Download Bio 226: Cell and Molecular Biology

Next Assignment?
In lab time Friday March 27 or in class starting March 27
or March 30?
•GMO plants?
• Herbicide resistance
• Pathogen/herbivore resistance
• Improving nutrition
• Making vaccines, other useful biochems
•Plant/Algal biofuels?
PSI and PSII work together in the “Z-scheme”
Light absorbed by PS II makes ATP
Light absorbed by PS I makes reducing power
Ultimate e- source
O2 released?
Terminal e- acceptor
Form in which energy is
temporarily captured
Photosystems required
cyclic
None
No
None
ATP
PSI
non-cyclic
water
yes
NADP+
ATP &
NADPH
PSI & PSII
Z-scheme energetics
PSII Photochemistry
1) LHCII absorbs a photon
2) energy is transferred to P680
PSII Photochemistry
3) P680* reduces pheophytin ( chl a with 2 H+ instead of
Mg2+) = primary electron acceptor
PSII Photochemistry
3) P680* reduces pheophytin ( chl a with 2 H+ instead of
Mg2+) = primary electron acceptor
charge separation traps the electron
PSII Photochemistry
4) pheophytin reduces PQA (plastoquinone bound to D2)
moves electron away from
P680+ & closer to stroma
PSII Photochemistry
5) PQA reduces PQB (forms PQB- )
PSII Photochemistry
6) P680+ acquires another electron ,
and steps 1-4 are repeated
PSII Photochemistry
7) PQA reduces PQB - -> forms PQB2-
PSII Photochemistry
8) PQB2- acquires 2 H+ from stroma
forms PQH2 (and adds to ∆pH)
PSII Photochemistry
9) PQH2 diffuses within bilayer to cyt b6/f
- is replaced within D1 by an oxidized PQ
Photolysis: Making Oxygen
1) P680+ oxidizes tyrZ ( an amino acid of protein D1)
Photolysis: Making Oxygen
2) tyrZ + oxidizes one of the Mn atoms in the OEC
Mn cluster is an e- reservoir
Photolysis: Making Oxygen
2) tyrZ + oxidizes one of the Mn atoms in the OEC
Mn cluster is an e- reservoir
Once 4 Mn are oxidized replace e- by
stealing them from 2 H2O
Shown experimentally that need 4 flashes/O2
Shown experimentally that need 4 flashes/O2
Mn cluster cycles S0 -> S4
Reset to S0 by taking 4 efrom 2 H2O
Electron transport from PSII to PSI
1) PQH2 diffuses to cyt b6/f
2) PQH2 reduces cyt b6 and Fe/S, releases H+ in lumen
since H+ came from stroma, transports 2 H+ across
membrane (Q cycle)
Electron transport from PSII to PSI
3) Fe/S reduces plastocyanin via cyt f
cyt b6 reduces PQ to form PQ-
Electron transport from PSII to PSI
4) repeat process, Fe/S reduces plastocyanin via cyt f
cyt b6 reduces PQ- to form PQH2
Electron transport from PSII to PSI
4) PC (Cu+) diffuses to PSI,
where it reduces an oxidized P700
Electron transport from PSI to Ferredoxin
1) LHCI absorbs a photon
2) P700* reduces A0
3) e- transport to ferredoxin via
A1 & 3 Fe/S proteins
Electron transport from Ferredoxin to NADP+
2 Ferredoxin reduce
NADP reductase
Electron transport from Ferredoxin to NADP+
2 Ferredoxin reduce
NADP reductase
NADP reductase
reduces NADP+
Electron transport from Ferredoxin to NADP+
2 Ferredoxin reduce
NADP reductase
NADP reductase
reduces NADP+
this also contributes
to ∆pH
Overall reaction for the Z-scheme
8 photons + 2 H2O
+ 10 H+stroma + 2 NADP+
= 12 H+lumen + 2 NADPH
+ O2
Chemiosmotic ATP synthesis
PMF mainly due to ∆pH
is used to make ATP
Chemiosmotic ATP synthesis
PMF mainly due to ∆pH
is used to make ATP
-> very little membrane
potential, due to transport
of other ions
Chemiosmotic ATP synthesis
PMF mainly due to ∆pH
is used to make ATP
-> very little membrane
potential, due to transport
of other ions
thylakoid lumen pH is < 5
cf stroma pH is 8
Chemiosmotic ATP synthesis
PMF mainly due to ∆pH
is used to make ATP
-> very little membrane
potential, due to transport
of other ions
thylakoid lumen pH is < 5
cf stroma pH is 8
pH is made by ETS, cyclic
photophosphorylation,water
splitting & NADPH synth
Chemiosmotic ATP synthesis
Structure of ATP synthase
CF1 head: exposed to
stroma
CF0 base: Integral
membrane protein
a & b2 subunits form stator that immobilizes a & b F1
subunits
a is also an H+ channel
c subunits rotate as H+
pass through
g & e also rotate
c, g & e form a rotor
Binding Change mechanism of ATP synthesis
H+ translocation through ATP synthase alters affinity of
active site for ATP
Binding Change mechanism of ATP synthesis
H+ translocation through ATP synthase alters affinity of
active site for ATP
ADP + Pi bind to subunit
then spontaneously form ATP
Binding Change mechanism of ATP synthesis
H+ translocation through ATP synthase alters affinity of
active site for ATP
ADP + Pi bind to subunit
then spontaneously form ATP
∆G for ADP + Pi = ATP is ~0
role of H+ translocation is to
force enzyme to release ATP!
Binding Change mechanism of ATP synthesis
1) H+ translocation alters affinity of active site for ATP
2) Each active site ratchets through 3 conformations that
have different affinities for ATP, ADP & Pi
due to interaction with the
subunit
Binding Change mechanism of ATP synthesis
1) H+ translocation alters affinity of active site for ATP
2) Each active site ratchets through 3 conformations that
have different affinities for ATP, ADP & Pi
3) ATP is synthesized by rotational catalysis
g subunit rotates as H+ pass through Fo, forces each
active site to sequentially adopt the 3 conformations
Evidence supporting chemiosmosis
1) ionophores (uncouplers)
2) can synthesize ATP if create ∆pH
a) Jagendorf expt: soak cp in pH 4 in dark, make ATP
when transfer to pH 8