Download anaerobic respiration

•
Figure 17.10a shows a typical phycobilin, and
Figure 7.10b shows the structure of a
phycobilisome.
• The absorption spectrum of a cyanobacterium
that has a phycobiliprotein (phycocyanin) as an
accessory pigment is shown in Figure 17.11.
17.4 Anoxygenic
Photosynthesis,
• In anoxygenic photosynthesis, a
series of electron transport reactions in
the photosynthetic reaction center of
anoxygenic phototrophs results in the
formation of a proton motive force and
the synthesis of ATP.
• The production of ATP in photosynthesis
is called photophosphorylation.
• Reducing power for CO2 fixation comes
from reductants in the environment and
requires reverse electron transport in purple
phototrophs. Figure 17.13 illustrates the
structure of the reaction center of purple
phototrophic bacteria.
•
A general scheme of electron flow in anoxygenic
photosynthesis in a purple bacterium is shown in Figure
17.14.
•
Figure 17.15 shows the arrangement of protein
complexes in the photosynthetic membrane of a
purple phototrophic bacterium.
Photosynthesis in Other
Anoxygenic Phototrophs
•
Figure 17.18 contrasts the photosynthetic
electron flow of purple and green bacteria and the
heliobacteria.
17.2 Concept Check
A series of electron transport reactions occur in the photosynthetic
reaction center of anoxygenic phototrophs, resulting in the
formation of a proton motive force and the synthesis of ATP.
Reducing power for CO2 fixation comes from reductants present in
the environment and requires reverse electron transport in purple
phototrophs.
 How does photophosphorylation compare with
electron transport phosphorylation in respiration?
 What is reverse electron flow and why is it
necessary? Which phototrophs need to use reverse
electron flow?
17.3 Oxygenic Photosynthesis
17.3 Concept Check
In oxygenic photosynthesis, water donates electrons to drive
autotrophy, and oxygen is produced as a by-product. Two separate
light reactions are involved, photosystems I and II. Photosystem I
resembles the system in anoxygenic photosynthesis. Photosystem II
is responsible for splitting H2O to yield O2.
 Why is the term noncyclic electron flow used in
reference to oxygenic photosynthesis?
 What are the major differences between the two
reaction center chlorophyll molecules in
photosystems I and II regarding absorption
properties and E0'?
17.4 Autotrophic CO2 Fixation:
The Calvin Cycle
RubisCO and the
Formation of PGA
Stoichiometry of the
Calvin Cycle
17.4 Concept Check
The fixation of CO2 by most phototrophic and other autotrophic
organisms occurs via the Calvin cycle, in which the enzyme
ribulose bisphosphate carboxylase (RubisCO) plays a key role. The
Calvin cycle is an energy-demanding process in which CO2 is
converted into sugar.
 What reaction does the enzyme ribulose
bisphosphate carboxylase carry out?
 Why is reducing power needed for autotrophic
growth?
 What is a carboxysome?
17.5 Autotrophic CO2 Fixation:
Reverse Citric Acid Cycle and
the Hydroxypropionate Cycle
Carbon Dioxide Fixation
* Calvin Cycle
+
. CO2 Fixation within many autotrophic microbial and plant cells
. 3 CO2 + 9 ATP + 6 NADPH -> glyceraldehyde-3-phosphate + 9 ADP
6 NADP+
. Ribulose-1,5-bisphosphate carboxylase (RuBisCo) Caraboxysomes
* Reductive Tricarboxylic Acid Cycle Pathway
. Some potoautotrophs, such as the green sulfur
bacterium Chlorobium, fix CO2 via a reverse (reductive) tricarboxylic
acid cycle.
. This is achieved at the expense of three ATP molecules.
. The normal enzymes of the TCA cycle work in
reverse of the normal oxidative direction of the cycle.
. One exception ; Citrate lyase Citrate synthase
Autotrophy in Chloroflexus
* Hydroxypropionate Pathway
. In the green nonsulfur bacterium, Chloroflexus,
Two CO2 molecules are fixed and converted into
one acetyl-CoA via the hydroxypropionate pathway.
. The net result is that three CO2 molecules are
converted into one pyruvic acid molecule.
* C4 Pathway
. The oxaloacetate formed in this pathway can
then be used in amino acid and nucleic acid
biosynthesis.
. Although all organisms fix CO2 as part of their
metabolism, heterotrophic organisms are unable
to form a significant portion of their
macromolecules from the C4 pathway alone.
Assimilation of Organic C-1 Compounds
* Methanotrophy
. Bacteria that have the ability to use methane (CH4)- the most
reduced
form of carbon- as their sole carbon souce are called
methanotrophs.
. All methanotrophs are obligate aerobes that require O2 ; they are
obligate C-1 utilizers.
. Some methanotrophs such as Methylomonas, Methylococcus, and
Methylosinus can grow on various C-1 compounds -methanol, for
example-rather than only methane.
. Methane monooxygenase
. wide range substrate specificity
. oxidation of ammonium ions, chloromethane, bromoethane,
ethane,
propane, trichloroethylene, and various other compounds
. CH4 + O2 + NADH ---> CH3OH + H2O + NAD+
CH4 + O2 + Cytochrome C (reduced) ---> CH3OH + H2O +
Cytochrome
C (reduced)
. The methanol formed by methane monooxygenase is further oxidized to
formaldehyde.
. Type 1 methanotrophs (Methylomonas, Methylococcus) ; Ribulose
monophosphate cycle
. Type II methanotrophs (Methylosinus) ; Serine pathway
* Methylotrophy
. The more general class of heterotrophic aerobes that can utilize onecarbon organic molecules other than methane, are called methylotrophs.
. Some Pseudomonas. Bacillus, and Vibrio species use methanol, formate,
or methylamine as a carbon source.
. The methylotrophs use the serine pathway for assimilating C-1
compounds into organic molecules.
17.5 Concept Check
The reverse citric acid cycle and the hydroxypropionate cycle are
pathways of CO2 fixation found in green sulfur and green nonsulfur
bacteria, respectively.
 Including the route of CO2 fixation, discuss at least
three ways that you could distinguish a purple
sulfur bacterium from a green sulfur bacterium.
 Including the route of CO2 fixation, what
similarities and differences exist between green
sulfur and green nonsulfur bacteria?
II CHEMOLITHOTROPHY
17.8 Inorganic Electron
Donors and Energetics
17.8 Concept Check
Chemolithotrophs are able to oxidize inorganic chemicals as their
sole sources of energy and reducing power. Most chemolithotrophs
are also able to grow autotrophically.
 For what two purposes are inorganic compounds
used by chemolithotrophs?
 Why does the oxidation of H2 yield more energy
with O2 as electron acceptor than with SO42– as
electron acceptor?
17.9 Hydrogen Oxidation
• The hydrogen bacteria can oxidize H2
compounds, thereby generating a proton
motive force and ATP synthesis (Figure
17.25). These chemolithotrophs are also
autotrophs and fix CO2 via the Calvin cycle.
17.10 Oxidation of Reduced
Sulfur Compounds
• The sulfur bacteria can oxidize
reduced sulfur compounds such as H2S
and S0 (Figure 17.27). These
chemolithotrophs are also autotrophs
and use the Calvin cycle to fix CO2.
17.9–17.10 Concept Check
Hydrogen (H2) and reduced sulfur compounds such as H2S and S0
are excellent electron donors for chemolithotrophs. These
compounds can be oxidized by the hydrogen bacteria or the sulfur
bacteria, respectively, thereby generating a proton motive force and
ATP synthesis. These chemolithotrophs are also autotrophs and fix
CO2 by the Calvin cycle.
 What special enzyme is needed for growth on H2?
 How many electrons are available from the
oxidation of H2S if S0 is the final product? If SO42–
is the final product?
17.11 Iron Oxidation
• The iron bacteria are chemolithotrophs
that use ferrous iron (Fe2+) as their sole
energy source (Figure 17.30).
• Most iron bacteria grow only at acid
pH and are often associated with acid
pollution from mineral and coal mining.
Some phototrophic purple bacteria can
oxidize Fe2+ to Fe3+ anaerobically.
17.11 Concept Check
The iron bacteria are chemolithotrophs able to use ferrous iron
(Fe2+) as sole energy source. Most iron bacteria grow only at acid
pH and are often associated with acid pollution from mineral and
coal mining. Some phototrophic purple bacteria can oxidize Fe2+ to
Fe3+ anaerobically.
 Why is only a very small amount of energy
available from the oxidation of Fe2+ to Fe3+ at
acidic pH?
 What is the function of rusticyanin and where is it
found in the cell?
 How can Fe2+ be oxidized anoxically?
17.12 Nitrification and
Anammox
• In anoxic ammonia oxidation
(anammox), the nitrifying bacteria can use
ammonia and nitrite as electron donors, a
process called nitrification.
• The ammonia-oxidizing bacteria produce
nitrite (Figure 17.32), which is then
oxidized by the nitrite-oxidizing bacteria to
nitrate (Figure 17.33). Anoxic NH3
oxidation is coupled to both N2 and NO3–
production in the anammoxosome.
17.12 Concept Check
Ammonia and nitrite can be used as electron donors by the
nitrifying bacteria. The ammonia-oxidizing bacteria produce nitrite,
which is then oxidized by the nitrite-oxidizing bacteria to nitrate.
Anoxic NH3 oxidation is coupled to both N2 and NO3– production in
the anammoxosome.
 What is the inorganic electron donor for
Nitrosomonas? For Nitrobacter?
 What are the substrates for the enzyme ammonia
monooxygenase?
 What do nitrifying bacteria use as a carbon source?
 What is the anammox reaction and how does it
differ from aerobic nitrification?
III
THE ANAEROBIC WAY OF LIFE:
ANAEROBIC RESPIRATIONS
17.13 Anaerobic Respiration
Alternative Electron Acceptors
and the Electron Tower
• Although oxygen is the most widely used
electron acceptor in energy-yielding
metabolism, a number of other
compounds can be used as electron
acceptors.
•This process of anaerobic respiration is less
energy efficient but enables respiration in
environments where oxygen is absent.
•
Examples of anaerobic
respiration are illustrated
in Figure 17.35.
Assimilative and
Dissimilative Metabolism
17.13 Concept Check
Although oxygen is the most widely used electron acceptor in
energy-yielding metabolism, a number of other compounds can
be used as electron acceptors. This process of anaerobic
respiration is less energy efficient but makes it possible for
respiration to occur in environments where oxygen is absent.
 What is anaerobic respiration?
 With H2 as electron donor, why is the reduction of
NO3– a more favorable reaction than the reduction
of S0?
17.14 Nitrate Reduction and
the Denitrification Process
• Nitrate is commonly used as an electron
acceptor in anaerobic respiration. Its use
requires the enzyme nitrate reductase,
which reduces nitrate to nitrite. Many
bacteria that use nitrate in anaerobic
respiration eventually produce N2, a
process called denitrification.
•
Figure 17.36 shows steps in the dissimilative
reduction of nitrate.
Biochemistry of Dissimilative
Nitrate Reduction
• Figure 17.37 shows electron transport
processes in the membrane of Escherichia coli
when O2 or NO3– is used as an electron
acceptor and NADH is the electron donor.
Other Properties of
Denitrifying Prokaryotes
17.14 Concept Check
Nitrate is a commonly used electron acceptor in anaerobic
respiration. Its use requires the enzyme nitrate reductase that
reduces nitrate to nitrite. Many bacteria that use nitrate in anaerobic
respiration eventually produce N2, a process called denitrification.
 For Escherichia coli, why is more energy released
in aerobic respiration than during NO3– reduction?
 Where is the dissimilative nitrate reductase found
in the cell? What unusual metal does it contain?
 Why does an organism like Pseudomonas stutzeri
derive more energy from NO3– respiration than
does Escherichia coli?
17.15 Sulfate Reduction
• The sulfate-reducing bacteria reduce
sulfate to hydrogen sulfide. A summary of
the oxidation states of the key sulfur
compounds is given in Table 17.3.
Biochemistry and Energetics
of Sulfate Reduction
• The reduction of sulfate first requires
activation by a reaction with ATP to
form the compound adenosine
phosphosulfate (APS) (Figure 17.38).
• Electron donors for sulfate reduction
include organic compounds, H2, and
even phosphite (HPO3–).
• Disproportionation of sulfur compounds is
an additional energy-yielding strategy for
certain members of this group.
•
Figure 17.39 illustrates electron transport and
energy conservation in sulfate-reducing bacteria.
Acetate Use and Autotrophy
Sulfur Disproportionation
Phosphite Oxidation
17.15 Concept Check
The sulfate-reducing bacteria reduce sulfate to hydrogen sulfide.
The reduction of sulfate first requires activation by a reaction with
ATP to form the compound adenosine phosphosulfate (APS).
Electron donors for sulfate reduction include organic compounds,
and even phosphite. Disproportionation of sulfur compounds is an
additional energy-yielding strategy for certain members of this
group.
 Identify the following: S0, SO42–, SO32–, S2O32–, H2S.
 How is sulfate converted to sulfite during dissimilative
sulfate reduction?
 Why is H2 of importance to sulfate-reducing bacteria?
 Give an example of disproportionation.
17.16 Acetogenesis
• In homoacetogenesis, anaerobes reduce CO2
to acetate, usually with H2 as the electron
donor. An overview of the processes of
methanogenesis and acetogenesis is shown in
Figure 17.40.
Organisms and Pathway
Reactions of the
Acetyl-CoA Pathway
• The mechanism of acetate formation is
the acetyl-CoA pathway (Figure 17.41),
a series of reactions widely distributed
in obligate anaerobes as either a
mechanism of autotrophy or for acetate
17.16 Concept Check
Homoacetogens are anaerobes that reduce CO2 to acetate, usually
with H2 as electron donor. The mechanism of acetate formation is
the acetyl-CoA pathway, a series of reactions widely distributed in
obligate anaerobes as either a mechanism of autotrophy or for
acetate catabolism.
 Draw the structure of acetate and identify the carbonyl group
and the methyl group. What key enzyme of the acetyl-CoA
pathway produces the carbonyl group of acetate?
 How do homoacetogens make ATP from the synthesis of acetate?
 If catabolism of fructose via glycolysis yields only two molecules
of acetate, how can Clostridium aceticum ferment fructose by
this pathway and produce three molecules of acetate?
17.17 Methanogenesis
C1 Carriers in
Methanogenesis
Redox Coenzymes
Biochemistry of CO2
Reduction to CH4
Methanogenesis from
Methyl Compounds and
Acetate