Transcript Anaerobic metabolism

1
Definitions
• Substrate level phosphorylation
– Chemical reaction coupled to ATP synthesis
– Example: Pyruvate synthesis in glycolysis
• Oxidative (respiratory) phosphorylation
– Pumping of protons powered by electron transport with
oxygen as terminal electron acceptor yields ATP
• Photophosphorylation
– Pumping of protons powered by absorption of light.
• Respiration:
– a redox process in which electrons are passed along an
electron transport chain.
Central Metabolism:
Funneling all nutrients into central pathways
2
•Many other
molecules
besides glucose
can serve as a
source of energy.
Central Metabolism:
A source of building blocks for biosynthesis
3
BUT, these
molecules can’t be
broken down to CO2
for energy AND
used for biosynthesis
Other ways to make ATP
4
• Photosynthesis: light driven ATP synthesis.
– Oxygenic (photosystem I and II)
• Uses chlorophyll, produces oxygen from water
– Anoxygenic (photosystem I only)
• H’s to reduce CO2 from other sources
Other ways to make ATP-2
5
• Inorganic molecules can be oxidized producing ATP
synthesis by e- transport and chemiosmosis.
– Examples: Fe+2 to Fe+3, NH3 to NO2– Requires O2 as terminal electron acceptor
http://dwb.unl.edu/Teacher/NSF/C11/C
11Links/www.bact.wisc.edu/microtext
book/metabolism/images/Nitrification
ET.jpg
Anaerobic metabolism to make ATP
6
• Anaerobic respiration: organic compounds oxidized,
electrons passed down e- transport chain to some
molecule other than oxygen (e.g. NO3-, SO4-2).
– Just like aerobic respiration but w/o O2
• Fermentation: common anaerobic pathway used by
many medically important bacteria.
– Electron transport not important in ATP production
– Organic molecules serve as electron acceptor (sink).
What’s Fermentation for?
Glucose can be oxidized to pyruvic acid with the synthesis of
2ATPs. This alone is enough energy to live on. It depends on
the oxidation of NADH to NAD so that NAD is available to
accept electrons during the oxidation of glucose.
7
Why fermentation-2
8
Fermentation: “life without air”
9
• Without O2 as an e- acceptor, NADH cannot be re-oxidized
to NAD.
• Even though aerobic metabolism can produce ~36 ATP
from 1 glucose, the 2 ATP from glycolysis is enough.
• But glycolysis requires that NAD be reduced to NADH;
what happens when ALL the NAD becomes NADH with no
O2 to accept the H?
• Pyruvic acid is reduced, and the product thrown away; NAD
restored, glycolysis can be repeated, more ATP made.
• A variety of ways of solving this problem exist; many types
of molecules can be produced from fermentation.
Examples for fermentations
10
• Lactic acid fermentation
– Lactic acid
• Alcoholic fermentation
– Ethanol, carbon dioxide
• Mixed acid fermentation
– Lactic acid, formic acid, succinic acid, ethanol, H2, CO2
• Propionic acid fermentation
– propionic acid, acetic acid, and carbon dioxide
Lessons from Fermentation
11
• Fermentation is inefficient. If C6H12O6 has lots of
energy-rich H’s, so does C3H5O3 (lactic acid); the
product cannot be further metabolized and is thrown
away! Only a couple of ATPs are made.
• Fermentation is quick. Even though few ATPs are
made, they are made quickly.
• Fermentation is wasteful. Large amounts of
substrate (e.g. sugar) is used, making large amounts
of product (e.g. lactic acid, ethanol, etc.)
Anaerobic respiration
12
• Not the same as fermentation
– Respiration involves the electron transport chain and
ATP synthesis by chemiosmosis.
– Most general biologists misapply the term.
• Anaerobic means without oxygen
• Anaerobic respiration: organic (or inorganic)
molecule is oxidized, the removed electrons are sent
down the electron transport chain, and something
OTHER than oxygen is the electron acceptor.
– Carried out by anaerobic bacteria, but some aerobes
(growing anaerobically) can reduce forms of N this way.
Anaerobic respiration-2
In this example, nitrate is reduced to nitrite. Other examples:
sulfate reduced to elemental sulfur (S) or S to sulfide (H2S).
13
Anaerobic respiration-3
14
• Molecules of electron transport chain different
– Some “aerobes” and facultative anaerobes carry out
anaerobic respiration
– A different set of electron carriers produced in response
to lack of oxygen, or
– Oxygen is preferred electron acceptor; others work if
oxygen is not available.
• Example: denitrification
• NO3- → NO2- → NO → N2O → N2
– Important environmentally; fixed nitrogen lost under
anaerobic conditions.
Anaerobic bacteria use
incomplete citric acid cycle for
production of biosynthetic
precursors. They do not
contain α-ketoglutarate
dehydrogenase.
cronus.uwindsor.ca/.../7a371e9af805f74e85256a4f0
0538021/$FILE/Citric%20acid%20cycle.ppt
Bacteria and the fragility of existence
• Bacteria use ATP or the proton motive force to:
–
–
–
–
–
Move
Synthesis proteins (lots of them)
Transport molecules into the cell
Synthesize cell materials
Homeostasis
• Bacteria do not store ATP
– Calculations: E. coli has enough ATP to last a few
seconds
– Thus, cells must keep on making it.
• Bacteria carefully regulate their use of energy!
16