Photophosphorylation
Light as an Energy Source
Joseph Priestly in 18th century
(plant and candle in a Bottle)
Validates Light as an Energy Source
hv
CO2 + H2O
CnH2nOn + O2
(starch, sucrose)
•Light energy is used to reduce C on CO2, reverse of oxidative phosphorylation
•This amazing ability changed the biosphere as we know it today (most ground
shattering events in earth’s history
Visible Light
Harnessed by life to sustain life
(produce 1011 tons of carbs=1018 Kj of Energy)
Photosynthesis is a two staged process by which light energy is
harnessed to oxidize H2O
Light Rxns
H2
O
hv
e-
NADPH +
O2
ATP
flow
H+
H+
H+
Dark Rxns
Ribulose-1,5-BiPhosphate +
CO2 + NADPH + ATP
Rubisco
3-Phosphoglycerate
Gluconeogenesis
CnH2nO
n
The Chloroplast
• very
similar to the mitochondria (permeable outer mem, impermeable
inner mem)
• Stroma contains enzymes involved in dark rxns, DNA/RNA/ribosomes
• Stroma surronds 3rd membrane of thylakoid
• Thylakoid mem contains proteins involved in, ATP production, light
harvesting, transport
• Harvest light using Photoreceptors comprised of pigments
Chlorophyll is a Porphyrin Like Heme
• chlorophyll (chl) is the principle photoreceptor in plants
• Variety of different chl exist throughout light harvesting organisms
• R group greatly affects the absorption spectra
• highly conjugated allowing strong absorption
• Chla is the most common chl in the plant kingdom
• Forms the LIGHT HARVESTING COMPLEX along with other pigments
Light Harvesting Complex (LHC)
hv
H2O
LHC
O2
e- mobilized
Photosynthetic
rxn center (PRC)
NADPH
ATP
chla is the principle
photoreceptor
Primary rxns of
photosynthesis
Highest amount of chla
LHC also contains accessory pigments to cover spectral regions
Chla cannot absorb
Accessory Pigments
Brilliant fall colours of
deciduous plants
Β-carotene
Water dwelling photosynthetic (long
λ)
Red algae which can be found well
below sea level (ocean flours)
Light Rxns
• Photosynthesis is the process by which excited e- from Chla are passed through
acceptor molecules converting electrical energy into chemical energy
• Energy of a photon can be determined by:
E = hv = hc/λ
Where E=energy, h=Plank’s constant
c=speed of light, λ=wavelength
Photons are absorbed by chromophores at a certain wavelengths, hence, the law
of conservation of energy applies ( energy difference between the ground state
and excited states of the e- must match energy of the photon)
During photosynthesis, light excited e- may dissipate energy in two fashions
A) Exciton transfer (LHC)
B) Photooxidation (PRC)
Upon absorption of photons, e- are excited to
another E level
This energy can be dissipated in several manners
Such as Photooxidation or Exciton transfer
Photosystem I (PS I) and Photosystem II
(PS II)
Photosynthesis is a noncyclic process that uses reducing power, provided by light
energy, to oxidize H2O. These events allow the production of ATP and NADPH
PS II = P 680, oxidize H2O produce O2 and QBH2
PS I = P 700, produce NADPH
hv
hv
NADP+
-
e
PS II
PS I
PROTONS
H2O
O2
NADPH
ATP
ATP, NADPH, and O2 production are at maximum when both complexes are absorbing
light
Mediators of Light Rxns
hv
NOTE THE SIMILARITIES WITH OXPHOS
Direction of e- flow
DCMU
8H+
4H+
Thylakoid
Tyrosyl radical (Z)
• the various prosthetic groups are organized in a Z-scheme
• depicts two photochemical events that drive e- from H2O to NADP+
Stroma
The OEC, H2O oxidation, and PS II
P 680+ most powerful oxidant in
nature. Captures e- from OEC
through tyrosyl radical (Z)
High positive redox potential
Mn (III-IV) during e- removal
4 step process requiring 4 photons
PS II* donates excited e- to Plastoquinone
Pheophytin a (Chl no Mg2+)
Plastoquinone (PQ)-Fe(II) complex
• PQ which accepts 2e- for transport to Cyt b6f
• QBH2 dissociates allowing anothor QB to bind
Cytochrome b6f
• Striking resemblance to Cyt C: ubiquinone oxidoreductase
• several prosthetics (including 2 hemes)
• 1 e- accepted, 2H+ are translocated (recall OEC)
• generates much of the electrochemical gradient that drives
ATP production
• Plastocyanin (PC, a Cu containing protein) transfers electrons
from cyt b6f to PS I
Met
Cu (II)
His
Cys
His
Std redox potential = 0.370V
Strained due to tetrahedral geometry
PS I Complex
•
•
•
•
Several peptides, numerous Chla (A0, A1) , 25
carotenoid molecules, series of [4Fe-4S] (Fx, Fa,
Fb), and 25 carotenoids
passage 1 e- at a time (from PC to P 700+)
electrons are passaged one at a time to Fd-NADP+
reductase in the stroma (once FAD is reduced it
passes the electrons and protons to NADPH
Ferredoxin (Fd) serves as the e- mediator between
PS I and Fd-NADP reductase
Photophosphorylation
• Chloroplasts produce ATP much in the same as mitochondria
– H+ gradient is dissipated
– intact thylakoid membrane
•
H+ translocating ATPase (cFocF1) resembles the mitochondrial
equivalent (FoF1)
– cFo and Fo are hydrophobic membrane spanners with H+ translocating
abilities
– cF1 and F1 are hydrophillic peripheral membrane proteins with several
subunits; Α3β3γδε
•
ATP production prevented by DCCD, oligomycin, and
protonophores (2,4-DNP, CCCP)
Stroma (alkaline)
Matrix (alkaline)
H+ translocation
H+ translocation
Thylakoid lumen (acidic)
Intermembrane space (acidic)
Summary
Dark Rxns
• Light rxns
– Directly dependent on light
– ATP, NADPH, O2
– Thylakoid lumen
• Dark rxns (Calvin Cycle)
– Dependent on products of light
rxns
– CnH2nOn
– Stroma
Rubisco (Ribulose-1,5 Biphosphate
Carboxylase/Oxygenase
Rubisco
(Lys)
Oxygen inhibits Rubisco:
Photorespiration
•
•
•
•
O2 can compete for the CO2 binding site on
Rubisco
Produce 3-Phosphoglycerate and 3Phosphoglycolate
The 3-Phosphoglycolate dissipates NADPH
and ATP through Photorespiration
Serves to protect the chloroplast from
photooxidative damage when CO2 is NOT
available
Regulation of Dark Rxns
1)
2)
3)
4)
5)
6)
CO2 and O2 compete for Rubisco
Rubisco functions optimally at pH of 8
3-phophoglycerate (alkaline-PG3-, acidic-PG2-). PG2- is an
allosteric inhibitor of Rubisco and is present when stroma is
alkaline
Rubisco activity is dependent on Mg2+
2-carboxyarabinitol-1-phosphate (CA1P)
Thioredoxin system (activates Fructose-1,6 Biphosphatase and
Sedohepulose-Biphosphatase)
Thioredoxin System