPH
•
•
•
•
–
e
TyrZ
e
–
e
Photosystem II
hν
OH
Fd
FNR
½ O2 + 2H+
e
stroma
+
2H
dimeric structure, ~640 kDa total
each monomer has ~C2 axis within & ⊥ to membrane
e.t. path follows that axis
photon excites P680 (Chla2) to P680* (Chl+Chl–),
initiates e.t. pathway through:
• another Chl
• pheophytin
• protein-bound plastoquinone
• non-heme Fe
• and a free PQ, which uptakes 2H+ from the stroma
upon reduction, and then migrates to cytb6f.
+
PQH2 (to b6f ) Thus, each photon generates P680 ,
which in turn oxidizes the Oxygen
PQ (free)
Evolving Cluster (OEC), via a nearby Tyr
+
residue. The OEC is a Mn4CaO5 cluster.
+2H
Four sequential photons yields four
sequential oxidations of the OEC Mn ions
(states called S0 to S4, oxidation state
+
P680
assignments tentative). O=O bond formation
oxidizes two water molecules by 4e, reducing the
P680
OEC back to the S0 state.
hν
A favoured O-O bond forming mechanism:
Overall reaction (per 2e–): H2O + PQ + 2H+(“in”) → ½ O2 + PQH2 + 2H+(“out”)
Reaction is uphill (+140 kJ/mol H2O plus [H+] gradient), driven by photon excitation
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Cytochrome b6f Complex
Very similar structure and function to cyt bc1 in mitochondria, nearly identical cofactors;
cyanobacteria use b6f in both photo- and oxidative phosphorylation et chains.
• dimer ~217 kDa (~½ the size of bc1)
• PQH2 is oxidized at Qp site near lumen (“out”), electrons transferred in two paths
• PQ reduced at Qn site near stroma (“in”), Q cycle drives proton gradient
• electrons passed to lumen-mobile plastocyanin (plants) or cytochrome c6 (cyanobacteria)
• overall reaction (per 4e–):
2 PQH2 + PQ + 2H+("in") + 2 PC CuII → 2 PQ + 4H+("out") + PQH2 + 2 PC CuI
cytochrome b6f complex
+
2H
e
e
f
–
2H
PQH2
2e
–
Qp
site
e
e
–
QH2
Qn
–
e
e
stroma
site
–
bH
e
–
2e
–
PQH•
PQH2
PQ
PQH•
+
intermembrane
space
Qo
site
–
e
–
e
bL
Qi
–
site
bH
e
–
QH•
QH2
QH•
Q
matrix
H
e
c1
–
e
e
heme x
–
c
–
e
[2Fe2S]
Q
–
bL
+
lumen
[2Fe2S]
PQ
cytochrome bc1 complex
PC
–
H+
H+
H+
Fd
Path 1: PQH2 at Qp to PC in lumen
• Rieske [2Fe2S] cluster: 2 cysS 2 hisN
• cytochrome f: c heme, hisN, N-terminus Tyr amine
Path 2: PQH2 at Qp to Q at Qn near stroma
• cytochrome bp (or bL)
• cytochrome bn (or bH), both 2×hisN
Additional cofactor: heme x
• between bn and stroma, c heme, one axial H2O, HS?
• likely involved in cyclic et (see later)
Additional cofactors: one chlorophyll a and one β-carotene
• unknown function: et is downhill, runs in the dark
different terminologies
for Q binding sites
Qo = Qp = QO
out, (+) side, oxidase site
near lumen
Qi = Qn = QR
in, (–) side, reductase site
near stroma
bL = bp bH = bn
low Eº, (+) side
high Eº, (–) side
Plastocyanin / Cytochrome c6
•
•
•
•
electrons transported from cyt b6f to PSI via lumen-mobile metalloprotein
either plastocyanin (plants) or cytochrome c6 (cyanobacteria)
PC: typical cupredoxin, Cu2+/+, distorted tetrahedral, 2×NHis, SCys, SMet
cyt c6: Fe3+/2+, heme c + NHis, SMet
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Photosystem I
in plants
• monomeric structure
• core subunit ~375 kDa with 93 Chl
• belt of four light-harvesting
“antennae” ~50kDa each, 74 Chl
•
•
•
•
•
•
in cyanobacteria
• trimeric structure
• each unit ~350kDa with 96 Chl, 22 carotenes
in each core unit ~C2 symm axis ⊥ to
membrane, e.t. path follows that axis
photon excites P700 (Chl2•2NHis) to P700*
(Chl+Chl–), initiates e.t. path through:
• a Chl (one axial H2O, may be in direct
conjugation with P700)
• a second Chl (axial SMet, A0)
• phylloquinone (A1)
• three [Fe4S4] clusters in sequence
(FX, FA, FB, all 4×SCys)
• ferredoxin (outside PSI in stroma)
P700+ then removes e– from PC
unlike PSII, both et pathways appear to be
operative, but at slightly different rates
(neither limiting)
overall reaction (per 2e–):
2 PC CuI + 2 Fd FeIII → 2 PC CuII + 2 Fd FeII
reaction is uphill (+75 kJ/mol PC),
driven by photon excitation
PC
hν
Cu
–
e
–
–
e
e
P700
(Chla2)
Chla
–
e
e
–
e
Chla
(A0)
–
e
O
O
O
phyllo
(A1)
O
e
–
–
–
e
FX
–
e
Ferredoxin / FNR / NADP+
[2Fe2S] ferredoxin (Fd) accepts
one of two oxidants:
–
e –,
e
delivers to
non-cyclic electron transport:
e– delivered to ferredoxin:NADP+ reductase
(FNR, an FAD-based non-metalloprotein),
reducing NADP+ to NADPH
FA
FB
–
Fd
e
FeFe
cyclic electron transport:
e– returns to b6f complex, reducing PQ to PQH2 at Qn site (via et path through heme x ?);
these e–- reenter the Q-cycle and can further increase the proton gradient
The overall reaction for non-cyclic photophosphorylation, per two e– transferred, can be
written as:
NADP+ + H2O + 5 H+(“in”) → NADPH + ½ O2 + 6 H+(“out”)
Doubling this equation gives 2 NADPH and a net 10H+ transferred, yielding 2.5 ATP,
somewhat short of the 3ATP:2NADPH ratio required by the Calvin Cycle. Recycling some of
the e– through b6f and the Q-cycle increases the amount of ATP generated.
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–1200
Photosynthetic Electron
Transport Chain
Photosystem I
P700*
A0
–1000
A1
–800
Photosystem II
Fx FeIII / FeII
P680*
–600
Chl a
pheo
–400
–200
E°'
FA FeIII / FeII
FB FeIII / FeII
cyclic
path
non-cyclic
path
Fd FeIII / FeII
NADP+ / NADPH
PQ / PQH2
PQ / PQH2
via heme x
plastoquinone
0
Calvin Cycle
cyt b6f complex
+200
Rieske FeIII / FeII
cyt f
+400
PC CuII / CuI
plastocyanin
+600
+800
+1000
P700
Photosystem I
H 2O
2 H + + ½ O2
Mn4 cluster
TyrO • / TyrOH
h
P680
+1200
Photosystem II
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h