Download Electron transport chain…

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chapter 9
Metabolism: Energy
Release and Conservation
1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chemoorganotrophic fueling
processess
3
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
acceptor such as NO3-, SO42-, CO2, Fe3+ or SeO42-.
– 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
4
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chemoorganic fueling processes
- fermentation
• uses an endogenous electron acceptor
– usually an intermediate of the pathway used
to oxidize the organic energy source (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
5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Overview of aerobic catabolism
• three-stage process
– large molecules (polymers)  small
molecules (monomers)
– initial oxidation and degradation to
pyruvate
– oxidation and degradation of pyruvate
by the tricarboxylic acid cycle (TCA
cycle)
6
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Many different
energy sources are
funneled into
common degradative
pathways
ATP is made
primarily by
oxidative
phosphorylation
7
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Amphibolic Pathways
•
•
function both as catabolic and
anabolic pathways
important ones
– Embden-Meyerhof pathway
– pentose phosphate pathway
(= hexose monophosphate
pathway)
– tricarboxylic acid (TCA)
cycle
8
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The Breakdown of
Glucose to Pyruvate
• Three common routes
– Embden-Meyerhof pathway
– Pentose phosphate pathway
– Entner-Doudoroff pathway
* Glucose breakdown (glycolysis):
- 3 + phosphoketolase reaction
9
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
10
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The Embden-Meyerhof Pathway
• occurs in cytoplasmic matrix of both
procaryotes and eucaryotes
• the most common pathway for glucose
degradation to pyruvate in stage two of
aerobic respiration
11
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Addition of phosphates
“primes the pump”
Oxidation step – generates
NADH
High-energy molecules –
used to synthesize ATP
by substrate-level
phosphorylation
12
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Summary of EM pathway
glucose + 2ADP + 2Pi + 2NAD+

2 pyruvate + 2ATP + 2NADH + 2H+
13
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The Pentose Phosphate Pathway
• also called hexose monophosphate
pathway
• can operate at same time as EM pathway
or Entner-Doudoroff pathway
• can operate aerobically or anaerobically
• an amphibolic pathway
14
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Oxidation
Steps
- produce
NADPH,
which is
needed for
biosynthesis
Sugar
transformation
reactions (blue
arrows)
- produce sugars
needed for
biosynthesis
Sugars can also
be further
degraded
15
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
glycoaldehyde group
dihydroxyacetone group
16
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Summary of pentose phosphate
pathway
glucose-6-P + 12NADP+ + 7H2O

6CO2 + 12NADPH + 12H+ Pi
17
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The Entner-Doudoroff Pathway
reactions of
pentose
phosphate
pathway
• Yield per glucose
molecule:
– 1 ATP
– 1 NADPH
– 1 NADH
reactions of
EM pathway
18
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The Tricarboxylic Acid Cycle
• also called citric acid cycle and Kreb’s cycle
• common in aerobic bacteria, free-living
protozoa, most algae, and fungi
(also in anaerobic organisms)
• major role
- complete oxidation of organic energy sources
- provide carbon skeletons for use in biosynthesis
19
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
20
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Summary of TCA cycle
• for each acetyl-CoA molecule
oxidized, TCA cycle generates:
– 2 molecules of CO2
– 3 molecules of NADH + H+
– one FADH2
– one GTP
21
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Electron Transport and
Oxidative Phosphorylation
• only 4 ATP molecules synthesized directly
from oxidation of glucose to CO2
• most ATP made when NADH and FADH2
(formed as glucose degraded) are
oxidized in electron transport chain
22
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The Electron Transport Chain
• series of electron carriers that operate
together to 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
23
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Electron transport chain…
• as electrons transferred, energy released
• in eucaryotes the electron transport chain
carriers are within the inner
mitochrondrial membrane
24
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
large difference in
E0 of NADH and
E0 of O2
large amount of
energy released
25
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Mitochondrial ETC
Electron transfer accompanied by proton movement across
inner mitochondrial membrane.
26
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Procaryotic ETCs
• located in plasma membrane
• some resemble mitochondrial ETC, but
many are different
–
–
–
–
27
different electron carriers
may be branched
may be shorter
may have lower P/O ratio
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Electron Transport Chain of E. coli
Branched pathway
Upper branch –
stationary phase and
low aeration
Lower branch – log
phase and high
aeration
28
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Oxidative Phosphorylation
• Process by which ATP is synthesized as
the result of electron transport driven by
the oxidation of a chemical energy source
29
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chemiosmotic hypothesis
• is the most widely accepted hypothesis to
explain oxidative phosphorylation
– electron transport chain organized so protons move
outward from the mitochondrial matrix as electrons
are transported down the chain
– proton expulsion during electron transport results in
the formation of a concentration gradient of protons
and a charge gradient
– The combined chemical and electrical potential
difference make up the proton motive force
30
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
PMF drives ATP synthesis
• diffusion of protons back across
membrane (down gradient) drives
formation of ATP
• ATP synthase
– enzyme that uses PMF down gradient to
catalyze ATP synthesis
31
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Importance of PMF
32
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a: 1
b: 2
c: 9-12
33
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Binding-change mechanism of ATP synthase during ATP synthesis
34
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
35
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ATP Yield During Aerobic Respiration
• maximum ATP yield can be calculated
– includes P/O ratios of NADH and FADH2
– ATP produced by substrate level
phosphorylation
• the theoretical maximum total yield of
ATP during aerobic respiration is 38
– the actual number closer to 30 than 38
36
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Maximum theoretic ATP yield
from aerobic respiration
37
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Theoretical vs. actual yield of ATP
• amount of ATP produced during aerobic
respiration varies depending on growth
conditions and nature of ETC
• under anaerobic conditions, glycolysis
only yields 2 ATP molecules
38
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Anaerobic Respiration
• uses electron carriers other
than O2
• generally yields less energy
because E0 of electron
acceptor is less positive
than E0 of O2
39
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ETC of Paracoccus denitrificans
- aerobic
40
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ETC of P. denitrificans anaerobic
41
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
An example…
• dissimilatory nitrate reduction
– use of nitrate as terminal electron acceptor
– the anaerobic reduction of nitrate makes it
unavailable to cell for assimilation or uptake
• denitrification
• reduction of nitrate to nitrogen gas
• in soil, causes loss of soil fertility
42
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Microbial Fermentations
• oxidation of NADH produced by glycolysis
- pyruvate or derivative used as
endogenous electron acceptor
• substrate only partially oxidized
• oxygen not needed
• oxidative phosphorylation does not occur
– ATP formed by substrate-level phosphorylation
(except succinate and oxalate fermentations by
Propionigenium modestum and Oxalobacter
formigenes, respectively)
43
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
44
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
45
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fermentations
46
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
homolactic
Fermenters
heterolactic
fermenters
- food
spoilage
- yogurt,
sauerkraut,
pickles, etc.
Propionic acid fermentation
47
alcoholic
fermentation
- alcoholic
beverages,
bread, etc.
mixed acid fermentation
butanediol fermentation
butyric acid fermentation
butanol-acetone (isopropanol)
-ethanol fermentation
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
48
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fermentations of amino acids
• Stickland reaction
– oxidation of one
amino acid with use of
second amino acid as
electron acceptor
– carried out by some
Clostridium spp.
49
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Catabolism of Carbohydrates
and Intracellular Reserves
• many different carbohydrates can
serve as energy source
• carbohydrates can be supplied
externally or internally (from
internal reserves)
50
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Carbohydrates
• monosaccharides
– converted to other
sugars that enter
glycolytic pathway
• disaccharides and
polysaccharides
– cleaved by
hydrolases or
phosphorylases
51
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Reserve polymers
• used as energy sources in absence of
external nutrients
– e.g., glycogen and starch
• cleaved by phosphorylases
(glucose)n + Pi  (glucose)n-1 + glucose-1-P
• glucose-1-P enters glycolytic pathway
– e.g., PHB
PHB    acetyl-CoA
• acetyl-CoA enters TCA cycle
52
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Lipid Catabolism
• triglycerides
– common energy sources
– hydrolyzed to glycerol
and fatty acids by
lipases
• glycerol degraded via
glycolytic pathway
• fatty acids often oxidized
via β-oxidation pathway
53
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
β-oxidation pathway
54
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CoA-SH
55
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Protein and Amino Acid Catabolism
• protease
– hydrolyzes protein to amino acids
• deamination
– removal of amino group from amino acid
– resulting organic acids converted to
pyruvate, acetyl-CoA, or TCA cycle
intermediate
• can be oxidized via TCA cycle
• can be used for biosynthesis
56
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Deamination often occurs by
transamination
Transfer of amino group from one amino acid to α-keto acid
57
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chemolithotrophy
• carried out by chemolithotrophs
• electrons released from energy source which
is an inorganic molecule
– transferred to terminal electron acceptor
(usually O2 ) by ETC
• ATP synthesis by oxidative phosphorylation
58
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
59
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Mycobacterium sp. strain JC1
CO
O2
Carboxidobacteria: aerobic bacteria that grow at the expense of CO
as the sole source of carbon and energy
CO + H2O → CO2 + 2H+ + 2e-
60
CO2 , H2O
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
61
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Nitrifying bacteria
oxidize ammonia to nitrate
- NH3  NO2-  NO3- requires 2 different genera of bacteria (NH3  NO2- / NO2-  NO3-)
62
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sulfur-oxidizing bacteria
ATP can be
synthesized by
both oxidative
phosphorylation
and substrate-level
phosphorylation
63
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
64
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
65
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Other chemolithotrophs
• Hydrogen bacteria
- hydrogenase: H2 → 2H+ + 2e-
• Iron-oxidizing bacteria: Fe2+ (ferrous iron)
• Carboxydobacteria
66
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Autotrophic growth by
chemolithotrophs
• Calvin cycle requires NADH as
electron source for fixing CO2
– many energy sources used by
chemolithotrophs have E0 more
positive than NAD/NADH
• use reverse electron flow to generate
NADH
67
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Metabolic flexibility of
chemolithotrophs
• many switch from chemolithotrophic
metabolism to chemoorganotrophic
metabolism
• many switch from autotrophic
metabolism (via Calvin cycle) to
heterotrophic metabolism
68
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Phototrophy
• photosynthesis
– energy from light trapped and converted to
chemical energy
– a two part process
• light reactions in which light energy is trapped
and converted to chemical energy
• dark reactions in which the energy produced in
the light reactions is used to reduce CO2 and
synthesize cell constituents
69
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
oxygenic photosynthesis: eucaryotes and cyanobacteria
anoxygenic photosynthesis: all other bacteria
70
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
halobacteria
71
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The light reaction in
oxygenic photosynthesis
• chlorophylls
– major light-absorbing pigments
• accessory pigments
– transfer light energy to chlorophylls
– e.g., carotenoids and phycobiliproteins
(phycoerythrin and phycocyanin in red
algae and cyanobacteria)
72
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
different
chlorophylls
have different
absorption
peaks
73
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Accessory pigments absorb different wavelengths of light than
chlorophylls
74
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Organization of pigments
• antennas
– highly organized arrays of chlorophylls and
accessory pigments
– captured light transferred to special
reaction-center chlorophyll
* RC: directly involved in photosynthetic electron
transport
• photosystems
– antenna and its associated reaction-center
chlorophyll
75
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Green plant photosynthesis
noncyclic
electron flow –
ATP + NADPH
made (noncyclic
photophosphorylation)
cyclic electron
flow – ATP
made (cyclic
photophosphorylation)
76
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
electron flow  PMF  ATP
77
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The light reaction in
anoxygenic photosynthesis
• H2O not used as an electron source;
therefore O2 is not produced
• only one photosystem involved
• uses different pigments and mechanisms to
generate reducing power
• carried out by phototrophic green bacteria,
phototrophic purple bacteria and
heliobacteria
78
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Reaction center of a purple nonsulfur bacterium
Polypeptide with
prosthetic groups
79
Reaction center prosthetic groups
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Purple nonsulfur bacteria
electron source for
generation of NADH
by reverse electron
flow
- H2S, S0,
organic compounds
80
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Green sulfur bacteria
81
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Bacteriorhodopsin-Based
Phototrophy
• Some archaea use a type of phototrophy
that involves bacteriorhodopsin, a
membrane protein which functions as a
light-driven proton pump
• a proton motive force is generated
• an electron transport chain is not
involved
82
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
83