Transcript Energy Metabolism V Autotrophy and Lithotrophy

Fermentation:
Catabolism of carbon in
the absence of a terminal
electron acceptor (like O2)
for electron transport chain
Compare the DEh for putting electrons onto O2 vs. lactate
The unusual fermentation of oxalate by
Oxalobacter formigenes
Thank goodness for this hard-working anaerobe in your gut: it degrades
oxalate from amino acid catabolism, coffee, tea, fruits, veggies… and
helps prevent kidney stones!! You can lose it by taking doxycycline and
other antibiotics, but can regain it by… guess how?
And now for something completely different!
Photosynthesis and Autotrophy
I. Photosynthesis
A. General Aspects
B. Classes of Photosynthetic Bacteria
C. Mechanism of Photosynthesis
1. Anoxygenic Photosynthesis
2. Oxygenic Photosynthesis
D. Halobacterium (light-driven H+ pump)
II.
Autotrophy
A. General Aspects
B. Types of Autotrophic Pathways
PHOTOSYNTHESIS
(Photoautotrophy)
Excited state
photon
X
CO2
NADP+
e-
Ground state
CH2O
NADPH
PHOTOAUTOTROPHY: 2 reactions
1. LIGHT

CHEMICAL ENERGY
(ATP)
2. CO2 reduction
→
Organic compounds
Phototrophic Prokaryotes:
the metabolic menu
Group
Reducing power
Oxidized product
Purple nonsulfur bacteria
H2, reduced organic
Oxidized organics
Purple sulfur bacteria
H2S
SO4-2
Green sulfur bacteria
H2S
SO4-2
Green non sulfur bacteria*
H2S
SO4-2
Heliobacteria**
Lactate, organics
Oxidized organics
Cyanobacteria
H2O
O2
Prochlorophytes***
H2O
O2
*Most ancient?
**Gram positive, heterotrophs
***Related to cyanobacteria
Three types of photochemical energy capturing
systems in microorganisms:
1. Carotenoid-based light-capturing system that
is structurally similar to rhodopsin in eyes. In
halophilic Archaea.
2. Anoxygenic (uses chlorophyll, no O2 made)
3. Oxygenic (uses chlorophyll, splits water,
generates oxygen)
Carotenoid-based (bacteriorhodopsin)
-no chlorophyll, no metals: protein with G-protein coupled receptor-like structure
plus chromophore (retinal)
-chromophore is a long-chain hydrocarbon with extensive conjugation
-ancient protection for oxygenic phototrophs against toxic O2
-light-powered ion transfer
Nagel et al. 2005. Mechanics of Biolenergetic Membrane Proteins 33: 863
Photosystems do not absorb at short enough wavelengths to split
water, so must get e-’s somewhere else.
Cyclic: electrons run in closed circuit
Photosystems can take light energy strong enough to split water.
Non-cyclic (although cyclic can occur)
Chlorophyll: Light Harvesting Molecule
Porphyrin (like heme in cytochromes, but Mg instead of Fe)
Bacteriochlorophyll: Absorbs at ~700 nm; allows light harvesting at depths
where light is low and environment is anoxic
Not enough energy to extract e- from H2O; must use H2S instead
Eventually, chlorophyll evolved. Utilizes a short enough wavelength (680
nm) to split H2O and generate O2.
Consequence of oxyenic photosynthesis in evolution:
*DNA absorbs UV at 260 nm; mutations occur
*Some exant organisms are resistant to damaging radiation
(e.g. Deinococcus radiodurans: survives 100 rad while 10 rads kills
us… D. radiodurans is resistant to chromosome shattering and
mutation)
-
O2 is a reactive molecule: ·O2- H2O2
-
At first, protected by Fe+2 (ferrous iron): Fe+2 + O2  FeOH3
Banded iron formations from
Wittenoom Gorge in Australia
OH ·
Consequence of oxyenic photosynthesis in evolution:
-
Bacteria began evolving carotenoids: protection against singlet
oxygen; convert to less toxic state
-
Eventually (at least 2 billion years ago), used up ferrous iron
-
Accumulation of O2 in atmosphere
-
O2 + sun (UV radiation) → O3 (ozone)
-
Ozone screened out wavelengths below 290 nm
-
Life could evolve on land, because water no longer necessary to
screen out damaging/mutagenic UV radiation
Production of Reactive Oxygen Species (ROS)
During normal cellular respiration, oxygen is reduced to water and highly
reactive superoxide ( ·O2- ).
O2 + 4H+ + 4 e-
2 H2O ( about 95% of the time)
2 O2
(about 5% of the time)
Reactive oxygen species react with nucleic acids, sugars, proteins and lipids eventually leading to molecular degradation.
Cellular Defense Mechanisms Prevent ROS Buildup.
-
Due to the oxygen rich environment
in which proteins exist, reactions with
ROS are unavoidable.
-
Superoxide dismutase, catalase, and
glutathione peroxidase are natural
antioxidants present in organisms
which eliminate some ROS. Other
molecules are antioxidants too (e.g.
ascorbic acid, or Ignose/Godnose!)
-
Glutathione peroxidase catalyzes the
reduction of peroxide by oxidizing
glutathione (GSH) to GSSG.
O
O-
2O2
H2O2 + O2
GSH + H2O2
O
SH
glutathione
peroxidase
superoxide
dismutase
H2O + O2 + GSSG
O-
S
2
O
O
H
N
NH3
H
N
O-
N
H
NH3
O-
N
H
O
GSH
GSSG
O
GSSG
Detection of algal blooms from satellites via remote sensing:
relies on reflected spectral properties of chlorophylls.
Nutrient upwelling (El Nino) = phytoplankton blooms
Photosynthesis in the open oceans
•Compared to freshwater, nutrients (N, P, Fe) are limiting. Fewer
cells found than in freshwater (only 106/mL prokaryotes and 104
eukaryotes)
•Because oceans are huge, collective O2 production and CO2 fixation
there is a major contributor to Earth’s carbon balance.
•Influence food chain, global climate
•Many marine microbes use light to drive ATP synthesis.
–Photic zone = upper 300 meters
–Oxygenic and anoxygenic photosynthesis
–Chlorophylls a and b (cyanobacteria and relatives; algae)
–Proteorhodopsin (very similar to bacteriorhodopsin but
Bacteria, not Archaea)
Phototrophic Primary Producers
(red = chlorophyll)
Phototrophic Prokaryotes:
1.
2.
3.
Purple nonsulfur bacteria
Green nonsulfur
Purple sulfur bacteria
(sulfur inside cell)
4.
Green sulfur bacteria
(sulfur outside cell
5.
Domain Bacteria
Heliobacteria
(G+ relatives of Clostridium, endospores, N2fixation)
5.
6.
7.
Cyanobacteria
Prochlorophytes
Halobacterium-type
1 group of “photocapable” prokaryotes in the Domain Archaea
(the halobacteria = extreme halophiles [salt-loving])
Photosynthetic Prokaryotes
Group
Reducing power
Oxidized product
Purple nonsulfur bacteria
H2, reduced organic
Oxidized organics
Purple sulfur bacteria
H2S
SO4-2
Green sulfur bacteria
H2S
SO4-2
Green non sulfur bacteria
H2S
SO4-2
Heliobacteria*
Lactate, organics
Oxidized organics
Cyanobacteria
H2O
O2
Prochlorophytes**
H2O
O2
*Gram positive, heterotrophs
**Related to cyanobacteria
Chlorophyll Diversity
Different absorbance maxima =
different niches… e.g. lower or higher
in water column.
Chlorophyll (cyanobacteria) = 680 nm
Bchl a (purple bacteria) = 805, 870
Structure of bacteriochlorophylls
Accessory
pigments:
Carotenoids
Accessory pigments:
Phycobilins
Photosynthetic Membranes
Reaction center chlorophyll
-few
-convert light energy to ATP
Light harvesting chlorophyll
-many
- “antenna”
-captures “faint signal” of low light
environments
Accessory pigments
Carotenoids
Phycobilins
… light harvesting complex in cyanobacteria, plants
Mechanism of Photosynthesis
1) Anoxygenic Photosynthesis
• Cyclic
• Your text: Fig. 17.14 , 17.15, and 17.18
• Purple Bacteria
• Green Bacteria
• Heliobacteria
Purple Bacteria
(within phylum Proteobacteria)
• photosynthetic membranes are lamellae or tubes with the
plasma membrane
• bacteriochlorophyll a or b
• accessory pigments are purple colored carotenoid pigments
(see Fig. 12.5 in your text)
• may live as photoheterotrophs
two types:
1. sulfur
2. nonsulfur
Green Bacteria
• photosynthetic membranes are vesicles attached to but not
continuous with the plasma membrane
• bacteriochlorophyll c, b, or e (small amount of a in LH and
RC)
• accessory pigments are yellow to brown-colored carotenoids
• two types: 1. sulfur (green sulfur bacteria phylum)
2. nonsulfur (green nonsulfur bacteria phylum)
Heliobacteria
• plasma membrane only (no specialized photosynthetic
membranes)
• bacteriochlorophyll g
• Photoheterotrophs: require organic carbon
• These are the only Gram-positive photosynthetic bacteria
Electron donors: H2S, Fe2+, S0, etc.
Anoxygenic Photosynthesis
strong e- donor
Purple bacteria
Purple bacteria
Cyclic
NAD(P)H and ATP can
be generated by PMF
Many cyanobacteria can use H2S as an electron donor for
anoxygenic photosynthesis.
Elemental sulfur globules outside
filamentous cyanobacterium
Oscillatoria limnetica
Green bacterium (Chlorobium):
external sulfur deposits
Purple bacterium (Chromatium):
internal sulfur deposits
Variation on the Theme
ATP & NAD(P)H
ATP only
ATP only
*
*
* Off to supply reducing power for CO2 fixation via reverse citric acid cycle
Green Sulfur Bacteria
(Chorobium, Chlorobaculum, Prosthecochloris)
Aquatic, anoxic environments
Most are facultative heterotrophs; strict autotrophy requires
reverse TCA cycle
Have chlorosomes: very efficient at light harvesting so live at
great depths
May form consortia – aggregates of cells that have differing
metabolic duties; chemotrophic and phototrophic
(epibiont) components. Example: Chlorochromatium
aggregatum (not a formal taxonomic name because not a
single species)
Green Non Sulfur Bacteria
(Choroflexus)
Filamentous, form microbial mats with cyanobacteria in
neutral to alkaline hot springs
Like Green Sulfur Bacteria: has chlorosomes
But reaction center of in cell membrane is like purple
bacteria
Earliest known photosynthetic bacterium: perhaps reaction
center first, chlorosome later by HGT
Most are facultative heterotrophs; CO2 fixation requires
hydroxypropionate pathway (unique to very ancient
organisms)
Light harvesting complex in green photosynthetic
bacteria (both sulfur and non-sulfur)
Chlorosome is a giant
antenna: Bchl c, d, or e
BP = baseplate (proteins)
LH = light harvesting
complex (Bchl a)
RC = reaction center (Bchl a)
Chlorosomes (EM, stained dark)
-in green sulfur bacteria
-lie along the inside of cytoplasmic membrane
-proteinaceous (nonlipid) membrane
-each vesicle contains ~ 10,000 bacteriochlorophyll c molecules in tubes/rods
-chlorosomes transmit energy via subantenna of bacteriochlorophyll a.
Mechanism of Photosynthesis
Oxygenic Photosynthesis
• Photosystems I & II
• Noncyclic
• Your text, Fig. 17.19
• Cyanobacteria
• Algae (protists)
• Plants
Cyanobacteria (phylum contains cyanobacteria
and prochlorophytes)
• Synechococcus, Oscillatoria, Nostoc, Anabaena
• photosynthetic mebranes are multilayered lamellae
• formerly called “blue-green algae” but now known to be
prokaryotic and possess peptidoglycan
• chlorophyll a only
• accessory pigments are carotenoids and phycobilin proteins
• Photosystem I and II are present (oxygenic photosynthesis)
• Autotrophs
• Gas vesicles frequent
• Some are filamentous, N2 fixing (heterocysts)
Lake Mendota up close:
eutrophic (nutrient-rich) lake
algal blooms July through September (ag runoff)
Electron donor: H2O
Halobacterium-type
• Use light-driven proton pump consisting of patches of the
pigment bacteriorhodopsin in cytoplasmic membrane
• bacteriorhodopsin resembles rhodopsin, the visual pigment
• Absorbs light near 570 nm (green region of spectrum)
• Extreme halophile (2-4M NaCl = 12-23%): balances Na+
outside with K+ inside to maintain osmotic equilibrium
• Heterotrophs (use amino acids and organic acids for growth)
• Most are obligate aerobes; some can do anaerobic respiration
or fermentation
Solar Salt
Evaporation
Ponds
(salterns) in
CA
Red
coloration
due to
carotenoids
of
halobacteria
Colonies of halobacteria isolated from Portsmouth salt piles.
Plates contain 25 % NaCl !
Halobacteria
• Domain Archaea
Oops, wrong, outdated hypothesis
• Not autotrophs - grow as
chemoheterotrophs but can
function as phototrophs
• Bacteriorhodopsin, proteorhodopsin =
cytoplasmic membrane-associated
photopigment similar to rhodopsin
of mammalian eye.
•Bacteriorhodopsin is a light driven ion
(proton) pump...
Homologous protein in Halobacteria is
called halorhodopsin; a chloride pump
Light at 570 nm excites the retinal chromophore of bacteriorhodopsin,
converting it from its normal all-trans conformation to a cis form.
Conversion instigates the movement of a proton across the membrane.
Proton loss returns retinal to its all-trans form.
Correct; see next slide
Chloride ions flow across
membrane in reverse
direction for halorhodopsin
Light + H+ = cis
Loss of H+ = trans
Arrangement of bacteriorhodopsin in the cytoplasmic membrane:
Purple structures are proteins (opsin) that hold the chromophore
(retinal)
Current model for how bacteriorhodopsin and halorhodopsin
work…
Biochemical studies show that rather than transporting H+ out,
bacteriorhodopsin (BR) may actually transport OH- in and
halorhodopsin (HR) may transport in a Cl- (from all that NaCl in
its environment)
Bacteriorhodopsin in the cell membrane. CP =
cytoplasm, EC = extracellular space. Arrows indicate
direction of ion transfer.
Bacteriorhodopsin and its retinal
chromophore. Yellow arrow indicates
direction of ion transfer.
Autotrophy
General Aspects
• Heterotrophs: organisms requiring
organic compounds as a carbon source
• Autotrophs: organism capable of
biosynthesizing all cellular material from
CO2; CO2 as a sole carbon source
Autotrophy
Types of Autotrophic Pathways
1. Calvin Cycle
Fig. 17.21 & 17.22
2. Acetyl-CoA Pathway
Fig. 17.41
3. Reverse TCA Cycle
Fig.17.24a
4. Hydroxypropionate Pathway
Fig. 17.24b
Calvin-Benson Cycle
• Fig. 17.21 & 17.22
Key enzymes:
A. Ribulose biphosphate carboxylase (RuBisCo)
• carboxyosomes : Inclusion bodies
B. Phosphoribulokinase
Calvin-Benson Cycle
Cyanobacteria
Key enzymes: ribulose biphosphate carboxylase (RuBisCo) = first enzyme,
phosphoribulokinase = final enzyme in cycle
Requires ATP and reducing power
Reverse TCA Cycle
some methanogens
Green Sulfur bacteria (Chlorobium)
Hydroxypropionate Pathway
Green Non-Sulfur Bacteria
(Chloroflexus)