Transcript Microbial Metabolism- Aerobic Respiration

Chapter 9
Metabolism: Energy
Release and Conservation
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Sources of energy
•most microorganisms use one
of three energy sources
•the sun
•reduced organic
compounds
•reduced inorganic
compounds
•the chemical energy obtained
can be used to do work
Figure 9.1
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Chemoorganotrophic fueling processess
Figure 9.2
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Chemoorganic fueling processesrespiration
• Most respiration involves use of an electron
transport chain
• aerobic respiration: final electron acceptor is oxygen
• anaerobic respiration
– final electron acceptor is different exogenous NO3-,
SO42-, CO2, Fe3+ or SeO42-.
– organic acceptors may also be used
• As electrons pass through the electron transport
chain to the final electron acceptor, a proton
motive force (PMF) is generated and used to
synthesize ATP
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Chemoorganic fueling processes fermentation
• Uses an endogenous electron acceptor
– usually an intermediate of the pathway
e.g., pyruvate
• Does not involve the use of an electron
transport chain nor the generation of a
proton motive force
• ATP synthesized only by substrate-level
phosphorylation
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Aerobic catabolism-An Overview
• Three-stage process
– large molecules (polymers)  small
molecules (monomers)
– oxidation of monomers to pyruvate
– oxidation of pyruvate by the
tricarboxylic acid cycle (TCA cycle)
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many
different
substrtaes
are
funneled
into
the TCA cycle
ATP made
primarily
by
oxidative
phosphorylation
Figure 9.3
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Amphibolic Pathways
• Function both as
catabolic and
anabolic pathways
• Examples:
– Embden-Meyerhof
pathway
– pentose phosphate
pathway
– tricarboxylic acid
(TCA) cycle
Figure 9.4
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The Breakdown of Glucose to
Pyruvate
• Three common routes
– Embden-Meyerhof pathway
– pentose phosphate pathway
– Entner-Doudoroff pathway
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The Embden-Meyerhof
Pathway (glycolysis)
• Occurs in cytoplasmic matrix
• Oxidation of glucose to pyruvate can be
divided in two stages
-glucose to fructose 1,6 -bisphosphate (6 carbon)
-fructose 1, 6-bisphosphate to pyruvate (two 3 carbon)
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Glycolysis
•oxidation step –
generates NADH
•ATP by substrate-level
phosphorylation
Figure 9.5
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Summary of glycolysis
glucose

2 pyruvate
2ATP
2NADH + 2H+
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The Pentose Phosphate
Pathway
• Can operate at same time as glycolytic
pathway
• Operates aerobically or anaerobically an
• Amphibolic pathway
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•produce
NADPH
•no ATP
•important
intermediates
Figure 9.6
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Figure 9.7
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Summary of pentose phosphate
pathway
glucose-6-P

6CO2
12NADPH
Glycolytic intermediates
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The Entner-Doudoroff Pathway
reactions of
pentose
phosphate
pathway
• yield per
glucose
molecule:
– 1 ATP
– 1 NADPH
– 1 NADH
Figure 9.8
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reactions of
glycolytic
pathway
The Tricarboxylic Acid Cycle
• Also called citric acid cycle and
Kreb’s cycle
• Common in aerobic bacteria
• Anaerobes contain incomplete TCA
cycle
• An Amphibolic pathway
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Figure 9.9
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Summary
• For each acetyl-CoA molecule
oxidized, TCA cycle generates:
– 2 molecules of CO2
– 3 molecules of NADH
– one FADH2
– one GTP
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Electron Transport and Oxidative
Phosphorylation
• Only 4 ATPs are synthesized directly from
oxidation of glucose to CO2 (by substratelevel phosphorylation)
• Most ATP made when NADH and FADH2
are oxidized in electron transport chain
(ETC)
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The Electron Transport Chain
• Series of electron carriers transfer
electrons from NADH and FADH2 to a
terminal electron acceptor
• Electrons flow from carriers with more
negative E0 to carriers with more positive
E0
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Electron transport chain…
• As electrons transferred, energy released
• In bacteria and archaea electron
carriers are in located plasma
membrane
• In eucaryotes the electron carriers are
within the inner mitochrondrial
membrane
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large difference in
E0 of NADH and
E0 of O2
large amount of
energy released
Figure 9.10
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Mitochondrial ETC
Figure 9.11
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electron transfer accompanied by
proton movement across inner
mitochondrial membrane
Electron Transport Chain of E. coli
branched pathway
upper branch –
stationary phase and
low aeration
lower branch – log
phase and high
aeration
Figure 9.12
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Oxidative Phosphorylation
Process by which ATP is synthesized
as the result of electron transport
driven by the oxidation of a chemical
energy source
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Proton Motive Force
• Is the most widely accepted hypothesis to
explain oxidative phosphorylation
– electron carriers are organized in the membrane
such that protons move outside the membrane as
electrons are transported down the chain
– proton expulsion results in the formation of a
concentration gradient of protons and a charge
gradient
– The combined chemical and electrical gradient
(electro chemical ) across the membrane is the
proton motive force (PMF)
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Chemiosmosis
Peter Mitchell in 1961 proposed that the electrochemical
gradient (proton and pH) across a membrane is
responsible for the ATP synthesis. He likened this process
to osmosis, the diffusion of water across a membrane,
which is why it is called chemiosmosis.
Peter Mitchell received the Nobel Prize in 1978 for this
concept.
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PMF drives ATP
synthesis(Chemiosmosis)
• Diffusion of protons back across
membrane (down gradient) drives
formation of ATP
• ATP synthase
– enzyme that uses PMF down gradient
to catalyze ATP synthesis
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ATP Synthase
Figure 9.14 (a)
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Figure 9.14 (b)
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Inhibitors of ATP synthesis
• Blockers
– inhibit flow of electrons through ETC
• Uncouplers
– allow electron flow, but disconnect it from
oxidative phosphorylation
– many allow movement of ions, including
protons, across membrane without activating
ATP synthase
• destroys pH and ion gradients
– some may bind ATP synthase and inhibit its
activity directly
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Maximum Theoretic ATP Yield from
Aerobic Respiration
Figure 9.15
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Theoretical vs. Actual Yield of
ATP
• Amount of ATP produced during aerobic
and anaerobic respiration varies depending
on growth conditions and nature of ETC
• Comparatively, anaerobic respiration
yields fewer ATP that aerobic respiration
• In fermentation yileds very few ATP
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