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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 processes-respiration

• 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 SeO4

2-.– 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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manydifferentsubstrtaesare funneledinto the TCA cycle

ATP madeprimarilybyoxidativephosphory-lation

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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Figure 9.5

•oxidation step – generates NADH

•ATP by substrate-levelphosphorylation

Glycolysis

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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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•produceNADPH

•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

• yield per glucose molecule:– 1 ATP– 1 NADPH– 1 NADH

reactions ofglycolyticpathway

reactions ofpentosephosphatepathway

Figure 9.8

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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 substrate-level 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 inE0 of NADH andE0 of O2

large amount ofenergy released

Figure 9.10

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Mitochondrial ETC

electron transfer accompanied byproton movement across innermitochondrial membraneFigure 9.11

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Electron Transport Chain of E. coli

branched pathway

upper branch – stationary phase andlow aeration

lower branch – log phase and highaeration

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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Figure 9.14 (a)

ATP Synthase

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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