Download 5.19.06 Electron Transport and Oxidative Phosphorylation Reading

5.19.06 Electron Transport and Oxidative
Phosphorylation
Reading Assignment:
Start reading Chapter 14: Energy Generation in
Mitochondria and Cholorplasts
See animation 14.3 on your text CD
ATPsynthase animation and lots of other stuff
http://vcell.ndsu.nodak.edu/animations/home.htm
1
What does the citric acid accomplish?
• Carbons in pyruvate are fully oxidized to CO2
• Some GTPs (which may be later converted to ATPs)
are generated by substrate level phosphorylation
• 8 NADH and 2 FADH2 are stockpiled
NADH and FADH2 are high energy molecules
and they can be used as reducing agents by the
cell.
But much of the stored energy is not directly
“accessible” to the cell in this form
What happens in the mitochondria to convert
the potential energy in NADH into the form of
ATP?
2
NADH and FADH2 are high energy molecules:
As electrons drop from the
top to the bottom of the scale,
energy is released
3
• Substances vary in
their tendancy to
become oxidized or
reduced.
• This tendancy is
expressed as the
reduction
potential. As
electrons drop
from the top to the
bottom of the scale,
energy is released.
• Glucose has a
reduction potential
of (-0.43 V) – so
the exergonic
oxidation of
glucose is coupled
to the endergonic
reduction of NAD+
How to power ATP synthesis
• build a dam
• pile up protons on one side
• poke a hole -- use the rush
of protons through the hole
to turn a turbine which then
makes ATP
4
The inner mitochondrial membrane is 70% protein and
30% phospholipid by weight
An electron micrograph of
the inside surface of the
inner mitochondrial
membrane in a plant cell.
Densely packed particles
are visible -- due to
protruding portions of
ATP synthase and the
respiratory enzyme
complexes
5
A Closer look at the inner mitochondrial membrane

 ATP synthase
ATP
Transporter
The inner mt membrane is 70% protein and 30% phospholipid
by weight
• many of the proteins belong to the electron transport chain
• also includes ATP synthase: converts the stored potential
energy in the electrochemical proton gradient into chemical
energy
matrix
space between inner membranes
6
THE ELECTRON TRANSPORT CHAINS CONSISTS
OF MEMBRANE ASSOCIATED ELECTRON
CARRIERS
These systems have two basic functions:
1. to accept electrons from an electron donor and to
transfer them to an electron acceptor
2. to conserve some of the energy released during
electron tranasfer for the synthesis of ATP
The electron transport chain reoxidizes the
coenzymes NADH and FADH and channels the free
energy into the synthesis of ATP
• NADH and FADH gained electrons when
oxidizing other compounds
• they transfer these electrons to the electrontransport chain: electron transport chain
reoxidizes the coenzymes NADH and FADH
and channels the free energy into the synthesis
of ATP
7
NADH is a high energy
molecule:
Brock 5.19
Reduction potential
of the components
of the electron
transport chain of
the mitochondrion
of eukaryotic cells
and the plasma
membrane of some
bacterial cells.
By breaking up the
complete oxidaton
into a series of
discrete steps,
energy “recapture”
is possible
• Substances vary in their tendancy to become oxidized or reduced.
• This tendancy is expressed as the reduction potential. As
electrons drop from the top to the bottom of the scale, energy is
released.
• NOTICE the molecule at the bottom!
8
How to power ATP synthesis
• build a dam
• pile up protons on one side
• poke a hole -- use the rush of protons through
the hole to turn a turbine which then makes ATP
Oxidative Phosphorylation:
the production of ATP using energy derived from the
redox reactions of an electron transport chain
Chemiosmosis:
the production of ATP from ADP using the energy of
hydrogen ion gradients
9
How protons can be
pumped across membranes:
As an electron passes along
an electron-transport chain
embedded in a lipidbilayer, it can bind and
release a proton at each
step. In this diagram
electron carrier B picks up
a proton (H+) from one side
of the membrane when it
accepts an electron from
carrier A. It releases the
proton to the other side of
the membrane when it
donates its electron to
carrier C
H-bonding is always with us
proton wires: net translocation of protons can occur over a long distance
through a protein by hopping between pairs of hydrogen bonded donor
and acceptor residues; a string of such residues connected by hydrogen
bonds can be thought of as a proton wire.....
Matrix (inside the inner membrane of the mt) is
above the membrane (gray bar). The
intermembrane space is below the membrane.
animation of electron transport
http://www.sp.uconn.edu/~terry/images/anim/ETS_slow.html
10
ALBERTS animation 14.2
• The energy released is used to transport H+ ions across the
inner mitochondrial membrane to the space between the two
membranes
• In this way, a gradient of H+ ions is maintained across the
inner membrane
• This gradient serves as a source of energy (like a battery) that
is tapped to drive a variety of energy-requiring reactions
• The most prominent of these reactions is the generation of
ATP
ADP + Pi -----> ATP
•
11
How to power ATP synthesis
• build a dam
• pile up protons on one side
• poke a hole -- use the rush of protons
through the hole to turn a turbine which
then makes ATP
12
Potential energy in gradient converted to
mechanical energy which via conformtional
changes in the cytoplasmic portion of the ATPase is
converted to chemical energy in ATP
• ATP synthase is
imbedded in the
inner mitochondrial
membrane
• below & left in this
figure is the matrix
of the mitochondria
(compartment
contained within the
inner mitochondrial
membrane)
• water soluble
catalytic domain is in
matrix
• spinning ion
transport channel
(embedded in lipid
bilayer)
13
ATP synthase: enzyme that uses energy from
the proton gradient to produce ATP from ADP
+ Pi
• inner mitochondrial membrane of all
eukaryotic cells
• the thylakoid membrane of chloroplasts of
plant cells
• the plasma membrane of prokaryotic cells
14
Now for some serious quaternary structure!
ATP synthase — energy converter. The enzyme consists of two rotary motors, F0
and F1 which are coupled via their drive shafts. The transmembrane F0 motor has
one a, two b and nine to twelve c subunits. The soluble F1 motor has three α and
three β subunits, and one each of the other subunits.
During ATP synthesis, F0 channels protons across the membrane to drive rotation.
Nature 410: 878 4/19/01
•
The rotating subunits are the c polypeptide in Fo and the γ
polypeptide in F1
• The rotation of Fo (caused by movement of protons) drives the
rotation of γ
• This rotation drives the conformational transitions of the
catalytic subunits which, in turn, alters the nucleotide binding
site affinities.
• As a consequence, conformational energy flows from the
catalytic subunit into the bound ADP and Pi to promote their
dehydration into ATP.
ALBERTS animation 14.3
HTTP://WWW.TCD.IE/BIOCHEMISTRY/IUBMB-NICHOLSON/SWF/GLYCOLYSIS.SWF
Step 32 in this animation substrate level ATP synthesis in glycolysis
15
MORE optional Stuff on ATP synthase for those who are
amused by this protein:
Look at First two links
ATP synthase
do cross section alpha, beta gamma
http://www.cnr.berkeley.edu/~hongwang/Project/ATP_synthase/
ATP synthase
http://rsb.info.nih.gov/NeuroChem/biomach/ATPsyn.html
This cartoon is adapted from fig. 2 of Cross. The 3 shades of red represent the 3 different
conformational states of the catalytic subunits. The central asymmetric black object represents
the gamma subunit which is caused to rotate by themitochondrial proton efflux. This rotation
drives the conformational transitions of the catalytic subunits which, in turn,alters the nucleotide
binding site affinities. As a consequence, conformational energy flows from the catalytic subunit
into
the bound ADP and Pi to promote their dehydration into ATP.
http://teddy.berkeley.edu:1024/ATP_synthase/
ATP synthase: the rotory engine in the cell:
http://www.res.titech.ac.jp/~seibutu/main.html?right/~seibutu/projects/f1_e.html
in vitro rotation of an actin filament attached to ATP synthase
16
What fraction of the potential energy can a respiring
cell extract from a glucose molecule?
A few billion years of evolution have ensured that
the aerobic system is
40 - 54%
efficient
17
The complete oxidation of 1 mole of glucose generates about 38 moles
of ATP (synthesized from ADP)
ATP yield from complete oxidation of glucose
Process
Direct product
Final ATP
Glycolysis
2 NADH (cytosolic)
2 ATP
2 NADH
(mitochondrial matrix)
3 or 5*
2
5**
Pyruvate oxidation
(2 per glucose)
Acetyl CoA oxidation
(Citric Acid cycle)
two per glucose
6 NADH
(mitochondrial matrix)
2 FADH2
2ATP or 2 GTP
Total ATP yield
per glucose molecule
15
3
2
30 -32***
* depends on which "shuttle system" transfers reducing equivalents into
the mitochondria
** 3 protons per ATP
*** This number varies from reference to reference..
18
Why is ATP “high energy”
ATP has stored potential energy:
ATP  ADP + Pi + energy
ΔGo = - 7.3 kcal/mole exergonic reaction
(corresponds to an Keq of >105
Under cellular conditions, the hydrolysis of ATP
creates two molecules of much lower energy and
releases a great deal of usable energy
• The phosphates in ATP can be considered to exist
in an activated state: the presence of four negative
charges in close proximity destabilizes the
molecule -- electrostatic repulsion between
negative charges favors hydrolysis
• Increased hydration ADP and P -- energetically
favored
• Release of a phosphate increases entropy because
the PO4 molecule released is capable of resonance
forms (delocalized proton and oxygen binding) not
possible when phosphate is bound to another
molecule
19
20
What do cells do with ATP?
21
• Drives anabolic (endergonic) chemical
reactions
• Used to do work (move stuff around –
molecular motors)
• Used for active transport to make/maintain
ion and solute gradients
• Lots of other roles
22
Cells drive active transport in three main ways
23
Remember lysosomes?
• Lysosomes contain hydrolytic enzymes that are
active under acidic conditions.
• The interior of this organelle is maintained at an
acidic pH (high proton concentration) by a H+
ATPase in the membrane that pumps protons
against the concentration gradient
24
WHAT HAPPENS DURING GLUCOSE OXIDATION IF
NO OXYGEN IS PRESENT?
AEROBIC AND ANAEROBIC ORGANISMS
One basic metabolic distinction among organisms is
whether or not they can use O2 as an electron
acceptor in energy producing pathways
AEROBES: CAN USE O2
Obligate Aerobe: O2 is obligatory for life
ANAEROBES: CAN SUBSIST WITHOUT O2
Facultative Anaerobe: can adapt to anaerobic
conditions by substituting other electron
acceptors for O2 (yeast, bacteria)
Obligate Anaerobe: cannot use O2 and are poisoned by it
25
BEAKER CONTAINS YEAST AND SUGAR
What is the gas being produced in the beaker?
WHAT HAPPENS DURING GLUCOSE OXIDATION IF
NO OXYGEN IS PRESENT?
Cell has a limited amount of NADH which must be
recycled if glycolysis is to continue under anaerobic
conditions
FERMENTATION: an anaerobic biological reaction
process
26
FERMENTATION: THE ANAEROBIC FATE OF PYRUVATE
Commercially Valuable fermentation reactions:
Alcoholic fermentation by yeast used in brewing
and winemaking
Bacteria also can carry out alcoholic
fermentation under anaerobic conditions
27
This type of fermentation occurs in some fungi and bacteria
(used to make yogurt and cheese) and in human muscle cells
when oxygen is limiting
28
The three common metabolic fates of pyruvate generated by
glycolysis:
• Under aerobic conditions, the pyruvate is completely
oxidized via the citric acid cycle to CO2 and H2O [NADH acts
as a high energy compound]
• Under anaerobic conditions, pyruvate must be converted to
a reduced end product in order to reoxidize the NADH
produced by the GAPDH reaction
• alcoholic fermentation: in yeast, pyruvate is converted to
ethanol + CO2 [free energy of NADH oxidation is dissipated
as heat]
• in muscle cells, under anaerobic conditions, pyruvate is
reduced to lactate [free energy of NADH oxidation is
dissipated as heat]
29
FERMENTATION: an anaerobic biological
reaction process in which a reduced organic
compound (like glucose) acts as an electron
donor and another organic compound acts as
an electron acceptor
An even more formal definition of fermentation
fermentation: catabolic reactions producing ATP in which
organic compounds serve as both the primary electron donor and
ultimate electron acceptor and ATP is produced by substrate
level phosophorylation
Note: fermentation is extremely inefficient
compared to aerobic respiration.
From first principles: Why?
30
The potential energy drop between glucose and an
electron acceptor like pyruvate is a fraction of the
potential energy drop that occurs during cellular
respiration where O2 is the electron acceptor
31