Microbial Metabolism:

Catabolic and Anabolic Pathways

Chapter 8(p. 211-231)

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Learning Objectives:

• Explain the overall function of metabolic pathway

• Describe the chemical reactions in glycolysis

• Explain the products of the Krebs cycle

• Describe the chemiosmotic model for ATP generation

• Compare and contrast aerobic and anaerobic respiration

• Describe the chemical reactions and list some products of fermentation.

• Describe how lipids and proteins undergo catabolism

• Define amphibolic pathways

• Contrast oxygenic and anoxygenic photosynthesis.

Metabolic Pathways

Carbohydrate Catabolism

• The breakdown of carbohydrates to release energy

• Glycolysis

• Krebs cycle

• Electron transport chain

Glycolysis

• Oxidation of glucose to pyruvic acid

• Produces ATP and NADH.

1

3

4

5

Preparatory Stage

• Two ATPs are used

• Glucose is split to form two phosphorylated 3-carbon sugars

9

Energy-Conserving

Stage• Two 3-carbon sugars

oxidized to two Pyruvic acid molecules

• Four ATP produced (substrate level phosphorylation)

• Two NADH produced

Glycolysis Summary

• One glucose is used

• Partial oxidation of the sugar, 2 pyruvates are end products

• Two NADH are reduced

• 2 ATP are consumed, 4 ATP total are made, net of 2 ATP produced

Intermediate Step

• Pyruvic acid (from glycolysis) is oxidized and decarboxylated.

Krebs Cycle

Complete oxidation

of acetyl CoA to CO2

produces NADH and

FADH2.

Krebs Cycle Summary

Pyruvate 1

• 3 CO2

• 4 NADH

• 1 FADH2

• 1 ATP

Pyruvate 2

• 3 CO2

• 4 NADH

• 1 FADH2

• 1 ATP• Glucose has now been completely oxidized to

carbon dioxide• Electrons are temporarily on carrier molecules• 2 ATP total made by substrate level phosphorylation

The Electron Transport Chain

• A series of carrier molecules that are, in turn, oxidized and reduced as electrons are passed down the chain.

• Energy released can be used to produce ATP by chemiosmosis.

Electron Transport System

• NADH oxidized

• Electrons pass through membrane carriers

• Protons pumped out (work is done)

• Electrons accepted by an inorganic molecule

H

HH H

H

HH

H H

HH HH

HHH

HH

H H

Cell wall

Cytochromes

Cytoplasm

Cell membranewith ETS

ATP

ADP

ATP synthase

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Aerobic Respiration

• The electron acceptor is an oxygen

• This is a very good acceptor

• Yields water upon reduction

• Because so much energy is released, the cell can pump out about 10 protons

• Occurs in bacterial membrane and mitochondria of eukaryotes

Anaerobic Respiration

• The electron acceptor is not oxygen

• Examples: nitrate, nitrite, and sulfate

• These are mediocre acceptors – not as good as oxygen

• Yields other inorganic molecules upon reduction

• Less favorable reactions pump out fewer protons

Electron acceptor Products

NO3– NO2

–, N2 + H2O

SO4– H2S + H2O

CO32 – CH4 + H2O

Anaerobic Respiration

Chemiosmosis

• Proton gradient is potential energy

• Allowing protons back into the cell can be coupled to work

• 3 protons entering drive the synthesis of 1 ATP

• Oxidative phosphorylation

Aerobic Respiration Yield

• 1 Glucose 6 CO2

• 2 ATP from glycolysis

• 2 NADH from glycolysis 6 ATP

• 2 ATP from Krebs cycle substrate level

• 8 NADH from Krebs cycle 24 ATP

• 2 FADH2 from Krebs cycle 4 ATP

• Total 36-38 ATP per glucose

Fermentation

• Performed by anaerobic microorganisms

• Does not use Krebs cycle or ETC

• Primary purpose: Regenerate NAD for reuse

• The electron acceptor is an organic molecule

• Secondary purpose: Generate additional energy

• Energy yields are very small

Fermentation

• NADH oxidized

• Organic molecule reduced

• Many possible end products

• Lactic acid

• Ethanol

• Vinegar

• Acetone

C C

OO

C

H

H

H

OHC C

H

H

H

H

H

C C

O

H

H

H

H

CC C

H

H

H

H

O HO

OH

System: Homolactic bacteria;human muscle

Glucose

System: Yeasts

Glycolysis

Lactic acidEthyl alcohol

Pyruvic acid

OH

NADH

NAD HNADH

NAD

NAD

CO2

Acetaldehyde

O2 is final electronacceptor.

ATP produced = 38 ATP produced = 2 to 36 ATP produced = 2

Non oxygen electron acceptors (examples: SO4

2–, NO3–, CO3

2– )

An organic molecule is finalelectron accept or (pyruvate,acetaldehyde, etc.).

AEROBIC RESPIRATION

Glycolysis

Glucose

ATP

NADH

2pyruvate (3C)

(6C)

CO2

Acety lCoA

FADH2

NADH

ATP

Krebs CO2

Electrons

Electron transport

ANAEROBIC RESPIRATION

Glycolysis

Glucose

ATP

NADH

2pyruvate (3C)

(6C)

CO2

Acety lCoA

FADH2

NADH

ATP

Krebs CO2

Electrons

Electron transport

FERMENTATION

CO2

Glycolysis

Glucose

ATP

NADH

2pyruvate

Lactic acid

Ethanol

Or other alcohols,acids, gases

Acetaldehyde

(6C)

(3C)

Fermentation

Fermentation

• Alcohol fermentation: Produces ethyl alcohol + CO2.

• Lactic acid fermentation: Produces lactic acid.

• Homolactic fermentation: Produces lactic acid only.

• Heterolactic fermentation: Produces lactic acid and other compounds.

Pathway Eukaryote Prokaryote

Glycolysis Cytoplasm Cytoplasm

Intermediate step Cytoplasm Cytoplasm

Krebs cycle Mitochondrial matrix Cytoplasm

ETC Mitochondrial inner membrane

Plasma membrane

Lipid Catabolism

Figure 5.20

Protein Amino acidsExtracellular proteases

Krebs cycleDeamination, decarboxylation, dehydrogenation

Organic acid

Protein Catabolism

Amphibolism

CO2

H2O

Gly

co

lys

is

An

ab

oli

sm

Ca

tab

oli

sm

Simpleproducts

Metabolicpathways

Building block

Macromolecule

Cellstructure

Chromosomes EnzymesMembranes

Cell wallStorage

MembranesStorage

LipidsFats

StarchCellulose

ProteinsNucleicacids

Nucleotides Amino acids Carbohydrates Fatty acids

Beta oxidationDeamination

GLUCOSE

Pyruvic acid

Krebscycle

Acetyl coenzyme A

NH3

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Photosynthesis

• Photo: Conversion of light energy into chemical energy (ATP)

• Light-dependent (light) reactions

• Synthesis: Fixing carbon into organic molecules

• Light-independent (dark) reaction, Calvin-Benson cycle

Oxygenic Photosynthesis

• Chlorophyll pigments

• Thylakoid membrane

• Capture light energy

• Electron transport

• Photophosphorylation

• Makes NADH and ATP

• Oxygen produced

• Algae, plants, and cyanobacteria

Anoxygenic Photosynthesis

• Purple bacteria (similar to photosystem II)

• Make ATP

• Can’t make NADH

• No oxygen produced

• Green bacteria (similar to photosystem I)

• Make ATP

• Also make NADH

• No oxygen produced

Calvin Benson Cycle

• Fix carbon dioxide

• Autotrophs

• Reverse of glycolysis

• 6 CO2 Glucose

P P

PP

PP

P P

P P

PP

P

PH

H

Splitting

ADP

ATP × 2

ADP

ATP

Series of 7 Carbonand 5 Carbonintermediates

Ribulose-1,5-bisphosphate5Carbon

CO2

6 Carbonintermediate

NADPH × 2

NADP

Glyceraldehyde-3phosphate

Glucose

Fructose intermediates

1,3-bisphosphoglyceric acid

Calvin Cycle

3-phosphoglycericacid

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