Prescott's Microbiology,4/e

		Microbiology, 4/e Prescott, Harley, Klein
		Instructor Resources

9 Metabolism: The Generation of Energy

CHAPTER OVERVIEW

This chapter presents an overview of metabolism beginning with carbohydrate degradation and the aerobic generation of ATP through electron transport. Fermentation and anaerobic respiration are examined, followed by the catabolism of lipids, proteins, and amino acids. The chapter concludes with discussions of the function of inorganic molecules as electron acceptors and the trapping of energy by photosynthesis.

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

o discuss the difference between catabolism and anabolism

o describe the various pathways for the catabolism of glucose to pyruvate

o discuss the tricarboxylic acid (TCA) cycle and its central role in aerobic metabolism

o describe the electron transport process, and compare and contrast the electron transport system of eucaryotes with that of procaryotes

o contrast the two major proposed mechanisms for oxidative phosphorylation (i.e., the chemiosmotic hypothesis and the conformational change hypothesis)

o discuss the different electron acceptors used during aerobic respiration, fermentation, and anaerobic respiration

o describe in general terms the catabolism of molecules other than carbohydrates

o discuss the photosynthetic light reactions, and compare and contrast the light reactions of eucaryotes (and cyanobacteria) with those of green (or purple) photosynthetic bacteria

CHAPTER OUTLINE

I. An Overview of Metabolism

A. Catabolism-the breakdown of larger, more complex molecules into smaller, simpler ones, during which energy is released, trapped, and made available for work

B. Anabolism-the synthesis of complex molecules from simpler ones during which energy is added as input

C. Chemolithotrophy and photosynthesis are included as energy-yielding metabolic processes, even though they do not involve degradation of complex molecules

D. Multi-stage process

1. Stage 1-breakdown of large molecules (polysaccharides, lipids, proteins) into their component constituents with the release of little (if any) energy

2. Stage 2-degradation of the products of stage 1 aerobically or anaerobically to even simpler molecules with the production of some ATP, NADH, and/or FADH₂

3. Stage 3-complete aerobic oxidation of stage 2 products with the production of ATP, NADH, and FADH₂; the latter two molecules are processed by electron transport to yield much of the ATP produced

E. Metabolic efficiency is maintained by the use of a few common catabolic pathways, each degrading many nutrients

F. Microorganisms are catabolically diverse, but are anabolically quite uniform

G. Amphibolic pathways function both catabolically and anabolically, and sometimes employ separate enzymes to catalyze the forward and reverse reactions; this separation enables independent regulation of the forward and reverse reactions

II. The Breakdown of Glucose to Pyruvate

A. The glycolytic (Embden-Meyerhof) pathway is the most common pathway and is divided into two parts:

1. The 6-carbon sugar stage involves the phosphorylation of glucose twice to yield fructose 1,6-bisphosphate, and requires the expenditure of two molecules of ATP

2. The 3-carbon sugar stage cleaves fructose 1,6-bisphosphate into two 3-carbon molecules, which are each processed to pyruvate; two molecules of ATP are produced by substrate-level phosphorylation from each of the 3-carbon molecules for a net yield of two molecules of ATP; 2 molecules of NADH are also produced per glucose molecule

B. The pentose phosphate (hexose monophosphate) pathway uses a different set of reactions to produce a variety of 3-, 4-, 5-, 6-, and 7-carbon sugar phosphates

1. These phosphates can be used to produce ATP and NADPH, as well as to provide the carbon skeletons for the synthesis of amino acids, nucleic acids, and other macromolecules

2. The NADPH can be used to provide electrons for biosynthetic processes or can be converted to NADH to yield additional ATP through the electron transport chain

C. The Entner-Doudoroff pathway can also be used to produce pyruvate with a lower yield of ATP, but is accompanied by the production of NADPH as well as NADH.

III. The Tricarboxylic Acid Cycle-a series of reactions

A. Acetyl-CoA (produced by decarboxylation of pyruvate) reacts with oxaloacetate to produce a 6-carbon molecule

B. Subsequently, two molecules of carbon dioxide are released, regenerating the oxaloacetate

C. ATP is produced by substrate-level phosphorylation

D. Three molecules of NADH and one molecule of FADH₂ are produced per acetyl-CoA, and can be further processed to produce more ATP

E. Even those organisms that lack the complete TCA cycle usually have most of the cycle enzymes because one of the TCA cycle's major functions is to provide carbon skeletons for use in biosynthesis

IV. Electron Transport and Oxidative Phosphorylation

A. The Electron Transport Chain

1. Electrons from NADH and FADH₂ are transported in a series of redox reactions to a terminal electron acceptor

2. Electron carriers are located within the inner mitochondrial membrane in eucaryotes, or within the plasma membrane in procaryotes

3. The bacterial electron transport chain may be extensively branched with several terminal oxidases

4. Bacterial electron transport chains may be shorter, and have lower P/O ratios than mitochondrial electron transport chains

5. Procaryotic and eucaryotic electron transport chains, therefore, differ in the details of construction, although they operate according to the same fundamental principles

B. Oxidative Phosphorylation

1. Some of the energy liberated during electron transport is used to drive the synthesis of ATP in a process called oxidative phosphorylation

a. The chemiosmotic hypothesis of oxidative phosphorylation postulates that the energy released during electron transport is used to establish a proton gradient, and that this protonmotive force is then used to drive ATP synthesis

b. The conformational change hypothesis of oxidative phosphorylation postulates that the energy released during electron transport causes conformational changes in the ATP-synthesizing enzyme, which drives ATP formation by increasing the substrates' binding affinities

c. There is evidence for conformational changes and molecular rotation in the ATP synthase complex during proton movement across the membrane and therefore the system exhibits features of both hypotheses

2. Inhibitors of ATP synthesis fall into two main categories:

a. Blockers that inhibit the flow of electrons through the system

b. Uncouplers that allow electron flow, but disconnect it from oxidative phosphorylation

C. The Yield of ATP in Glycolysis and Aerobic Respiration

1. The yield of ATP in glycolysis and aerobic respiration varies with each organism, but has a theoretical maximum of 38 molecules of ATP per molecule of glucose catabolized

2. Anaerobic organisms using glycolysis can only produce two molecules of ATP per molecule of glucose catabolized

3. Aerobic respiration yields between 2 and 38 ATP molecules per glucose molecule, depending on the precise nature of the electron transport system

4. The Pasteur effect is a regulatory phenomenon by which organisms lower their rate of sugar catabolism when conditions cause a shift from anaerobic to aerobic metabolism because the aerobic process is more efficient and generates greater energy per glucose molecule

V. Fermentations

A. In the absence of oxygen, NADH is not usually oxidized by the electron transport chain because no external electron acceptor is available

B. However, NADH must still be oxidized to replenish the supply of NAD⁺ for use in glycolysis

C. Fermentations are reactions that regenerate NAD⁺ from NADH in the absence of oxygen

1. Fermentations involve pyruvate or pyruvate derivatives as electron acceptors

2. Fermentations may or may not produce additional ATP for the cell

D. Alcoholic fermentations produce ethanol and CO₂

E. Lactic acid fermentations produce lactic acid (lactate)

1. Homolactic fermenters reduce almost all pyruvate to lactate

2. Heterolactic fermenters form substantial amounts of products other than lactate

F. Formic acid fermentation produces either mixed acids or butanediol

VI. Anaerobic Respiration

A. Uses inorganic molecules other than oxygen as terminal electron acceptors; this produces additional ATP for the cell, but not usually as much as is produced by aerobic respiration

B. The major electron acceptors are nitrate, sulfate, and CO₂ but some metals can also be reduced

VII. Catabolism of Carbohydrates and Intracellular Reserve Polymers-proceeds by either hydrolysis or phosphorolysis to produce molecules that can enter the common catabolic pathways already discussed

VIII. Lipid Catabolism-proceeds by the b-oxidation pathway, which produces acetyl-CoA, which can enter the TCA cycle

IX. Protein and Amino Acid Catabolism

A. Proteins are degraded by secreted proteases to their component amino acids, which are transported into the cell and catabolized

B. The amino group is removed by deamination or transamination

C. The resulting organic acids are converted to pyruvate, acetyl-CoA, or a TCA-cycle intermediate

X. Oxidation of Inorganic Molecules

A. A pathway used by a small number of microorganisms called chemolithotrophs

B. Produces a significant but low yield of ATP

C. The electron acceptor is usually O₂, but sulfate and nitrate are also used

D. The most common electron donors are hydrogen, reduced nitrogen compounds, reduced sulfur compounds, and ferrous iron (Fe²⁺)

XI. Photosynthesis-energy from light is trapped and used to produce ATP and NADPH (light reactions), and to reduce carbon dioxide to form carbohydrates (dark reactions)

A. Photosynthetic organisms serve as the base of most food chains in the biosphere

B. Photosynthesis is also responsible for replenishing our supply of O₂

C. Cyclic photophosphorylation is performed by photosystem I; an excited electron is recycled to the chlorophyll of origin with concomitant production of ATP

D. Noncyclic photophosphorylation is performed by photosystem II; excited electrons from chlorophyll molecules are used in the production of NADPH, with concomitant production of ATP; electrons are replaced from water and oxygen gas is produced as a waste product

E. Photosynthesis in green and purple photosynthetic bacteria differs from photosynthesis in eucaryotes and cyanobacteria: water is not used as an electron source and oxygen is not produced; thus, these organisms are said to be anoxygenic

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