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210
MCB 3020, Spring 2005
Chapter 5:
Nutrition and Metabolism I
211
The Generation of Energy:
I. Metabolism (metabolic reactions)
II. Nutrients
III. Energy
IV. Review of free energy
V. Enzymes
VI. Energy generation:
oxidation and reduction reactions
I. Metabolism (metabolic reactions)
212
• all of the biochemical reactions
in a cell
• includes catabolic (degradative)
and anabolic (biosynthetic) reactions
213
1. Catabolism
• the breakdown of complex
molecules into simpler compounds
with the release of energy
2. Anabolism
• the biosynthesis of complex
molecules from simpler compounds
with the input of energy
B. Catabolic reactions generate ATP. ATP is 214
used for biosynthesis and cell maintenance.
energy
source
Catabolism
waste
products
ATP,
reductant
small
molecules
Anabolism
macromolecules
(polymers)
215
C. ATP is called the energy currency
of the cell.
• catabolic reactions release energy
and store it as ATP.
• anabolic (biosynthetic) reactions
require energy in the form of ATP.
II. Nutrition
A. Nutrients
chemicals taken up from
environment and used for
cellular reactions
1. macronutrients
2. micronutrients
3. growth factors
216
1. Macronutrients: chemicals taken
up and required in relatively large
amounts
C
+
K
H
Mg2+
O
+
Na
N
2+
Ca
P
Fe2+/Fe3+
S
217
218
Where do macronutrients occur in cells?
C
H
O
N
P
S
Fe
many organic molecules
amino acids, nucleic acids, cell walls, etc.
nucleic acids, phospholipids
cysteine, methionine, vitamins like CoA
Electron transport proteins
2. Micronutrients: inorganic
required in small amounts
219
chemicals
• also called trace elements
• usually metals in metabolic enzymes
• examples
Co (the metal center of vitamin B12)
Cu (found in electron transport proteins)
Se (found in selenocysteine)
Ni, Zn, Mn, V, W
220
3. Growth factors: organic chemicals
required in small amounts by some
(but not all) cells
a. Examples:
vitamins, like B1, B6, B12, biotin
some amino acids
purines, pyrimidines
b. Many vitamins are precursors of
coenzymes used in metabolism.
Vitamin
B2 (riboflavin)
niacin (nicotinic acid)
B12
folate
221
Coenzyme
FAD, FMN
NAD, NADP
cobalamin
tetrahydrofolate
Coenzymes are molecules that work together
with enzymes to catalyze chemical reactions.
222
B. Cells can be grown in laboratory cultures.
Two classes of culture media
1. Chemically defined medium
exact chemical composition is known;
contains precise amounts of pure
chemicals added to distilled water
2. Complex (undefined) medium
exact chemical composition is not known;
contains digests of milk proteins, yeast,
soybeans, etc. that have growth factors
Different organisms can have vastly
different nutritional requirements.
223
Escherichia coli can grow on a simple defined
medium. It can synthesize most of the
organic molecules required for biosynthesis.
Leuconostoc mesenteroides needs added
amino acids, purines, pyrimidines, and
vitamins for growth because it cannot
synthesize these molecules by itself.
Laboratory growth medium for E. coli
Glucose K2HPO4 (NH4)2SO4
H2O
KH2PO4 MgSO4
224
CaCl2
minerals
Growth medium for L. mesenteroides
Glucose, H2O, K2HPO4, KH2PO4, NH4Cl, MgSO4, Na acetate,
alanine arginine asparagine, aspartate, cysteine, glutamate,
glutamine, glycine, histidine, isoleucine, leucine, lysine,
methionine, phenylalanine, proline, serine, threonine,
tryptophan, tyrosine, valine, adenine, guanine, uracil,
xanthine biotin, folate, nicotinic acid, pyridoxal, pyridoxamine
pyridoxine, riboflavin, thiamine, pantothenate,
para-aminobenzoic acid, trace elements (don't memorize)
III. Energy
Why do cells need energy?
Where do organisms get energy?
How do cells use energy sources?
225
A. Why do cells need energy?
• growth and biosynthesis
• motility
• nutrient uptake
• reproduction
• maintenance, etc.
nutrients
polysaccharides
226
B. Where do organisms get energy?
Chemotrophs
chemicals
227
Phototrophs
light
Chemoorganotrophs
Chemolithotrophs
organic chemicals
(eg. sugars)
inorganic chemicals
(eg. H2, NH3, H2S)
C. How do chemotrophs derive energy228
from energy sources?
Organisms capture energy that is
released when an organic or
inorganic chemical is oxidized.
Remember:
oxidation is the
loss of electrons
glucose + 6 O2 
6 CO2 + 6 H2O
G°’ = - 686 kcal/mol
D. Units of energy
229
kcal (kilocalorie)
• a unit of energy
• amount of heat energy required
to raise the temperature of 1 kg
of water 1°C
• 1 kcal = 4.184 kilojoules (kJ)
• 1 kcal = 1 “nutritional” calorie
IV. Review of free energy (G)
• energy that is available to do
useful work
Review from General Chemistry:
G = H - T S
230
change in entropy
change in enthalpy (total energy)
change in free energy
231
standard
For biological reactions, the
conditions for measuring the change
in free energy (G°’ ) are
• 25°C
• pH 7
• reactants and products initially
present at 1 M concentration
A. The  G°’ can tell us about the 232
direction a reaction tends to occur.
A+B
Free
Energy
C+D
A+B
 G°’ is
negative
C+D
Progress of reaction
If G°’ is (-)
products have
lower free
energy than
substrates

233
1. If  G°’ is negative
• free energy is released
• the reaction is exergonic
• the reaction tends to occur in the
direction written
Examples:
H2 + 1/2 O2  H2O
glucose + 6 O2  6 CO2 + 6 H2O
ATP + H2O
 ADP + PO4-
- 57 kcal/mol
- 686 kcal/mol
- 7.3 kcal/mol
2. If  G°’ is positive
• energy input is usually required
• the reaction is endergonic
• the reaction does not tend to
occur in the direction written
Free
Energy
C+D
A+B
 G°’ is
positive
Progress of reaction
234
If  G°’ is (+)
products have
higher free
energy than
substrates
B. Coupled reactions
235
Exergonic reactions (-G°’) can be used to
"drive" endergonic reactions (+G°’) to make
the overall "coupled" reaction favorable.
Reaction 1
A
B
Go’ = +20 kJ/mole
D
Go’ = -30 kJ/mole
Reaction 2
C
Reactions 1 and 2 coupled
A+C
B+D
Go’ = -10 kJ/mole
236
C. Equilibrium
A+B
C+D
• equilibrium occurs when the rates of the
forward and reverse reactions are equal
• usually at equilibrium, the concentrations
of the products and reactants are not equal
C. Equilibrium (contd.)
• if the G°’ is large and negative,
equilibrium lies towards product;
very little of the reactants remain
A+B
C+D
237
D. “Activation energy” is required to
break bonds.
238
H2 + 1/2 O2  H2O G°’ = - 57 kcal/mol
If H2 and O2 are mixed without a
catalyst, no detectable amount of
water is formed in our lifetime. Why?
Because before water is formed,
chemical bonds have to be broken.
239
Activation energy: energy required to
bring molecules to the reactive state
Free
Energy
H2 + 1/2 O2
Activation
energy
H2O
Progress of reaction
240
E. Catalysts
chemicals that increase the
reaction rate by lowering the
activation energy
Free
Energy
H2 + 1/2 O2
G
Activation
energy of
catalyzed
reaction
H2O
Progress of reaction
Properties of catalysts
241
• increase the rate of the reaction,
• but DO NOT change the G,
• DO NOT change the equilibrium
• many reactions in living organisms
are catalyzed by biological
molecules called enzymes
V. Enzymes
• biological catalysts
• most enzymes are proteins,
a few are nucleic acids
(ribozymes or catalytic RNAs)
• most enzymes catalyze specific
reactions or sets of reactions
242
A. Enzyme catalysis
Enzyme (E): usually a protein
Substrates (S): reactants, S
starting materials
Products (P): ending materials
243
Substrate(s) first combine with the
enzyme to form an enzyme-substrate
(E-S) complex.
244
B. Typical enzymatic reaction sequence:
S
E
E
E
E
E + S  E-S  E-P  E + P
Enzyme-substrate
complex
At end of reaction, the enzyme
returns to its original form
C. Important notes on enzymes
245
• Enzymes DO NOT alter the equilibrium of
the reaction.
• Enzymes can catalyze exergonic and
endergonic reactions.
• Substrates bind at the enzyme active site.
• Many enzymes contain nonprotein
components: coenzymes (loosely bound) or
prosthetic groups (tightly bound).
Important notes on enzymes (contd.) 246
• Enzymes tend to be sensitive to pH and
temperature.
• Enzymes are often named after the substrate
or the reaction catalyzed, plus the ending
“-ase” (eg. cellulase breaks down cellulose,
ATP synthase makes ATP).
D. Sometimes enzymes change shape247
when substrates bind (“induced fit”)
glucose + hexokinase (a protein used in glycolysis)
Active site
248
E. Metabolic reactions are catalyzed
by enzymes.
CH OH
2
glucose
12 enzymes
HO OH
O
OH
OH
ethanol + CO2
glucose fermentation (anaerobic)
Respiration of glucose (aerobic)
glucose + 6 O2
~36 [ADP + Pi]
~30 enzymes
~36 ATP
6 CO2 + 6 H2O
249
250
VI. Energy generation:
A. Oxidation and reduction reactions
For chemotrophs, utilization of a
chemical energy source involves
oxidation and reduction reactions
(redox reactions).
Oxidation and reduction reactions
“LEO says GER”
Loss of Electrons = Oxidation
H2  2 H+ + 2 eGlucose (C6H12O6)  12 H+ + 12 e- + 6 CO2
Gain of Electrons = Reduction
1/2 O2 + 2 H+ + 2 e-  H2O
251
B. Complete redox reactions can be divided 252
into oxidative and reductive half reactions.
Oxidative half-reaction:
H 2  2 H + + 2 e-
Reductive half-reaction:
1/2 O2 + 2 H+ + 2 e-  H2O
Complete reaction:
H2 + 1/2 O2
 H2O
e- donor e- acceptor
H2 and H+ are called a redox couple.
253
C. Because electrons do not
typically exist alone in solution,
complete redox reactions need
an electron donor (eg. H2) and
an electron acceptor (eg. O2)
glucose + 6 O2
primary
terminal
electron donor e- acceptor
6 CO2 + 6 H2O
D. Energy is released when an
energy source is oxidized.
H2 + 1/2 O2

H2O
glucose + 6 O2  6 CO2 + 6 H2O
254
G°’
- 57 kcal/mol
- 686 kcal/mol
Oxidative half-reaction
H2  2 H+ + 2 e-
255
E. Cells oxidize energy sources and
harness the energy released to make ATP.
H2
1/2 O2
H2
Explosive
release of
energy as
heat can't
be
harnessed
to do work
2 H+
H2 O
H2 + 1/2 O2  H2O
2 e-
Hydrogen atoms separated into
protons & electrons
Some
released
energy is
harnessed
to make ATP
Electron
transport
system
2 e2 H+
1/2 O2
H2 O
G°’ = - 57 kcal/mol
Study Objectives
256
1. Understand metabolism, catabolism, anabolism, and the role of ATP in
metabolism.
2. Know the differences between macronutrients, micronutrients, and growth
factors. Know where they occur in biological molecules and the examples
presented in class.
3. Contrast defined and complex media. Know one reason why nutritional
requirements differ among organisms.
4. Give examples of energy-requiring processes in the cell.
5. Define chemotrophs, phototrophs, chemoorganotrophs, chemolithotrophs.
(eg. chemotrophs are organisms that use chemicals as an energy source.)
Given an energy source (eg. NH3), be able to identify the type of
catabolism being used (eg. chemolithotrophy).
6. Understand the terms kcal and free energy. What predictions can be made
from the Go' value of a reaction. What is reaction coupling and how can it
be used by the cell?
257
7. Understand equilibrium, activation energy, catalysts and their properties.
Understand the effect of catalysts on equilibrium. Can catalysts make a
nonspontaneous reaction spontaneous?
8. Understand enzymes and all the properties presented in class. What is the
function of enzymes in the cell?
9. Define oxidation, reduction, half reactions, redox couples, electron donor,
electron acceptor.
10. Describe how cells derive energy from an energy source. What are the
roles of the primary electron donor and the terminal electron acceptor?
Energy generation and glycolysis
I. Oxidation of the energy source
II. Reduction of NAD+
III. Making ATP through
substrate level phosphorylation
IV. Glycolysis
V. Reoxidation of NADH
258
I. Oxidation of the energy source:
259
A. Energy released when an energy
source is oxidized can be conserved in the
form of high energy chemical bonds.
oxidation
waste
glucose
products
ADP + Pi
ATP
chemicals with high energy bonds
260
B. Electrons are transferred during catabolism.
glucose
[carbon]
energy source
primary electron donor
electrons
one or more intermediate
electron carriers, e.g. NAD+
terminal electron acceptor
(the last molecule to accept
electrons), e.g. O2
261
C. Redox terminology
1. Oxidation is the loss of electrons
Glucose (C6H12O6)  12 H+ + 12 e- + 6 CO2
Compounds become oxidized after
losing electrons.
An oxidant is a compound that
accepts electrons. It can
oxidize other compounds.
TB
262
2. Reduction is the gain of electrons
Compounds become reduced after
gaining electrons.
A reductant is a compound that
donates electrons. It can reduce
other compounds.
TB
D. Redox reactions
263
electron donor
A(red) + B(ox)
A(ox) + B(red)
electron acceptor
e.g.
glucose + 6 O2
6 CO2 + 6 H2O
TB
1. Redox couples are substances
interconverted by redox reactions
A(red) + B(ox)
264
A(ox) + B(red)
A(ox)/A(red) is a redox couple (CO2/ glucose)
B(ox)/B(red) is a redox couple (O2/ H2O)
Note: the oxidized substance is written to the left.
Two redox couples are needed for a redox reaction.TB
Example: pyruvate and lactate are a
redox couple.
pyruvate/lactate
half-reaction (hypothetical)
pyruvate + 2H+ + 2e-
lactate
pyruvate can be reduced to lactate
lactate can be oxidized to pyruvate
o
E'
(reduction potential) = -0.19 volts
265
2. Redox couples have associated
standard reduction potentials (Eo').
266
(Eo') is a measure of the tendency of a redox
couple to donate electrons in a redox reaction.
Eo' values can be summarized in a "table of
reduction potentials."
In this table, the REDUCED substance of the
redox couple is written on the right.
TB
3. Partial table of reduction potentials 267
Oxidized form / Reduced form
Reduction potential
Eo' (Volts)
CO2 / glucose (C6H12O2)
2 H+ / H 2
NAD+ / NADH
pyruvate / lactate
fumarate / succinate
NO3- / NO2-
(- 0.43)
(- 0.42)
(- 0.32)
(- 0.19)
(+ 0.03)
(+ 0.42)
O2 / H2O
(+ 0.82)
a. In a table of reduction potentials,
the reduced compound of redox
couple with a more negative Eo'
268
can give electrons to
the oxidized compound
of a redox couple lower
in the table
b. Example
Two redox couples
269
NAD+/NADH
Eo’ = –0.32 V
pyruvate/lactate
Eo’ = -0.19 V
In a redox reaction, NADH can donate
electrons to pyruvate.
NADH + pyruvate
NAD+ + lactate
4. Eo' is the change in standard
reduction potential.
270
Reduction potential
Eo' (Volts)
CO2 / glucose (C6H12O2)
(- 0.43)
Eo'
O2 / H2O
(+ 0.82)
5. A large  Eo' corresponds to a large Go'.271
Go' = -nF Eo' (don't memorize equation)
Reduction potential
Eo' (Volts)
small  Eo'
CO2 / glucose
(- 0.43)
= -0.24 V
pyruvate / lactate
(- 0.19)
not much energy
O2 / H2O
(+ 0.82)
large  Eo'
= -1.25 V
(lots of energy)
6. Electrons can be transferred to intermediate272
electron carriers in a series of redox reactions.
(glucose)
(H2O)
A(red)
A(ox)
B(ox)
B(red)
C(red)
C(ox)
(CO2)
(O2)
A(red) = primary electron donor (energy source)
B = intermediate electron carrier
TB
C (ox) = terminal electron acceptor
II. NAD+ is an intermediate electron
carrier.
A(red)
NAD+
C(red)
273
A(ox)
NADH
C(ox)
"A" and "C" can be many numerous compounds
many of which are catabolic intermediates.
TB
A. NAD+ and NADP+
274
1. NAD+
nicotinamide adenine dinucleotide
carries 2 electrons and a proton;
usually involved in catabolic rxns
2. NADP+
similar to NAD+ with an extra PO4-;
usually involved in biosynthesis
B. The NAD+/NADH couple (Eo' = –0.32V)275
+
NAD
NADH +
H
NH2
adenine
O
OH HO
O
P-P
O
N+
HO OH
2e– + 2H+
+
H
H H O
NH2
N
+ H+
R
TB
(look at but don't memorize structures)
C. NAD+ must be recycled
276
NAD+ is made by cells in limited
amounts.
The reduction of NAD+ to NADH
depletes NAD+.
NAD+ must be regenerated by the
oxidation of NADH to NAD+.
TB
III. Making ATP by substrate level
phosphorylation (SLP)
277
A. Substrate Level Phosphorylation:
*ATP synthesis driven by a
high-energy compound,
NOT the proton motive force (PMF).
Example
PEP + ADP
pyruvate + ATP
TB
Ex. of Substrate level phosphorylation278
COO-
P ~ P OCH2 O
R
CO~ P
CH2
PEP + ADP
COO-
pyruvate + ATP
C=O
CH3
P ~ P ~ P OCH2 O
R
B. High energy compounds
279
Compounds that can release large
amounts of energy when they react.
Catabolism conserves energy in the
form of high energy compounds which
can be used to perform cellular work.
TB
280
High energy compounds
Go' of hydrolysis
(kJ / mol)
phosphoenolpyruvate
1,3-bisphosphoglycerate
acetyl phosphate
succinyl CoA, acetyl CoA
ATP
ADP
-52
-52
-45
-32
-32
1. ATP is the most important high
energy compound in cells.
ATP + H2O
o
G '
281
ADP + Pi
= - 32 kJ / mol
2. ADP
ADP + H2O
AMP + Pi
o
G ' = – 32 kJ/mol
TB
3. Phosphoenolpyruvate (PEP)
PEP + H2O
o
G '
COOCO~PO3
CH2
282
pyruvate + Pi
= - 52 kJ / mol
COOC=O + PO43CH3
TB
4. 1,3-bisphosphoglycerate (BPG)
283
BPG + H2O
3-phosphoglycerate
+ Pi
o
G ' = – 52 kJ/mol
The hydrolysis of the above high energy
compounds is coupled to energy-consuming
cellular reactions to drive them forward.
TB
SLP and glycolysis
284
During glycolysis, the hydrolysis of
the high-energy compounds PEP or
1,3-bisphosphoglycerate (BPG)
is "coupled" to ATP synthesis.
This is an example of SLP.
PEP + ADP
pyruvate + ATP
TB
AMP and glucose-6-phosphate
are examples of compounds with
low energy bonds.
o
G '
285
of hydrolysis  -14 kJ / mol
IV. Glycolysis
286
A. Overall reaction of glycolysis:
Glucose
2 pyruvate + 2 NADH + 2 ATP
• one pathway of making energy from glucose
• glucose is partially oxidized to pyruvate
• NAD+ is the intermediate electron
carrier that accepts the electrons
• ATP is made by substrate level
phosphorylation (SLP)
• glycolysis occurs in the cytoplasm
B. Important steps in glycolysis
glucose (C6)
energy
input
287
ATP
ATP
hexose splitting
redox
2 NADH step
ATP
2 ATP
synthesis
2 ATP
by SLP
2 pyruvate (C3)
C. Individual steps of glycolysis
(1)
glucose
hexokinase
(1)
288
ATP*
ADP
glucose-6-phosphate
energy input*
TB
(1)
glucose-6-phosphate
(1)
fructose-6- phosphate
289
TB
(1)
fructose-6- phosphate
290
ATP*
ADP
(1)
fructose-1,6- bisphosphate
energy input*
TB
(1)
fructose-1,6- bisphosphate
(C6 molecule)
291
splitting reaction
dihydroxyacetone
phosphate
(C3 molecule)
glyceraldehyde
3- phosphate
(C3 molecule)
TB
(2) glyceraldehyde-3- phosphate
Pi
(2)
+
NAD
(2) NADH + (2)
(2)
292
+
H
1,3-bisphosphoglycerate
Redox reaction
TB
(2)
1,3 bisphosphoglycerate (BPG)
293
2 ADP
(2)
2 ATP
3-phosphoglycerate
substrate level phosphorylation
TB
(2)
2-phosphoglycerate
(2)
phosphoenolpyruvate
294
TB
(2)
phosphoenolpyruvate
295
2 ADP
2 ATP
(2)
pyruvate
substrate level phosphorylation
TB
V. Reoxidation of NADH to NAD+
Important: in the cell
NAD+ is limited, so NADH
must be reoxidized
296
A. The reoxidation of electron carriers297
All organisms on earth that have been
studied use one or more of 3 general
methods to reoxidize electron carriers
1. Fermentation
2. Aerobic respiration
3. Anaerobic respiration
Organisms that use all three methods
usually prefer aerobic respiration.
TB
Glycolysis
298
glucose
2 pyruvate
2 NAD+ 2 NADH + H+
re-oxidation
1. Fermentation reactions
2. Aerobic respiration
3. Anaerobic respiration
TB
1. Fermentation
299
• catabolic process in which NADH
is re-oxidized using a compound
derived from the growth substrate
• ATP synthesis is by substrate level
phosphorylation (SLP) only.
• Generally used when O2 is
not available
TB
a. Fermentation producing ethanol
300
2 NAD+ 2 NADH + H+
glucose
2 pyruvate
2 CO2
2 CH3CH2OH
2 Ethanol
+
2NAD
2 Acetaldehyde
2NADH +
+
H
TB
b. Fermentation producing lactate
301
COO2 NAD+ 2 NADH +
C=O
glucose
2 pyruvate
CH
+
3
2NADH + H
H+
+
2NAD
COOHC-OH
2 lactate
CH3
TB
302
Fermentation does not use electron
transport chains for the reoxidation of
electron carriers. Cytoplasmic enzymes
catalyze the reoxidation of NADH.
Many different fermentations are known.
Some of the products of fermentation
are valuable.
TB
Study objectives
303
1. Describe how cells derive energy from an energy source. What are the
roles of the primary electron donor and the terminal electron acceptor?
2. Understand redox reactions and the terminology used to talk about them.
3. Understand redox couples.
4. Be able to use the table of standard reduction potentials to predict the
direction of a redox reaction.
5. Understand the relationship between Eo' and Go'. (Basically, a large Eo'
corresponds to a large Go' ) You will NOT be asked to do a calculation.
6. Understand how NAD functions in cells.
7. Compare and contrast NAD and NADP.
8. Understand high energy compounds. Know the examples of high energy and
presented in class. Know that GTP is a high energy compound.
9. Describe substrate level phosphorylation. Understand the difference
between substrate level phosphorylation and oxidative phosphorylation.
10. Understand the process of glycolysis. Know the overall reaction. Memorize
all the steps. Know which steps involve energy input, hexose splitting, redox
reactions, substrate level phosphorylation, ATP synthesis.
304
11. What are the 3 general methods microbes use to reoxidize reduced
electron carriers formed during catabolic processes?
12. Why must reduced electron carriers be reoxidixed?
13. Understand fermentation and its purpose. Memorize the examples and
reactions presented in class.
Respiration and the TCA cycle:
I. Aerobic respiration of glucose
II. TCA cycle
III. Electron carriers
IV. Electron transport system
V. Oxidative phosphorylation
305
Reoxidation of NADH
Growth
substrates
306
Oxidized
products
Oxidized
electron carriers
Reduced
electron carriers
re-oxidation
1. Fermentation
2. Aerobic respiration
3. Anaerobic respiration
TB
I. Aerobic respiration of glucose
307
one way to get more energy out of
glucose than by fermentation
glucose + 6 O2  6 CO2 + 6 H2O
Fermentation: ~2 ATP / glucose
Respiration: ~36 to 38 ATP / glucose
308
A. Respiration
1. Oxidation of an organic energy
source in the presence of an
external terminal electron acceptor
"external" terminal electron acceptor
glucose + 6 O2  6 CO2 + 6 H2O
organic energy source
1. terminal electron acceptor:
the last molecule to receive the
electrons during catabolism
309
2. "external" terminal electron
acceptor:
terminal electron acceptor that
is NOT derived from the energy
source
310
B. Aerobic respiration
1. Terminal electron acceptor is O2
Anaerobic respiration
"external" terminal electron
acceptor is NOT O2
eg. NO3- (nitrate), Fe3+, SO4-,
CO2, CO32-, succinate or
another organic molecule
311
B. Aerobic respiration (continued)
2. Reoxidation of reduced electron
carriers with O2 occurs via
intermediate electron carriers
arranged as electron transport
chains (respiratory chains).
3. ATP synthesis occurs mainly
by oxidative phosphorylation
312
C. Aerobic respiration of glucose
complete oxidation of glucose to CO2
higher energy yield than fermentation
C6H12O6
6 O2
6 CO2 + 6 H2 O
Respiration:
36 to 38 ATP
glucose
2 C3H6O3
lactic acid
Fermentation:
2 ATP
313
D. Oxidative phosphorylation
(electron transport phosphorylation)
ATP synthesis at the expense of a proton gradient
(proton motive force) produced across
a membrane by an electron transport system
H+
ATP
H+
H+
H+
H+
H+ H+
+
+
H
H
H+
ADP + Pi
Cytoplasmic membrane in prokaryotes
Inner mitochondrial membrane in eukaryotes
E. Overview: aerobic glucose respiration 314
glucose
membrane
NADH
acCoA
pyruvate
NADH
TCA
FADH2
GTP
NADH
NADH
NADH
e-
outside
2 H+
NAD+
proton
motive force
2 H+
H+
315
II. Glucose respiration to CO2 and the
TCA cycle glucose
1
NADH
pyruvate
Glucose respiration
CO2
2
acCoA
TCA CO2
3
NADH
4
NAD+
CO2
1. Glycolysis
2. Conversion of pyruvate (3C) to acetyl CoA (2C)
3. Oxidation of acetyl CoA in TCA cycle
4. Reoxidation the intermediate electron acceptors
5. ATP synthesis by oxidative phosphorylation
316
A. Conversion of pyruvate to acetyl CoA
• pyruvate oxidation produces NADH
• decarboxylation makes CO2
glucose
glycolysis
2 pyruvate
(3C)
2 CoA +
+
2 NAD
2 NADH
2 acetyl CoA
(2C)
2 CO2
317
B. Oxidation of acetyl CoA in the
TCA cycle (tricarboxylic acid cycle)
acetyl CoA
also called the
citric acid cycle
NADH
NADH
TCA CO2
FADH2
GTP
CO2
NADH
• two carbons are oxidized to CO2 per acetyl CoA
• 3 NADHs and 1 FADH2 are made per acetyl CoA
• one GTP is made by substrate level phosphorylation
1. Acetyl CoA (C2) condenses with 318
oxaloacetate (C4) to form citrate (C6).
O
CH3C~SCoA
acetyl CoA (2C)
COOO=CH oxaloacetate
CH2
COO-
citrate
(6C)
(4C)
(5C)
CH2COOHOCH2COOCH2COO-
2. Redox reactions, decarboxylations, SLP319
CoA + NAD+ NADH
pyruvate
acetyl CoA (2C)
CO2
(3C)
citrate
oxaloacetate
NADH
NAD+
(6C)
(4C)
FADH2
NAD+
(5C)
NADH
FAD
*SLP
CoA
GTP GDP + Pi
NAD+
NADH
CO2
CO2
3. The TCA Cycle
320
acetyl CoA (2C)
oxaloacetate
NADH
NAD+
FADH2
malate
fumarate
FAD
SLP
citrate
aconitate
(6C)
(4C)
succinate
CoA
GTP GDP + Pi
(5C)
isocitrate
NAD+
NADH
CO2
 -ketoglutarate
succinyl
CoA
NAD+
NADH
CO2
4. In the TCA cycle, there are 4 redox 321
reactions (3 NADH and 1 FADH2)
and two decarboxylations.
Four oxidative steps in the TCA cycle
isocitrate  -ketoglutarate
-ketoglutarate  succinyl CoA
succinate  fumarate (FADH2)
malate  oxaloacetate
2 decarboxylations (CO2 removed)
d. There is one substrate level
phosphorylation in the TCA cycle.
succinyl CoA
succinate
GTP is made and is
easily converted to ATP
322
e. Sum of reactions for
NADH
pyruvate oxidation
CO
acCoA
pyruvate
and TCA cycle
323
2
pyruvate  3 CO2
NADH
4 NADH
must be
1 FADH2
reoxidized
1 GTP by SLP
TCA
FADH2
GTP
NADH
CO2
NADH
CO2
15 ATP
equivalents
per pyruvate
III. Reoxidation of NADH and FADH2
with O2 occurs via intermediate
electron carriers arranged as electron
transport chains in the membrane.
324
e–
2H
2H
NAD+
NADH + H+
e–
Q
e–
1/2 O2 +
e–
2H+
H2O
TB
A. Intermediate electron carriers
325
1. NADH dehydrogenases
Protein complexes that accept
protons and electrons from NADH.
2H
NAD+
NADH + H+
[2H] = 2 protons + 2 electrons TB
2. Flavoproteins
326
Proteins with FAD or FMN (flavin
adenine dinucleotide or flavin
mononucleotide) as a prosthetic group.
Flavoproteins carry protons and electrons.
2H
TB
FMN and FAD (isoalloxazine ring)
H3C
H3C
N
N
327
O
NH
N
Oxidized form
O
R
Flavin couples
FMN / FMNH2
FAD / FADH2
FMN and FAD are
functionally equivalent, but
have a different R-group TB
Iron-sulfur center
S
E-cys-S
Fe
E-cys-S
328
S-cys-E
Fe
S
S-cys-E
2Fe2S
E-cys-S = the sulfur of a cys residue of
the protein is bonded to the iron
TB
329
Iron-sulfur center
E-cys-S
S
Fe
Fe
S
Fe
S
S
Fe
S-cys-E
S-cys-E
E-cys-S
4Fe4S
TB
4. Quinones
Small molecules (nonprotein)
330
Quinones carry both protons and
electrons.
Quinones can diffuse within the
membrane.
TB
331
Quinone
OH
O
O
CH3O
CH3O
R
CH3O
R
HO
CH3O
Q
QH2
Oxidized
Reduced
TB
Diffusion of quinones within the cell
membrane.
332
cytoplasm
TB
5. Cytochromes
333
Proteins that contain the heme
prosthetic group.
Cytochromes carry
electrons only.
cytochrome c
e–
e–
e–
cytochrome cytochrome
bc1
aa3 TB
334
Cytochrome
heme
protein
Fe
Fe3+/Fe2+
The iron carries the electrons
TB
B. Electron Transport Chains
A series of electron carriers
arranged within a membrane.
Many different electron transport
chains are known and they all
function similarly.
335
• electron transport chains can oxidize
intermediate electron carriers like NADH and
FADH2 and create proton gradients (PMF) TB
1. Electron transport chain of E. coli.
flavoprotein
quinone
336
cytochrome c
Q
NADH
dehydrogenase
iron-sulfur
protein
cytochrome
cytochrome
bc1
aa3
cytoplasm
TB
a. In aerobic respiration, the electron transport337
chain is used to reoxidize NADH with O2.
2H = 2 protons and 2 electrons
e–
2H
2H
2e–
Q
e–
e–
NAD
NADH + H+
cytoplasm
1/2 O2 + 2H+
H2O
TB
b. Oxidation via electron transport allows338
proton pumping. A proton gradient
(PMF) is formed across the membrane.
2H+
2H
2H
NAD
NADH +
H+
2H+
Q
2e–
e–
e–
e–
2H+ +
222H
cytoplasm
1/2 O2 + 2H+
H2O
TB
339
2. Proton motive force (PMF)
an energized state of the membrane created
by a proton gradient
+ + + +
+ -- - - - + -+ + - - - - +
+ + + + +
+ + + + + +
- - - - - - - H+
OH-
H+
+
H
- - - - - - -
+ + + + + +
H+
H+
H+ H+
+
+
H
H
H+
PMF
about -20 kJ/mol
In prokaryotes, H+ are pumped out of the cell.
The outside becomes slightly acidic and
positively charged relative to the inside.
340
3. Results of the electron transport chain
a. intermediate electron carriers (e.g.
NADH and FADH2) are reoxidized
b. electrons are ultimately transferred
to O2, making water
c. proton motive force (PMF) is
created, which can be used for ATP
synthesis
341
V. Oxidative phosphorylation
(electron transport phosphorylation)
ATP synthesis driven by PMF
The F1F0 ATPase synthesizes
ATP using the PMF.
A. Chemiosmosis (Peter Mitchell, 1961):
Use of an ion gradient (like PMF) to
drive ATP synthesis
TB
B. ATP synthesis using PMF
342
Energy is released when the H+ gradient is
dissipated. The energy can do work (make
ATP, rotate flagella, take up nutrients).
H+
ATP
2 to
4H+
ADP + Pi
H+
H+
H+ H+
+
H+ H
H+
PMF
-20 kJ/mol
ATP synthase
F1F0 ATPase
H+
H+
343
H+
Fo
F1: catalyzes
ATP
synthesis
cytoplasm
ADP + Pi
H+
+
+
H
H
ATP
TB
C. How many ATPs can be
synthesized when NADH and
FADH2 are reoxidized through an
electron transport chain
(respiratory chain)?
NADH
FADH2
~ 3 ATP
~ 2 ATP
344
D. Comparison of glucose fermentation 345
and respiration in bacteria
glucose fermentation: 2 ATP per glucose
glucose respiration (bacteria): 38 ATP per glucose
glycolysis:
2 ATP (net)
2 NADH (2 x 3 ATP)
2 pyr  2 acCoA: 2 NADH
2 ATP
6 ATP
6 ATP
2 acCoA  (2) x TCA cycles:
(2) x 3 NADH (2 x 3 x 3 ATP) 18 ATP
(2) x 1 FADH2 (2 x 1 x 2 ATP) 4 ATP
(2) x 1 GTP
2 ATP
Total:
38 ATP
Study objectives for lecture 9
346
1. Understand respiration. Contrast fermentation and respiration.
2. Understand oxidative phosphorylation. Contrast oxidative phosphorylation
and substrate level phosphorylation.
3. Describe the overall process of glucose respiration and the five steps
presented in class.
4. Memorize and understand the reaction of pyruvate conversion to acetyl CoA.
5. Understand the TCA cycle. Know that the TCA cycles begins with the
reaction of acetyl CoA and oxaloacetate to make citrate. How does the TCA
cycle help cells produce energy?
6. Memorize ALL the steps in the TCA cycle. Know which steps involve redox
reactions, substrate level phosphorylation, decarboxylation. You do not need
to memorize the structures.
7. In respiration, glucose is oxidized completely to CO2. How is this done?
Where is CO2 released? What happens to the electrons? What is the role of
oxygen in respiration? How is energy conserved as ATP? How do cells
derive energy from glucose in respiration? In fermentation?
8. What are electron transport chains? What is their role in metabolism? 347
9. Compare and contrast the electron carriers used in electron
transport chains. Understand the particular features of each electron carrier.
You do not need to memorize the structures.
10. Which electron carrier is nonprotein?
11. Can cytochromes carry protons?
12. Describe how electron transport chains are used to synthesize ATP.
Understand how electron transport in the membrane generates proton
motive force. Recall that proton motive force can be used to produce ATP.
Continued on next slide
348
1. Describe oxidative phosphorylation. Define chemiosmosis.
2. Know the general structure of the F1/F0 ATPase. What is its function?
3. How is the reoxidation of intermediate electron carriers related to ATP
synthesis?
4. Which method allows production of more ATP: aerobic respiration or
fermentation?
5. Starting with glucose, describe how ATP is made from glucose in fermentation
and respiration.
6. Describe how glycolysis, the TCA cycle, electron transport chains, and ATP
synthesis are connected in respiration.