Transcript Transport of molecules into a bacterial cell

An overview of bacterial catabolism
• Model compound: glucose, C6H12O6
• Aerobic metabolism
– With oxygen gas (O2)
– Fermentation and anaerobic respiration – later
• Four major pathways
– Glycolysis, Krebs cycle, electron transport, and
chemiosmosis
• The Goal: gradually release the energy in the
glucose molecule and use it to make ATP.
– The carbons of glucose will be oxidized to CO2.
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Glucose, showing
numbering system.
Fructose: same atoms,
but rearranged.
http://nov55.com/scie/fructose.gif
http://www.biotopics.co.uk/as/glucosehalf.png
Glycolysis: glucose is broken
Glucose is activated
Glucose Glu-6-P
2 ATPs are “invested”
Glu (6 C’s) broken into two 3-C
pieces
2 oxidations steps
NAD
NADH
4 ATPs are produced, net gain:
2 ATPs
2 molecules of pyruvic acid are
produced.
http://members.tripod.com/beckysroom/glycolysis.jpg
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3 ways bacteria use glucose
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EMP = EmbdenMeyerhof-Parnas pathway
Traditional glycolysis, yields 2
ATP plus 2 pyruvic acids.
Pentose Phosphate
Complicated pathway, produces 5
carbon sugars and NADPH for use
in biosynthesis.
Entner-Doudoroff
Yields only 1 ATP per glucose, but only used by aerobes such
as Pseudomonas which make many ATP through aerobic
respiration.
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Usually used in addition to
EMP or Entner-Doudoroff.
http://www.cellml.org/examples/images/metabolic_models/the_pentose_phosphate_pathway.gif
Krebs Cycle
detailed
http://www.personal.kent.edu/~cearley/PChem/Krebs1.gif
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What happens: carbons of
glucose oxidized completely
to CO2
Preliminary step: pyruvic acid
oxidized to acetyl-CoA.
Things to note: several redox
steps make NADH, FADH2
One ATP made
OAA remade, allowing cycle to
go around again.
http://www.sp.uconn.edu/~bi107vc/images/mol/krebs_cycle.gif
About Coenzyme A
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The vitamin CoA is way bigger
than the organic acids acted on
by the enzymes. CoA serves as a
handle; an acid attaches to it,
chemistry is done on the acid.
Acids (e.g. acetate, succinate)
attach to this –SH group here.
This piece here = acetyl group.
www.gwu.edu/~mpb/ coenzymes.htm
Electron transport
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• Metabolism to this point, per molecule of glucose:
– 2 NADH made during glycolysis, 8 more through the end of Krebs
Cycle (plus 2 FADH2)
• What next?
– If reduced NAD molecules are “poker chips”, they contain energy
which needs to be “cashed in” to make ATP.
– In order for glycolysis and Krebs Cycle to continue, NAD that gets
reduced to NADH must get re-oxidized to NAD.
– What is the greediest electron hog we know? Molecular oxygen.
– In Electron transport, electrons are passed to oxygen so that these
metabolic processes can continue with more glucose.
– Electron carriers in membrane are reversibly reduced, then reoxidized as they pass electrons (or Hs) to the next carrier.
About Hydrogens
• A hydrogen atom is one proton and one electron.
• In biological redox reactions, electrons are often
accompanied by protons (e.g. dehydrogenations)
• In understanding metabolism, we are not only
concerned with electrons but also protons.
– Also called hydrogen ions or H+
– H+ (hydrogen ions) and electrons are opposites!! Don’t
get them confused!
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Electron transport
Electrons are
passed carrier to
carrier, releasing
energy.
All occurs at the cell membrane
NADH is oxidized to NAD
H’s are passed to next electron
carrier; NAD goes, picks up
more H.
Process can’t continue
without an electron
acceptor at the end. In
aerobic metabolism,
the acceptor is
molecular oxygen.
http://employees.csbsju.edu/hjakubowski/classes/ch331/oxphos/oxphos.gif
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Electron Transport molecules
Molecule
Cofactor
Carry H?
Flavoproteins Yes
Flavins
yes
Quinones
No
Small lipids
yes
NA
yes
Fe-S groups
no
yes
Heme (Fe)
no
Iron-Sulfur
proteins
Cytochromes
Protein?
Chemiosmosis: electron transport used to
make ATP
The membrane acts as an insulator;
protons can only pass through via
the ATPase enzyme; the energy
released is used to power synthesis
of ATP from ADP and Pi.
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Energy released
during
electron
transport used
to “pump”
protons
(against the
gradient) to the
outside of the
membrane.
Creates a proton current (pmf)
much like the current of
electrons that runs a battery.
Proton motive force
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Electrochemical gradient establishes a voltage across the
membrane. Flow of protons (instead of electrons) does work
(the synthesis of ATP).
http://www.energyquest.ca.gov/story/images/chap04_si
mple_circuit_light.gif
Overview of aerobic metabolism
• Energy is in the C-H bonds of glucose.
• Oxidation of glucose (stripping of H from C atoms)
produces CO2 and reduced NAD (NADH)
– Energy now in the form of NADH (“poker chips”)
• Electrons (H atoms) given up by NADH at the membrane,
energy released slowly during e- transport and used to
establish a proton (H+) gradient across the membrane
– Energy now in the form of a proton gradient which can do work.
– Electrons combine with oxygen to produce water, take e- away.
• Proton gradient used to make ATP
– Energy now in the form of ATP. Task is completed!
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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.
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Central Metabolism:
Funneling all nutrients into central pathways
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•Many other
molecules
besides glucose
can serve as a
source of carbon.
Central Metabolism:
A source of building blocks for biosynthesis
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BUT, these
molecules can’t be
broken down to CO2
for energy AND
used for biosynthesis