Download light-activated electron transport with proton extrusion: photosynthesis

LIGHT-ACTIVATED ELECTRON TRANSPORT
WITH PROTON EXTRUSION: PHOTOSYNTHESIS
PHOTOSYNTHETIC METABOLISM
Outside reading for extra information: 1. Excellent review of Reaction center
structure-function. J. Deisenhofer and H. Michel, "The Photosynthetic reaction
center from the purple bacterium Rhodopseudomonas viridis." Science 245: 14631473 (Nobel Prize lecture/article).
2. Outstanding, short, and very highly recommended overview of methanogenesis by
Ralph S. Wolfe: Alessandro Volta's combustible air. ASM News 62: 529-534.
Carbon dioxide reduction (carbon fixation) is conceptually the inverse of respiration.
There is no requirement for light in CO2 fixation, after all the chemolithoautotrophs use
the same carbon reduction pathway as cyanobacteria and higher plants: the Calvin Cycle.
6CO2 + 18 ATP + 12NADPH --> C6H12O6 + 18ADP + 12 NADP+
Nearly all autotrophs utilize the Calvin Cycle pathway (see in-class handout for details).
Exceptions: green sulfur bacteria, which utilize the reverse TCA path, and Chloroflexus
aurantiacus, an enigmatic filamentous organism that uses the hydroxypropanoate
pathway (see in class handout for details).
Melvin Calvin received the Nobel Prize for working out the details of the Calvin
pathway; it was the first major application of radioisotopes to the solution of a
biochemical pathway (used 14CO2 to label intermediates of the pathway).
The key enzyme of the Calvin cycle is RuBisCO, suggested to be the most abundant
enzyme on Earth (?) :
Ribulose 1,5-Bisphosphate Carboxylase-Oxygenase. It carboxylates the 5-carbon sugar
ribulose-1,5-bisphosphate and produces 2 molecules of 3-phosphoglycerate; oxygen is a
competitive inhibitor of the reaction, competing with CO2 for binding. Activation by
phosphorylation, followed by reduction produces glyceraldehyde-3-phosphate which can
be converted to glucose. Sugar rearrangements in steady-state can regenerate ribulosebis-phosphate starting material (sugar rearrangement reactions similar to those of the
pentose phosphate pathway).
4 Groups of Photosynthetic Bacteria
1. Oxygen-evolving
a. cyanobacteria
b. prochlorophytes
2. Purple bacteria
a. purple sulfur bacteria
b. purple non-sulfur bacteria
3. Green bacteria
a. green sulfur bacteria
b. green gliding bacteria
4. Heliobacteria
1. Oxygen-evolving
A. CYANOBACTERIA (G- in wall type, but similar to G+ in many biochemical and
genetic properties.
Characterized by presence of Chlorophyll a and water-soluble light-harvesting proteins
known as PHYCOBILIPROTEINS. Form unique light-harvesting structure, known as
phycobilisomes.
Examples: Synechococcus, Nostoc, Anabaena, Synechocystis,Oscillatoria
B. PROCHLOROPHYTES: very similar to cyanobacteria, and closely related to them.
Lack phycobiliproteins, but contain Chlorophylls a +b like higher plants.
Examples: Prochloron, Prochlorothrix, Prochlorococcus (an abundant marine organism)
2. PURPLE BACTERIA (true G-, proteobacteria, like E. coli). Reaction centers related
to those of PS II of higher plants
A. PURPLE SULFUR BACTERIA. Autotrophs that utilize H2, H2S, S0, S2O3-2 as
electron sources. Contain either Bacteriochlorophyll a or b. More reduced that
Chlorophyll a--chemically distinct with distinct absorption properties (longer
wavelengths than chlorophyll a, absorbs in the blue and the near-infrared). The
characteristic reddish-purple color does not come from Bchl (pale blue-gray) but from
carotenoids.
Examples: Chromatium, Ectothiorhodospira, Thiocapsa, Thiopedia,
B. PURPLE NON-SULFUR BACTERIA
Photoheterotrophs under anaerobic conditions; typically chemoheterotrophs under
aerobic conditions.
Examples: Rhodobacter, Rhodospirillum, Rhodopseudomonas, Rhodocyclus, Rhodoferax
3. GREEN BACTERIA
A. Green sulfur bacteria. Contain specialized light-harvesting structures,
CHLOROSOMES, that contain either Bchl c, d, or e in addition to Bchl a. Typically
either green or brown in color. Reaction centers similar to PS I of higher plants. Unique
carbon fixation pathway--the reverse TCA cycle. Electrons from H2, S0, S2O3-2, etc.
Examples: Chlorobium, Pelodictyon, Prosthecochloris
B. Green gliding bacteria. Contain chlorosomes with Bchl c + Bchl a. However, have
reaction centers like purple bacteria (quinone acceptors) and have another carbon fixation
pathway--the hydroxypro-panoate pathway (see handout).
Examples: Chloroflexus, Oscillochloris, Heliothrix
4. HELIOBACTERIA
True G+ bacteria, closely related to Clostridium sp. Have reaction centers related to green
sulfur bacteria and higher plant photosystem I RC. Form heat-resistant endospores, and
are photoheterotrophs under all conditions--no known autotrophs to date. Contain Bchl g
(unique, related to Chl a), and are green in color.
Examples: Heliobacillus, Heliobacterium, Heliophilum
PROCESSES IN PHOTOSYNTHESIS
1. Chromophores absorb light energy, and energy migrates to "reaction centers". Very
rapid, usually ~1-100 picoseconds.
2. PHOTOCHEMISTRY = Light-driven electron transport reaction. Absolutely
dependent upon Chlorophylls. Occurs at special protein complex known as REACTION
CENTER (also referred to as PHOTOSYSTEMs in cyanobacteria and higher plants).
The primary oxidized species in all of photosynthesis is a special pair of chlorophylls, the
"special pair."
3. Dark electron transport reactions follow initial charge separation event. No
involvement of light. Very similar to respiration.
4. PHOTOPHOSPHORYLATION, ATP synthesis, occurs via chemiosomotic
coupling.
5. Other dark biochemical reactions: CO2 fixation produces biomass).
CLASSES OF CHROMOPHORES AND PROTEINS
Chromoproteins: proteins with light-absorbing prosthetic groups.
1. CHLOROPHYLLS
Proteins that bind chlorophylls have 2 functions:
1. Antenna--absorb photons, transfer energy (not electrons) to reaction centers.
2. Reaction centers perform PHOTOCHEMISTRY -- UNIQUE!! Light absorption leads
to electron transfer from donor chlorophyll to an acceptor. Charge separation occurs.
It should be noted that most chlorophyll proteins also bind carotenoids--for protection
(see below).
2. CAROTENOIDS
Also have 2 functions:
1. Antenna--harvest light energy, transfer the energy to reaction centers for
photochemistry
2. Photoprotection.
P + light --> 3P* + O2 --> 1O2 (singlet oxygen)
3
P* + carotenoids --> P
1
O2 + carotenoids --> O2 + carotenoid
1
O2 + carotenoids --> oxidized carotenoid
3. PHYCOBILIPROTEINS.
Unique to cyanobacteria. Only function as antenna proteins. Have linear tetrapyrroles as
chromophores (heme ring cleaved to form linear molecule). Very high absorbtivity for
visible light. Form PHYCOBILISOMES.
REACTION CENTERS
Site of photochemistry, and hence, are Bchl/Chl containing proteins. Differ in
organization from antenna proteins by having appropriate electron acceptors near a pair
of chlorophylls. Light absorption at special pair causes oxidation of chlorophyll,
reduction of acceptor = charge separation = electron tranfer.
Two types occur in nature:
1. Type I Reaction centers.
All have Fe-S centers as electron acceptors and produce very strong reductants. Can
directly reduce NAD(P)H
Examples: Photosystem I (PS I) of cyanobacteria, higher plants; green-sulfur bacterial
RC; heliobacterial RC
2. Type II Reaction Centers
All have quinone acceptors and CAN NOT reduce NAD(P)+ directly--requires reverse
electron flow.
Examples: Photosystem II (PS II) of cyanobacteria and higher plants; purple bacterial
and Chloroflexus sp. RC
In most cases EXCEPT cyanobacteria/
prochlorophytes: reaction center used primarily to produce a proton gradient for ATP
synthesis and to provide driving force for reverse electron flow. Light-driven electron
transport is CYCLIC, and no net oxidation/reduction can take place. Reductant for CO2
fixation comes from organic compounds ("non-sulfur" bacteria) or from inorganic
sources ("sulfur" bacteria). Defines ecological niches for these organisms.
CYANOBACTERIA and PROCHLOROPHYTES
The presence of 2 RCs, PS II and PS I allows NON-CYCLIC electron transport to occur.
PS II oxidizes water, producing a reduced quinone and oxygen as products. ET chain
delivers electrons to oxidized PS I and creates a proton gradient for ATP synthesis. PS I
can reduce Fe-S protein ferredoxin, which in turn can reduce NADP+ to form NADPH.
Electron flow from H2O to NADP+ follows the so-called "Z-scheme." CYCLIC ET can
also occur around PS I, producing only a proton gradient for ATP synthesis. For some
details, see in-class handout).