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BIOL 3702 Lecture Outline
Chapter 9: Metabolism: Energy, Release, and Conservation
Fueling Processes
◆ Chemoorganotrophs differ in the types of electron acceptors they use in their various
energy-yielding processes
◆ The process involving exogenous (external) acceptors in the electron transport chain
is termed respiration
✳ Final electron acceptor is oxygen, the process is termed aerobic respiration
✳ Final electron acceptor is not oxygen, the process is termed anaerobic respiration
◆ The electron transport system uses the donated electron to make ATP via oxidative
phosphorylation
◆ When the electron transport system is not used and endogenous (internal) acceptors
are employed, the process is termed fermentation and ATP is formed via substratelevel phosphorylation
Aerobic Respiration
◆ Aerobic catabolism can be divided into three different stages:
✳ Stage I - larger molecules are broken down into their constituent parts with little
energy released
✳ Stage II - either an aerobic or anaerobic process whereby even simpler molecules
are produced as is ATP as well as NADH and/or FADH2
✳ Stage III - complete oxidation of molecules under aerobic conditions to form CO2
as well ATP, NADH, and FADH2 (the latter two molecules generate even more
ATP through the electron transport system
◆ Some of the pathways in metabolism are amphibolic, i.e., they function both
catabolically and anabolically
Breakdown of Glucose
◆ Microbes use a number of pathways to catabolize glucose to pyruvate, among which
include the following:
✳ Glycolysis
✳ Pentose phosphate pathway
✳ Entner-Doudoroff Pathway
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BIOL 3702 Lecture Outline
Chapter 9
◆ Glycolysis (Embden-Meyerhof or Glycolytic Pathway)
✳ Most common pathway found in all major groups of microbes
✳ Functions in the absence or presence of oxygen
✳ Located in the cytoplasmic matrix of both procaryotes and eucaryotes
✳ Divided into two parts
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Six carbon stage - glucose (6 carbon [C6] molecule) is phosphorylated twice
and converted to fructose 1,6-bisphosphate [C6]
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Input of 2 ATP molecules
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No energy produced
Three carbon stage - catabolism of fructose 1,6-bisphosphate [C6] to two
molecules of pyruvate [C3] and generate per pyruvate
–
One molecule of NADH
–
Two molecules of ATP by substrate-level phosphorylation
✳ Substrate-level phosphorylation is the synthesis of ATP by coupling ADP
phosphorylation with an exergonic reaction
✳ Overview of process
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Six-carbon stage
–
ATP + Glucose [C6] = glucose-6-phosphate [C6]
–
Glucose-6-phosphate [C6] + ATP = fructose-1,6-bisphosphate [C6]
Three-carbon stage
–
Fructose-1,6-bisphosphate [C6] cleaved into glyceraldehyde-3-phosphate
[C3]
–
Glyceraldehyde-3-phosphate [C3] converted to pyruvate [C3] + 2 ATP +
NADH (this process occurs twice - balance the carbon atoms!!! 2 x C3 =
C6)
✳ Summary of glycolysis
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2 pyruvate molecules
2 ATP molecules - sum total: 2 used to begin glycolysis, 4 produced by
substrate-level phosphorylation
2 NADH molecules
◆ Pentose Phosphate Pathway (Hexose monophosphate pathway)
✳ May be used simultaneously with glycolysis and Enter-Doudoroff pathway
✳ Operates under either aerobic or anaerobic conditions
✳ Important in both catabolism and biosynthesis
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BIOL 3702 Lecture Outline
Chapter 9
✳ Summary of pentose phosphate pathway
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NADPH formed serves as a source of electrons for reduction of molecules
during biosynthesis
Four- and five-carbon sugars are synthesized that will be used for a variety of
purposes
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Intermediates are used to produce ATP
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Serves as a catabolic pathway for five-carbon sugars
●
More important as an anabolic pathway than as a catabolic pathway to produce
energy
◆ Entner-Doudoroff Pathway
✳ Begins the same way as the pentose phosphate pathway to produce
glyceraldehyde-3-phosphate
✳ Glyceraldehyde-3-phosphate catabolized to pyruvate via the 3-carbon stage of
glycolysis
✳ Total yield per glucose molecule
●
One ATP molecule
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One NADH molecule
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One NADPH molecule
Tricarboxylic Acid Cycle
◆ Tricarboxylic Acid Cycle (Krebs Cycle, TCA Cycle, Citric Acid Cycle) is widely
distributed among microbes
◆ Comprised of five basic steps:
✳ Oxidation of pyruvate [C3] to acetyl-coenzyme A (AcCoA) [C2] with the release of
CO2 and the formation of NADH
✳ AcCoA [C2] condenses with oxaloacetate (OAA) [C4] to form citric acid (citrate)
[C6], the first compound of the 6-carbon stage
✳ Through a series of reactions in the six-carbon stage,citric acid [C6] loses a
carbon as CO2 to form α-ketoglutarate [C5] while generating one NADH molecule
✳ In the five-carbon stage, α-ketoglutarate [C5] loses a carbon as CO2 to form
succinyl coenzyme A (succinyl-CoA) [C4] while generating one NADH molecule
✳ Through a series of reactions in the four-carbon stage, succinyl-CoA [C4] is
converted to OAA [C4] while generating one NADH molecule, one FADH2
molecule, and one GTP molecule (equivalent to ATP; produced via substrate-level
phosphorylation)
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BIOL 3702 Lecture Outline
Chapter 9
◆ Summary of TCA cycle per molecule of pyruvate
✳ 4 NADH molecules
✳ 1 FADH2 molecule
✳ 1 ATP (as GTP) molecule
✳ 3 CO2 molecules
Electron Transport Chain
◆ In eucaryotes, the electron transport chain is located in the mitochondrial inner
membrane
✳ Comprised of a series of electron carriers arranged into four complexes connected
by coenzyme Q and cytochrome c
✳ Electrons flow from electron carrier to another
✳
Final electron acceptor is to O2
◆ Energy that is released during these transfers is captured as ATP via the formation of
proton and electrical gradients
◆ The ATP is made via the process of oxidative phosphorylation - ATP formation driven
by electron transfer between carriers
✳ NADH will drive the synthesis of 3 ATPs
✳ FADH2 will drive the synthesis of 2 ATPs
◆ The same principles apply to the electron transport system in procaryotes, but the
structural details differ as do physiological responses
✳ Vary in cytochrome composition
✳ Electrons can enter at several different points in the chain
✳ Chains may be branched
✳ Different branches may operate under different environmental conditions
Oxidative Phosphorylation
◆ Several theories have been postulated for this mechanism of ATP synthesis
◆ Most widely accepted theory is the chemiosmotic theory
✳ During electron transport in eucaryotes, protons move outward from the
mitochondrial matrix and electrons are transported inward
✳ Results in the formation of a gradient of protons and a membrane potential due to
the unequal distribution of charges
✳ This unequal distribution of charges, known as the proton motive force, drives the
formation of ATP when protons return to the mitochondrial matrix
✳ ATP synthesis occurs through the action of ATP synthase (F1 particle) attached to
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BIOL 3702 Lecture Outline
Chapter 9
the inner mitochondrial membrane
◆ In procaryotes, a similar process takes place with the F1 particle located in the
plasma membrane
◆ Inhibitors of aerobic ATP synthesis fall into one of two categories:
✳ Blockers - prohibit electron transport from one carrier to another
✳ Uncouplers - permit electron transport, but disconnect the link to oxidative
phosphorylation (energy is released as heat)
Yield of ATP
◆ Eucaryotes theoretically generate 36-38 total ATP molecules per glucose molecule
catabolized
◆ Procaryotes may produce less ATP (about 30 molecules per glucose molecule
catabolized) due to less efficient electron transport chain, but still produces more
energy in this manner than in anaerobic respiration
Anaerobic Respiration
◆ Anaerobic respiration is an energy-yielding process that involves the use exogenous
electrons acceptors other than O2
◆ Major electron acceptors in anaerobic respiration include nitrate, sulfate, and CO2,
though various metals and organic compounds also function as final electron
acceptors
◆ Nitrate as an electron acceptor
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✳ Dissimilatory nitrate reduction - nitrate (NO3 ) is reduced to nitrite (NO2 ) via the
transfer of two electrons
✳ Not very efficient in the production of ATP because only two electrons are donated
-
✳ NO2 is also toxic and must be eliminated
-
✳ Some bacteria use the process of denitrification to convert NO3 to nitrogen gas
(nontoxic)
◆ Some obligate anaerobes use CO2 (or carbonate) or sulfate as final electron
acceptors
✳ Methanogens are able to reduce CO2 to methane (CH3)
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✳ Sulfur bacteria reduce SO4 to sulfide (S2 ) or hydrogen sulfide (H2S)
◆ In summary, anaerobic respiration is less efficient than aerobic respiration in the
synthesis of ATP
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BIOL 3702 Lecture Outline
Chapter 9
Fermentations
◆ In the absence of aerobic and anaerobic respiration, NADH is not oxidized by the
electron transport chain due to the absence of an external electron acceptor
◆ NADH must still be oxidized back to NAD+ or glycolysis will stop
◆ Many microbes use pyruvate or its derivatives as the electron acceptor
◆ Various types of fermentations exist, but all have two common themes:
✳ NADH is oxidized to NAD+
✳ Electron acceptor is often pyruvate or a derivative thereof that, when partially
oxidized, can lead to ATP production
◆ Some chemoorganoheterotrophs will only produce energy via fermentation even
though exogenous electron acceptors are available
Carbohydrate Catabolism
◆ Microbes catabolize other types of carbohydrates than glucose
✳ Monosaccharides
✳ Disaccharides
✳ Polysaccharides
◆ Monosaccharide sugars are initially phosphorylated, then fed into glycolysis
(sometimes after slight modifications)
◆ Disaccharides are first cleaved into monosaccharides by hydrolysis (addition of water
across a covalent bond) or phosphorolysis (phosphate attack on covalent bond), then
enter glycolysis
◆ Polysaccharides are similarly cleaved, usually via the catalysis of exogenous
enzymes produced by microbes, then again fed into the glycolytic pathway
◆ Moreover, internal reserve polymers are processed into monosaccharides prior to
being fed into other metabolic pathways
✳ Glycogen and starch are cleaved into one glucose molecule at a time which enters
the glycolytic pathway
✳ Poly-β-hydroxybutyrate is converted into acetoacetate which is then converted to
acetyl-CoA (enters the TCA cycle)
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Lipid Catabolism
◆ Lipids, which are comprised of glycerol and fatty acids, are hydrolyzed into these
components
✳ Glycerol is phosphorylated, oxidized to dihydroxyacetone phosphate, then fed into
the glycolytic pathway
✳ Fatty acids are oxidized by the β-oxidation pathway into acetyl-CoA, which then
feeds into glycolysis; NADH and FADH2 are also produced
Protein Catabolism
◆ Some microbes produced proteases that hydrolyze proteins and peptide chains into
amino acids which are further catabolized in a two step process:
✳ Deamination - removal of the amino group
✳ Transamination - transfer of the amino group to an α-keto acid acceptor (this step
does not always occur)
◆ The resulting product(s) can be used by the TCA cycle or as a carbon source for
making other compounds
Chemolithotrophy
◆ Chemolithotrophs generate energy using electrons from inorganic nutrients
◆ Commonly used electron donors are hydrogen, reduced nitrogen compounds,
reduced sulfur compounds, and ferrous iron (Fe2+)
◆ Final electron acceptor is usually O2
◆ Three major chemolithotrophic groups:
✳ Hydrogen gas utilizers
✳ Nitrifying bacteria (oxidize ammonia to nitrite, then to nitrate)
✳ Sulfur-oxidizing bacteria
◆ The nitrifying and sulfur-oxidizing bacteria do not generate efficient yields of ATP, but
do produce some NADH
Photosynthesis
◆ Photosynthesis is the process of trapping light energy and converting it to chemical
energy in terms of ATP and NADH (or NADPH) molecules
◆ Photosynthesis if composed of two parts:
✳ Light reactions - trapping light and making chemical energy
✳ Dark reactions - using energy to fix CO2 and make cellular constituents
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◆ Light reactions in eucaryotes/cyanobacteria
✳ Photosynthetic microbes trap light energy using a group of green pigments termed
chlorophyll
✳ Accessory photosynthetic pigments include:
✳
✳
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Carotenoids (usually yellowish)
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Phycobiliproteins (Cyanobacteria and red algae) of two types:
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Red (phycoerythin)
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Blue (phycocyanin) in color
Accessory proteins function to cover wavelengths of light which are not absorbed
by chlorophylls, and transfer the energy to these molecules
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Makes photosynthesis more efficient in capturing light energy
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Helps protect the organism from harmful oxidizing affects of sunlight
Chlorophylls and accessory pigments arranged in highly organized arrays that
transfer all energy to a reaction-center chlorophyll involved in electron transport
✳ Organized into two photosystems:
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Photosystem I - absorbs energy from wavelengths of light ≥680 nm using a
special chlorophyll a molecule termed P700
Photosystem II - absorbs energy from wavelengths of light ≤680 nm transferring
it to a special chlorophyll molecule termed P680
✳ The photosystems cooperate to form ATP and NADP+ by two mechanisms
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Cyclic photophosphorylation
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Non-cyclic photophosphorylation
✳ Cyclic photophosphorylation
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P700, excited by light energy, eventually donates its high-energy electron to
ferredoxin
Ferredoxin donates the electron through a series of electron carriers to produce
ATP - this type of ATP synthesis is termed photophosphorylation
Electron returns to P700
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BIOL 3702 Lecture Outline
Chapter 9
✳ Non-cyclic photophosphorylation
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P700, excited by light energy, eventually donates its high-energy electron to
ferredoxin
Ferredoxin donates the electron to reduce NADP+ to NADPH (these electrons
are now not available to reduced the oxidized P700)
P680, which has also been excited by light energy, donates its electrons to
P700 via a series of electron carriers that also produce an ATP molecule
Electrons lost from P680 are replace from the oxidation of water to O2
(oxygenic process)
✳ Photophosphorylation takes place in the thylakoid membrane of the chloroplast in
eucaryotes and in similar structures in cyanobacteria
✳ ATP production occurs by the previously described chemiosmotic mechanism
◆ Light reaction in green and purple bacteria
✳ Differs significantly from that performed by eucaryotes and cyanobacteria
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Anoxygenic process - do not use water as an electron source or produce O2
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NADH/NADPH synthesis uses quite different electron donors
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Different photosynthetic pigments (bacteriochlorophylls) are used that are more
effective for particular environmental niches
✳ Absence of a photosystem II restricts these microbes to ATP production by cyclic
photophos-phorylation
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