1

Chapter 9 Metabolism: Energy Release and Conservation

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 2 Figure 9.1

Chemoorganotrophic fueling processess

3 Figure 9.2

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 NO 3 , SO 4 2 , CO 2 , Fe 3+ or SeO 4 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) synthesize ATP is generated and used to

4

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

5 •

ATP synthesized only by substrate-level phosphorylation

6

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)

many different substrtaes are funneled into the TCA cycle 7 Figure 9.3

ATP made primarily by oxidative phosphory lation

Amphibolic Pathways

8 • •

Function both as catabolic and anabolic pathways Examples:

–

Embden-Meyerhof pathway

–

pentose phosphate pathway

–

tricarboxylic acid (TCA) cycle

Figure 9.4

9

The Breakdown of Glucose to Pyruvate

•

Three common routes

–

Embden-Meyerhof pathway

–

pentose phosphate pathway

–

Entner-Doudoroff pathway

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)

10

Glycolysis

• oxidation step – generates NADH • ATP by substrate-level phosphorylation Figure 9.5

11

12

Summary of glycolysis

glucose



2 pyruvate 2ATP 2NADH + 2H +

13

The Pentose Phosphate Pathway

• • •

Can operate at same time as glycolytic pathway Operates aerobically or anaerobically an Amphibolic pathway

• produce NADPH • no ATP • important intermediates Figure 9.6

14

Figure 9.7

15

16

Summary of pentose phosphate pathway glucose-6-P



6CO 2 12NADPH Glycolytic intermediates

The Entner-Doudoroff Pathway

•

yield per glucose molecule:

–

1 ATP

–

1 NADPH

–

1 NADH

reactions of glycolytic pathway Figure 9.8

17 reactions of pentose phosphate pathway

The Tricarboxylic Acid Cycle

18 • • • •

Also called citric acid cycle and Kreb’s cycle Common in aerobic bacteria Anaerobes contain cycle incomplete TCA An Amphibolic pathway

Figure 9.9

19

Summary

20 •

For each acetyl-CoA molecule oxidized, TCA cycle generates:

–

2 molecules of CO 2

–

3 molecules of NADH

–

one FADH 2

–

one GTP

Electron Transport and Oxidative Phosphorylation

•

Only 4 ATPs are synthesized directly from oxidation of glucose to CO 2 (by substrate level phosphorylation)

•

Most ATP made when NADH and FADH 2 are oxidized in electron transport chain (ETC)

21

The Electron Transport Chain

22 •

Series of electron carriers transfer electrons from NADH and FADH terminal electron acceptor 2 to a

•

Electrons flow from carriers with more negative E E 0 0 to carriers with more positive

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

23

large difference in E 0 E 0 of NADH and of O 2 large amount of energy released Figure 9.10

24

Mitochondrial ETC

Figure 9.11

25 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

26

27

Oxidative Phosphorylation

Process by which ATP is synthesized as the result of electron transport driven by the oxidation of a chemical energy source

Proton Motive Force

28 •

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)

29

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

Figure 9.14 (a) 31 ATP Synthase

Figure 9.14 (b) 32

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 33

Maximum Theoretic ATP Yield from Aerobic Respiration

Figure 9.15

34

35

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