III. Metabolism Photosynthesispeople.uleth.ca/~steven.mosimann/bchm3300/Bchm3300_L10.pdf ·...
Transcript of III. Metabolism Photosynthesispeople.uleth.ca/~steven.mosimann/bchm3300/Bchm3300_L10.pdf ·...
Biochemistry 3300 – Photosynthesis I Slide 1
III. Metabolism
Photosynthesis
Department of Chemistry and BiochemistryUniversity of Lethbridge
Biochemistry 3300
Biochemistry 3300 – Photosynthesis I Slide 2
Overall Scheme of Photosynthesis
Photosynthesis can be separated into“light” and “dark” reactions:
CO2 + H2O → O2 + (CH2O)
Yield Energy
ConsumeEnergy
Dark reaction (consume energy):
2H2O + 2NADP+ → 2NADPH + 2H+ + O2
Light reaction (yield energy):
Light Reactions
Dark Reactions
Biochemistry 3300 – Photosynthesis I Slide 3
Photosynthesis Takes Place in Chloroplasts
Stroma contains flattenedvesicles (thylakoids), stackedin grana.→ contains membrane-associated
ATP synthesis machinery
Light reactions: thylakoid membranes
Dark reactions: stroma
They have two membranes:→ outer membrane is porous
the inner is not.
Biochemistry 3300 – Photosynthesis I Slide 4
Electromagnetic Radiation
The Spectrum of electromagnetic radiation, and the energy of photons in thevisible range of the spectrum.
Note: 1 mole of photons = einstein
Biochemistry 3300 – Photosynthesis I Slide 5
Determination of the Action Spectrum of Photosynthesis
Oxygen electrode allows measurement of O2 production.Action spectrum describes the relative rate of photosynthesis forillumination with a constant number of photons of different wavelengths.→ correlates with absorption spectra of photosynthetic chromophores
Biochemistry 3300 – Photosynthesis I Slide 6
Absorption of Visible Light by Chromophores.
A combination of chlorophylls together with accessory pigments allow most of the sun’s energy to be harvested.
Absorption in the red and blueareas by plants (chlorophylls)gives them their green color.→ only green light is left to
reflect or transmit
Absorption of 1 mol of photons yields 170-300 kJ
Biochemistry 3300 – Photosynthesis I Slide 7
Important Chromophores in Photosynthesis
Primary pigments are Chlorophylls a, b (plants), Bacteriochlorophyll (bacteria),and phycobilins (algae, cyanobacteria)
Biochemistry 3300 – Photosynthesis I Slide 8
Important Chromophores in Photosynthesis
Primary pigments are Chlorophylls a, b (plants), Bacteriochlorophyll (bacteria),and phycobilins (algae, cyanobacteria)
Biochemistry 3300 – Photosynthesis I Slide 9
PhycobilisomesLight Harvesting Complex of cyanobacteria
Phycobilins are covalently linkedto specific binding proteins
→ phycobiliproteins
Phycobiliproteins associate in highly ordered complexes.
→ phycobilisomes→ light-harvesting structure in e.g. red algae
Phycobilin pigments bound to specific proteins form complexes calledphycoerythrin (PE), phycocyanin (PC), and allophycocyanin (AP)
Biochemistry 3300 – Photosynthesis I Slide 10
Important Chromophores in Photosynthesis
β-Carotene (a Carotenoid) and Lutein (a Xanthophyll) are accessory pigmentsin plants.
Biochemistry 3300 – Photosynthesis I Slide 11
The Light-harvesting Complex from Plants
The functional complexis a trimer of LHCII.
→ containing 6 Lutein and36 Chlorophyllmolecules
7 Chlorophyll a5 Chlorophyll b2 Lutein
View along the membrane plane
Biochemistry 3300 – Photosynthesis I Slide 12
Light HarvestingComplex (non-plant)
Green – chlorophyllYellow – accessory pigments
Biochemistry 3300 – Photosynthesis I Slide 14
Photosystems and the Thylakoid Membrane.
Thylakoid membranes arefunctionally arranged into photosystems with 50-200chromophores per system
Light is transduced to chemicalenergy only in the photochemicalreaction center.→ a special pair of chlorophyll a
molecules
The function of most of thechromophores in a photosystem isto harvest light photons and totransfer, by excitation transfer,the energy successively to thereaction center.
Biochemistry 3300 – Photosynthesis I Slide 15
Photosystems and the Thylakoid Membrane.
Thylakoid membranes arefunctionally arranged into photosystems with 50-200chromophores per system
Light is transduced to chemicalenergy only in the photochemicalreaction center.→ a special pair of chlorophyll a
molecules
The function of most of thechromophores in a photosystem isto harvest light photons and totransfer, by excitation transfer,the energy successively to thereaction center.
Biochemistry 3300 – Photosynthesis I Slide 24
Functional Modules of the Photosynthetic Machinery
There are several classes of photosynthetic reaction centers
I. Single reaction centers → Type I Fe-S and → Type II Pheophytin
-Quinone
II. Double reaction centers (Cyanobacteria, algae, and vascular plants)
Biochemistry 3300 – Photosynthesis I Slide 25
Functional Modules of the Photosynthetic Machinery
Purple bacteria – Light photons excite the P870 chromophore
→ Electron flow is cyclic
→ Proton gradient is generated
→ ATP synthesis
Biochemistry 3300 – Photosynthesis I Slide 26
Photoreaction Center of Purple Bacteria
PDBid 1PRC
X-ray structure of purple bacterium reaction center.
→ 3 integral membrane subunits→ Cytochrome c outside the membrans
Space-filling models are thechromophores in the membrane andthe cytochrome c part.
Biochemistry 3300 – Photosynthesis I Slide 27
The Electron Transfer
The Structure together with rapid reaction kineticsallowed the assignment of the sequence of eventsin electron transfer.
1. Lights excites the Chl2 pair2. e- pass to Pheophytin3. e- move to Quinone A4. e- flow through non heme iron
to Quinone B (diffusible)
5. The electron hole in Chl2 is filled by e- transfer a heme of Cyt c
6. Fully reduced Quinone B diffuses to Cyt bc1 and initiate the formation of the proton gradient
very slow!
Biochemistry 3300 – Photosynthesis I Slide 28
Functional Modules of the Photosynthetic Machinery
→ Electron flow is cyclic
→ Proton gradient is generated
→ ATP synthesis
Green-sulfur bacteria also have a P840 and Cytochrome bc1
Coupled reaction:H2S is oxidized to S, then to SO4
2- toprovide electrons to P840
But some of the e- are passed to theredox protein ferredoxin, through a reductaseto produce NADH
That’s a problem! What could be the solution?
Biochemistry 3300 – Photosynthesis I Slide 29
Two Reaction Centers Act in Plants
In plant chloroplasts photosynthesisrequires two reaction centers.
The “Z Scheme” is not cyclic.
H2O is oxidized and NADP+ isreduced to NADPH.
Two photon absorptions are neededto drive the reduction in PSI/PSII
Oxygen is evolved in a specificcomplex (water-splitting complex)
A proton gradient is developed
Biochemistry 3300 – Photosynthesis I Slide 30
Two Reaction Centers Act in Plants
PS II resembles the purple bacteriareaction center.PSI resembles the green-sulfurbacteria center.
H2O oxidation in plants is analogous to the H2S oxidation by green-sulfur bacteria.→ activated e- are replaced→ oxygenic photosynthesis
Plastocyanin is a diffusible smallprotein (one e- carrier) thatshuttles between PSII and PSI
All O2 evolving organisms havetwo coupled photosystems.
Biochemistry 3300 – Photosynthesis I Slide 32
Two Reaction Centers Act in Plants
Overall Z-scheme
2 H2O + 2 NADP+ + 8 hv → O2 + 2 NADPH + 2 H+
One electron transferred per 2 photons absorbed, forming O2 requires4 electron transfers (8 photons)
Biochemistry 3300 – Photosynthesis I Slide 33
PSII Mechanism
Transmembrane protein (D1/D2 dimer), but electrons flow only through D1.
e- flow from the Mn4
center (active site ofthe water-splitting enzyme) to replacee- excited by lightin P680.
After excitations, e-
flow through carriersto quinone PQB and tocytochrome b6f
Biochemistry 3300 – Photosynthesis I Slide 34
PSI Mechanism
Excited e- from P700 [(Chl)2pair] is transferred through ferredoxin.→ Ferredoxin:NADP+ oxidoreductase transfers e- to NADP+
→ NADPH and ferredoxin(ox)
Plastocyanin (soluble Cu-containig transporter) resupplies P700+ with e-
Biochemistry 3300 – Photosynthesis I Slide 35
The PSI Complex is Associated with Antenna Chlorophylls
PDBid 1JBO
The proteins in the complex hold the components rigidly in orientations thatmaximize efficient exciton transfer betweenexcited antenna molecules towards P700
Trimeric structure viewed from thethylakoid lumen.
PSI monomer with proteins removed
Fe-S centers
Biochemistry 3300 – Photosynthesis I Slide 36
Cytochrome b6f Complex Links to Photosystems I and II
Connection between PSII and PSI centers→ Plastoquinol formed in PSII is oxidized
by the cytochrome b6f complex→ a Q-cycle generates a proton gradient→ H+ are pumped from stroma to lumen
PDBid 1VF5
Biochemistry 3300 – Photosynthesis I Slide 37
Proton and Electron Circuits in Thylakoids
An ATP synthase in Chloroplasts harnesses theproton gradient generated by the cytochrome b6fcomplex to synthesize ATP.
Biochemistry 3300 – Photosynthesis I Slide 38
Water-splitting Activity in the Oxygen-evolving Complex
The ultimate source of e- is Water → splitting requires energy of 4 photons.
Each photon into P680 ejects an e- → replaced by e- from Mn complex
The Mn4+ complex accepts four e- from 2 H2O → Oxygen (O2)
Biochemistry 3300 – Photosynthesis I Slide 39
Alternatives
An alternative path of light-inducedelectron flow allows variation inthe amounts of ATP and NADPHthat are made.
→ Cyclic pathway e- partitioned at Ferrodoxin some return to Cyt b6f
→ only PSI is involved in this pathway→ resembles the green-sulfur bacteria
pathway
→ No O2 or NADPH is made, butH+ is pumped and ATP is synthesized.
Biochemistry 3300 – Photosynthesis I Slide 40
Cyanobacteria
Cyoanobacteria use cyt b6f, cyt c6
and plastoquinone for both
- oxidative phosphorylation→e- flow from NADH to O2
- photophosphorylation →e- flow from O2 to NADPH
→ occure at the same membrane(no motiochondria or chloroplasts exist)
Both processes are accompanied byH+ movement across the membrane
Biochemistry 3300 – Photosynthesis I Slide 41
ATP Synthase of Chloroplasts is similarto Mitochondrial Enzyme
ATP is synthesized in chloroplasts by a large complex with thetwo functional components CFO and CF1
CFO is a transmembrane proton pore andCF1 is a peripheral membrane protein similarin structure and function to mitochondrial F1
ADP + Pi → ATP + H2Olight
Biochemistry 3300 – Photosynthesis I Slide 42
A Single Protein Absorbs Light in Halophilic Bacteria
PDBid 1C8R
Bacteriorhodopsin has seven membran-spanning α-helices. The chromophore all-trans retinal is covalently attached
via a Schiff base to an ε-amino groupof Lysine.
A series of Asp and Glutogether with closeassociated H2O providea transmembrane pathfor protons.
Biochemistry 3300 – Photosynthesis I Slide 43
Light-driven Proton Pumping in Bacteriorhodopsin
In the dark the Schiff base is protonated.→ Illumination photoisomerizes the all trans-retinal → H+ is transferred to Asp85