Download Cellular metabolism

Cellular
metabolism
George Howell III, Ph.D
Basics of cellular metabolism
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Uptake of sugars (e.g. glucose)
Glycolysis
Citric acid cycle
Electron transport chain
Oxidative phosphorylation
Metabolic overview (aerobic respiration)
ATP synthase
Figure 2-86 Molecular Biology of the Cell (© Garland Science 2008)
Chapter 2
HOW CELLS OBTAIN ENERGY
FROM FOOD
• Most Animal Cells Derive Their Energy from
Fatty Acids Between Meals
• Sugars and Fats Are Both Degraded to
Acetyl CoA in Mitochondria
• The Citric Acid Cycle Generates NADH by
Oxidizing Acetyl Groups to CO2
• Electron Transport Drives the Synthesis of
the Majority of the ATP in Most Cells
© Garland Science 2008
Pathways of production of acetyl CoA from sugars and fats
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Occurs in mitochondrion in eucaryotic cells
Produced from both types of major food molecules
Where most of the cell’s oxidation reactions occur
Where most of the cell’s ATP is made
Figure 2-80 Molecular Biology of the Cell (© Garland Science 2008)
Stored fats are mobilized for
energy production (animals)
HSL
Low glucose levels in
the blood
Hydrolysis of
triacylglycerol in fat
droplets
Free fatty acids and
glycerol enter bloodstream
Bind to serum albumin
(blood protein)
Fatty acid transporters in
fat oxidizing cells pass
fatty acids to cytosol
To mitochondria for
energy production
Figure 2-78 Molecular Biology of the Cell (© Garland Science 2008)
Outline of
glycolysis
Phosphofructokinase = rate
limiting step
Fructose 6 phosphate to
fructose 1,6 - bisphosphate
•2 ATP in…..4 ATP out
= net gain of 2 ATP
•2 molecules of NADH
out
Glycolysis (cont.)
Figure 2-70 Molecular
Biology of the Cell (©
Garland Science 2008)
Figure 2-71a Molecular Biology
of the Cell (© Garland Science
2008)
2 pathways for anaerobic
breakdown of pyruvate
Inadequate oxygen
In some anaerobic organisms
TCA cycle
Net production:
• 3 NADH
• 1 FADH2
• 1 GTP
• 2 CO2
Pyruvate dehydrogenase = 1
NADH
Harnessing Energy for Life
(A)
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The essential requirements for chemiosmosis are:
a membrane—in which are embedded a pump protein
an ATP synthase
a source of high-energy electrons (e -)
(B) The proton gradient
• Serves as an energy store
• Used to drive ATP synthesis
by the ATP synthase enzyme.
Protons (H+)
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Freely available from water molecules
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The pump harnesses the energy of electron transfer to pump protons
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Creates a proton gradient across the membrane.
Figure 14-1 Molecular Biology of the Cell (© Garland Science 2008)
The relationship between mitochondria and microtubules
Why be highly associated with microtubules??
(A) A light micrograph of chains of
elongated mitochondria in a
living mammalian cell in culture.
The cell was stained with a
fluorescent dye (rhodamine 123)
that specifically labels
mitochondria in living cells.
Figure 14-5 Molecular Biology of the Cell (© Garland Science 2008)
(B) An immunofluorescence
micrograph of the same cell
stained (after fixation) with
fluorescent antibodies that bind
to microtubules. Note that the
mitochondria tend to be
aligned along microtubules.
The general organization of a mitochondrion
•In the liver, an estimated 67% of the total mitochondrial protein is located in the matrix,
21% is located in the inner membrane, 6% in the outer membrane, and 6% in the
intermembrane space.
•Each of these four regions contains a special set of proteins that mediate distinct functions.
Figure 14-8 Molecular Biology of the Cell (© Garland Science 2008)
A summary of energy-generating metabolism in
mitochondria
•Pyruvate and fatty acids enter the mitochondrion
(bottom) and are broken down to acetyl CoA.
•The acetyl CoA is then metabolized by the citric acid
cycle, which reduces NAD+ to NADH (and FAD to FADH2).
•In the process of oxidative phosphorylation, highenergy electrons from NADH (and FADH2) are then
passed along the electron-transport chain in the inner
membrane to oxygen (O2).
•This electron transport generates a proton gradient
across the inner membrane, which is used to drive the
production of ATP by ATP synthase.
•The NADH generated by glycolysis in the cytosol also passes electrons to the respiratory chain.
•NADH cannot pass across the inner mitochondrial membrane
•the electron transfer from cytosolic NADH must be accomplished indirectly
•done by means of one of several “shuttle” systems that transport another reduced compound into the
mitochondrion.
•After being oxidized, this compound is returned to the cytosol, where it is reduced by NADH again.
Figure 14-10 Molecular Biology of the Cell (© Garland Science 2008)
Table 14-1 Molecular Biology of the Cell (© Garland Science 2008)
The major net energy conversion catalyzed by the mitochondrion
•In this process of
oxidative phosphorylation,
the inner mitochondrial
membrane serves as a
device that changes one
form of chemical bond
energy to another
•Converts a major part of
the energy of NADH (and
FADH2) oxidation into
phosphate-bond energy in
ATP.
Figure 14-11 Molecular Biology of the Cell (© Garland Science 2008)
The general mechanism of oxidative phosphorylation
A high-energy electron is passed along the electron-transport chain
• Some of the energy released is used to drive the three respiratory enzyme
complexes that pump H+ out of the matrix.
• The resulting electrochemical proton gradient across the inner membrane
drives H+ back through the ATP synthase, a
• transmembrane protein complex that uses the energy of the H+ flow to
synthesize ATP from ADP and Pi in the matrix.
Figure 14-14 Molecular Biology of the Cell (© Garland Science 2008)
The path of electrons through the three respiratory enzyme
complexes
•During the transfer of electrons from NADH to oxygen (red lines), ubiquinone and
cytochrome c serve as mobile carriers that ferry electrons from one complex to the
next.
•Protons are pumped across the membrane by each of the respiratory enzyme
complexes.
Figure 14-26 Molecular Biology of the Cell (© Garland Science 2008)
Redox potential changes along the mitochondrial electrontransport chain
•The redox potential (E′0) increases as electrons flow down the respiratory chain to
oxygen.
•Electrons flow through a respiratory enzyme complex by passing in sequence through
the multiple electron carriers in each complex.
•Part of the favorable free-energy change is harnessed by each enzyme complex to
pump H+ across the inner mitochondrial membrane.
•NADH is not the only source of electrons for the respiratory chain.
•The flavin FADH2 is also generated by fatty acid oxidation and by the citric acid
cycle.
•Its two electrons are passed directly to ubiquinone, bypassing NADH
dehydrogenase.
•Cause less H+ pumping than the two electrons transported from NADH.
The standard
free-energy
change, ΔG°, for
the transfer of
each of the two
electrons donated
by an NADH
molecule can be
obtained from
the left-hand
ordinate.
NADH
dehydrogenase
and cytochrome
b-c1 complexes
each pump two
H+ per electron
Figure 14-29 Molecular Biology of the Cell (© Garland Science 2008)
ΔG = -n(0.023) ΔE′0,
where n is the
number of
electrons
transferred across a
redox potential
change of ΔE′0 mV
Cytochrome
oxidase complex
pumps one H+ per
electron
Quinone electron carriers in the respiratory chain
Ubiquinone picks up
one H+ from the
aqueous environment
for every electron it
accepts
Ubiquinone can
carry either one
or two electrons
as part of a
hydrogen atom
When reduced
ubiquinone donates its
electrons to the next
carrier in the chain, the
protons are released.
A long hydrophobic
tail confines
ubiquinone to the
membrane and
consists of 6–10
five-carbon
isoprene units, the
number depending
on the organism.
•The corresponding electron carrier in the photosynthetic membranes of chloroplasts is
plastoquinone, which is almost identical in structure.
•For simplicity, both ubiquinone and plastoquinone are referred to in this chapter as quinone
(abbreviated as Q).
Figure 14-24 Molecular Biology of the Cell (© Garland Science 2008)
The reaction of O2 with electrons in cytochrome
oxidase
Four protons are pumped out of the
matrix for each O2 molecule that
undergoes the reaction
4e - + 4H+ + O2 → 2H2O.
Figure 14-27
Molecular Biology of
the Cell (© Garland
Science 2008)
ATP synthase
•Both F1 and F0 are formed from multiple subunits.
•A rotating stalk turns with a rotor formed by a ring of 10 to 14 c subunits in the membrane
(red).
•The stator (green) is formed from transmembrane a subunits, tied to other subunits that create
an elongated arm.
•This arm fixes the stator to a ring of 3α and 3β subunits that forms the head. (B) The threedimensional structure of the F1 ATPase, determined by x-ray crystallography. This part of the ATP
synthase derives its name from its ability to carry out the reverse of the ATP synthesis reaction—
namely, the hydrolysis of ATP to ADP and Pi, when detached from the transmembrane portion.
transmembrane
H+ carrier, called
F0
head portion,
called the F1
ATPase
Figure 14-15 Molecular Biology of the Cell (© Garland Science 2008)
The three-dimensional
structure of the F1 ATPase,
determined by x-ray
crystallography.
•This part of the ATP
synthase derives its name
from its ability to carry out
the reverse of the ATP
synthesis reaction—
namely, the hydrolysis of
ATP to ADP and Pi, when
detached from the
transmembrane portion.
Inhibitors of ETC
Cyanide
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Hydrogen cyanide is most toxic (gas at room
temp.)
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Most common industrial use is in mining industry
to dissolve gold and silver
Used in Nazi Germany in gas chambers during
Holocaust
May have a bitter almond smell
Cyanide ion can come from cyanide salts,
apricot, peach, cherry pits (contain toxic
amounts of cyanide that must be metabolized
by intestinal bacteria to release it), fire smoke
(especially burning plastics), vapors from
industrial metal plating
Main MOA is to bind metalloenzymes
(cytochrome oxidase) and inhibit oxidative
respiration/phosphorylation
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Shifts to anaerobic respiration
Death occurs quickly
Causes metabolic acidosis
Tissues with high oxygen demand (brain and
heart) are first damaged
Rotenone
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Broad spectrum pesticide and insectide
Interferes with electron donation from
complex I to ubiquinone
Oral ingestion produces GI irritation
Can cause conjunctivitis, dermatitis,
pharyngitis, rhinitis
Possible link with Parkinson’s upon chronic
exposure
Treatment is symptomatic
The two components of the electrochemical proton gradient
•The total proton-motive force across the inner mitochondrial membrane consists of …
•a large force due to the membrane potential (traditionally designated ΔΨ by experts, but
designated ΔV in this text)
•a smaller force due to the H+ concentration gradient (ΔpH). Both forces act to drive H+ into
the matrix.
Figure 14-13
Molecular Biology
of the Cell (©
Garland Science
2008)
The organization of the human mitochondrial genome
•The human mitochondrial genome contains…
•2 rRNA genes
•22 tRNA genes
•13 protein-coding sequences
•The DNAs of many other animal mitochondrial genomes have also been completely
sequenced.
•Most encode precisely the same genes as humans
•Gene order is identical for animals that range from mammals to fish
Figure 14-60 Molecular Biology of the Cell (© Garland Science 2008)
An electron micrograph
of an animal
mitochondrial DNA
molecule caught during
the process of DNA
replication:
•The circular DNA
genome has replicated
only between the two
points marked by red
arrows.
•The newly synthesized
DNA is colored yellow.
Figure 14-54 Molecular Biology of the Cell (© Garland Science 2008)