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Electron transport chain
Cellular respiration is a series of reactions that:
-are oxidations – loss of electrons
-are also dehydrogenations lost electrons are accompanied by hydrogen
what is actually lost is a hydrogen atom (1 electron, 1 proton).
• A hydrogen atom is equivalent to a hydrogen ion plus an electron
H = H+ + e-electrons carry energy from one molecule to another.
-electrons are shuttled through electron carriers to a final electron
acceptor
aerobic respiration:
final electron receptor is oxygen (O2)
NAD+ is an electron carrier.
-NAD accepts 2 electrons + 1 proton to become NADH.
-the reaction is reversible
The goal of respiration is to produce ATP.
- energy is released from oxidation reaction in the form of electrons
- electrons are shuttled by electron carriers (e.g. NAD+) to an electron transport chain
- electron energy is converted to ATP at the electron transport chain
Oxidation of Glucose
Cells are able to make ATP via:
1. substrate-level phosphorylation –
transferring a phosphate directly from substrate molecules to
ADP.
2. oxidative phosphorylation –
use of ATP synthase and energy derived from a proton (H+)
gradient to make ATP, occurs only in O2 presence.
The complete oxidation of glucose proceeds in stages:
1. glycolysis
2. pyruvate oxidation
3. Krebs cycle
4. electron transport chain & chemiosmosis
For glycolysis to continue, NADH must be recycled to NAD+ by:
1. Fermentation – occurs when oxygen is not available; an
organic molecule is the final electron acceptor.
2. Aerobic respiration – occurs when oxygen is available as the
final electron acceptor.
The fate of pyruvate depends on oxygen availability.
When oxygen is present, pyruvate is oxidized to acetyl-CoA
which enters the Krebs cycle.
Without oxygen, pyruvate is reduced in order to oxidize NADH
back to NAD+.
Electron Transport Chain
The electron transport chain (ETC) is a series of membrane-bound
electron carriers.
-embedded in the mitochondrial inner membrane
-electrons from NADH and FADH2 are transferred to complexes of the
ETC
-each complex transfers the electrons to the next complex in the chain
As the electrons are transferred, some electron energy is lost with each
transfer
This energy is used to pump protons (H+) across the membrane from the
matrix inner membrane space.
A proton gradient is established. The energy of the proton gradient is
known as the chemiosmotic potential == proton motive force PMF.
• Krebs cycle in the matrix, produces NADH and FADH2
• NADH and FADH2 transfers its hydrogens to the ETC, where they
are split into H+ ions and electrons
• The ETC acts as a H+ ion pump, pulling H+s into the intermembrane
space
• This causes intermembrane space pH to drop to 7 and raises matrix
pH to 8
• electrons are conducted laterally through the membrane from one ETC
complex to the next, each time donating some of their energy to
"pump" H+ ions.
• Finally the de-energized electrons are combined with hydrogen ions
and oxygen to produce water in the matrix
• This reaction is catalyzed by cytochrome oxidase
• The rxn is inhibited by cyanide, which is why cyanide is a poison
An overview of electron transport chain operations
2 mitochondrial compartments formed by 2 bilayer membranes. The light
blue circles represent the (ETC), embedded in the membrane between the
2 compartments (the inner mitochondrial membrane) embedded in this
membrane is the enzyme ATP synthase (pink)
• These reactions store energy in a hydrogen gradient: H+
concentration is 10 X >>in intermembrane space than in the
matrix
• H+ diffuses back into the matrix through a channel in ATP
synthase, producing an electric current
• The shaft of the ATP synthase complex rotates
(counterclockwise) when it conducts an electric current.
• ATP is formed in the inner matrix: it must be transported
across 2 membranes to get into the cytosol
• Specific integral (intrinsic) membrane proteins accept electrons and H from NADH
and FADH2 in the presence of O2.
• NADH --> NAD+
– NADH from rxn in krebs and from the pyruvate dehydrogenase complex rxn.
– The name of the protein/enzyme that oxidizes NADH is NADH reductase.
– This enzyme dumps H+ into the intermembrane space.
• FADH2 --> FAD
– FADH2 from the succinate dehydrogenase rxn in Krebs.
– The protein/enzyme that oxidizes FADH2 is succinate reductase.
– This enzyme dumps H+ into the intermembrane space.
Cytochrome oxidase is the name of the protein/enzyme which interacts with
oxygen.
• This enzyme dumps H+ into the intermembrane space.
• Any chemical interferring with the exchange of electrons and protons
between cytochrome oxidase and oxygen will halt the electron transport
chain function and will cause respiration to stop.
• Proteins in the inner mitochondrial membrane and cristae
that have Fe, S, Cu as part of their structures are reduced
and oxidized as electrons and H are moved along.
• Each electron transport protein works in a one way
direction. It 'grabs' electrons away from the previous
component of the chain.
• Protons dumped into the intermembrane space create a
proton gradient across the inner mitochondrial
membrane. The proton gradient is the stored energy that
catalyzes the oxidative phosphorylation of ADP.
• More ATP produced in oxidative phosphorylation vs
anaerobic fermentation.
Cytochrome
oxidase
NADH dehydrogenase
Succinate
dehydrogenase
Ubiquinone
Cytochrome c
oxidoreductase
Cyanide inhibits cytochrome oxidase
• Comparing the Energy Capture of Glycolysis and
Respiration
• Most of the energy captured from the Krebs cycle is in the
bonds of the coenzymes, NADH and FADH2
• In the electron transport chain the coenzyme energy is
"cashed in" for ATP similar to currency exchange
• The exchange rate different for different coenzymes
– FADH2 gives 2 ATPs
– NADH made in the mitochondria gets 3 ATPs
– NADH made outside the mitochondria (in glycolysis)
gets only 2 ATPs (some energy is lost in transporting this
NADH into the mitochondria)
The higher negative charge in the matrix attracts the protons (H+)
back from the intermembrane space to the matrix.
Most protons move back to the matrix through ATP synthase.
ATP synthase is a membrane-bound enzyme that uses the energy
of the proton gradient to synthesize ATP from ADP + Pi.
theoretical energy yields
- 36 ATP per glucose for eukaryotes
Glycolysis
2 net ATP from substrate-level phosphorylation
2 NADH yields 6 ATP (assuming 3 ATP per NADH) by oxidative phosphorylation
Transition Reaction
2 NADH yields 6 ATP (assuming 3 ATP per NADH) by oxidative phosphorylation
Citric Acid Cycle
2 ATP from substrate-level phosphorylation
6 NADH yields 18 ATP (assuming 3 ATP /NADH) by oxidative phosphorylation
2 FADH2 yields 4 ATP (assuming 2 ATP /FADH2) by oxidative phosphorylation
Total Theoretical Maximum Number of ATP Generated per Glucose in
Prokaryotes
38 ATP: 4 from substrate-level phosphorylation; 34 from oxidative phosphorylation.
In eukaryotic cells, the theoretical maximum yield of ATP generated per glucose is
36 to 38, depending on how the 2 NADH generated in the cytoplasm during glycolysis
enter the mitochondria and whether the resulting yield is 2 or 3 ATP per NADH.
• The NADH dehydrogenase of the inner mitochondrial
membrane accept electrons only from NADH in the matrix.
Problem: the inner membrane is not permeable to NADH,
how can the NADH generated by glycolysis in the cytosol
be reoxidized to NAD by O2 via the respiratory chain?
Solution: Special shuttle systems carry reducing
equivalents from cytosolic NADH into
mitochondria by an indirect route.
The most active NADH shuttle, which functions in liver,
kidney, and heart mitochondria, is the malate-aspartate
shuttle
Malate Aspartate shuttle
• In contrast to oxidation of mitochondrial NADH, cytosolic
NADH, when it is oxidized via the electron transport system
• if it proceeds via the malate aspartate shuttle gives rise to 3
ATPs
• if it is oxidized by the glycerol phosphate shuttle gives rise
to 2 equivalents of ATP
Malate-Aspartate Shuttle
Glycerol-3-phosphate shuttle :
In skeletal muscles and Brain: