Electron Transport Chain Flashcards
ETC is essentially chain of
Redox reactions
Goal: To reduce molecular oxygen
Flow of electrons (overall)
NADH —> Complex 1 —> CoQ —> Complex III —> Cyt c —> Complex IV —> O2
FADH2 also donates its electrons to Complex II which donates to CoQ
Synthesis of ATP is coupled to
Electron transfer
No electron transfer = no proton gradient = no energy = no ATP synthesis
Electrons move in one direction because? (2)
Trend?
- Of changes in standard reduction potential.
— As you move down the chain, each e- acceptor is better than the previous as indicated by their more positive standard reduction potentials. - Successive transfer of electrons are exergonic.
— Standard free energy change is related to the change in reduction potential
—For every 2 electrons passed —> 220 kJ/mol energy generated
Electron transfer potential of NADH and FADH2 is converted to __________ of _______
High phosphoryl transfer potential of ATP
Oxidation-reduction reactions must _______
Occur together.
One molecule is oxidized —> something else is reduced
Redox reactions occur in “ ________”
Two half reactions
Ultimate electron donor =
Carbon source
Ultimate electron acceptor =
O2
A more negative standard reduction potential means _______
Better electron DONOR
Ex. NADH is better than FADH2
A more positive standard reduction potential means ____
Better electron ACCEPTOR
Each complex of the ETC contains _____
Fe
- coordinated because very reactive
- good in redox reactions
All prosthetic groups accept and pass _____ electron at a time except _______ which can _________
All prosthetic groups accept and pass 1 electron at a time except FMN which can accept 2 electrons (still passes 1)
4 Main Molecules in ETC
- Flavoproteins
- Cytochromes
- Iron-Sulfur proteins
- Protein bound copper
Prosthetic groups do the ___________
Chemistry
Flavoproteins
2
FMN and FAD
- tightly bound
FMN
Flavoproteins - Prosthetic group in Complex I
Need b/c NADH is carrying 2 e- and 1 H+ and can accept 2 electrons
Still only passes one electron, though
FAD
Flavoprotein- Prosthetic group on Complex II
Cytochromes (b,c,c1, a,a3)
Contain heme (Fe2+,Fe3+) prosthetic groups
Complexes III and IV
All heme prosthetic groups:
Absorb light around 415 nm
Just the substitutions on the protoporphyrin ring differ leading to different cytochromes
Iron Sulfur Proteins
Prosthetic groups in complexes I, II, III
- Has one or more Fe-S clusters
Fe2+ , Fe3+
cysteine groups
Protein bound copper
Prosthetic groups in complex IV only
Cu2+ and Cu+
Coenzyme Q / Ubiquinone important characteristics
- A lipid
- Mobile electron carrier
- Has an isoprenoid hydrophobic tail
Quinone functional group of CoQ
Undergoes “redox cycling” - can continuously cycle from oxidized to reduced state as radicals in the process
Therefore, CoQ is a source of O2 free radicals in the ETC
How can CoQ be a source of O2 free radicals?
Quinone functional group:
Accept 1 e- and 1 H+ —> form semi quinone intermediate. If this loses the H+ it forms the semi quinone radical which is bad
But, if followed by another acception of 1 e- and 1 H+ —> form reduced CoQ which is fine
Isoprene functional unit
- A 5-C repeating unit that produces a LONG carbon hydrophobic tail
- Allows molecule to move laterally in the membrane
- Made in biosynthesis pathway for cholesterol
Isoprene is made in the biosynthetic pathway for _______________.
Cholesterol
Patients on statin drugs (to reduce high cholesterol) often experience muscle weakness, why?
Statin drug inhibits cholesterol synthesis therefore not making isoprene units —> block availability of isoprene for CoQ
CoQ function
Accepts electrons from complex I and complex II —> transfer to complex III
Further down chain = ____________ electron acceptor
Why?
BETTER
More positive standard reduction potential
Complex I location
Part of the complex is embedded in the inner mitochondrial membrane
— each of the helices contributes to action of proton pump
The rest of the complex protruding into the matrix portion.
— FMN located here
Complex I functions as _____
How can it do this?
A proton pump with 3 channels
- As the electrons move down the prosthetic groups, protons are pumped from the matrix into the inter membrane space
- Can do this because the of the amino acid composition of the helices —> Charged amino acids can change their charge depending on whether they are binding to a proton or releasing it
In Complex I, _________ and _________ happen together.
Electron flow and proton pumping
Explain electron transfer in complex I
NADH donates 2 electrons to FMN —> FMN passes one electron at a time to various Fe-S centers —> electrons continue to move and as they move protons are pumped into the IMS space —> two additional protons from the matrix side reduce CoQ that is embedded in the meme brand
2 main things at play in complex I
- Exergonic nature of free energy change as electrons are passed
- Proton pumping - beginning of generation of proton gradient
Proton gradient contains _____ called ______
Purpose?
Energy. Called proton motive force
Powers ATP synthase
Complex I can be a source of ________ that are associated with
reaction oxygen species ; neurodegenerative diseases
Complex II structure & location & groups
- Includes Succinate dehydrogenase (part of TCA cycle)
- Has flavoprotein (FAD) and Fe-S center
- Located in inner mitochondrial membrane
What happens in complex II?
FAD accepts electrons from FADH2 —> prosthetic groups —> CoQ
Reaction is exergonic
CoQ gets electrons from ? And take them to?
Complexes I and II
Takes to Complex III
What is unique about complex II?
NO proton pumping!!
Therefore, does not contribute to proton motive force.
Do you get more protons from NADH or FADH2? Why?
NADH. Because complex II does not do proton pumping.
Complex III- prosthetic groups
- Cytochrome bc1 and Fe-S center
What happens at complex III?
CoQ passes its electrons to cytochrome c
Net 2 H+ pumped into IMS
Cytochrome c properties
- Small protein on outer side of inner membrane
- Water soluble
- Can move laterally within membrane
- Carries electrons to complex IV
Complex 4 prosthetic groups
- Cytochome a and a3
- Heme a3 & CuB form the active center that reduces oxygen
What happens at complex IV ***?
- Two molecules of cytochome c sequentially transfer electrons to reduce CuB and heme a3
- O2 enters and forms a peroxide bridge between the Fe and Cu
- 2 additional cytochrome c come and transfer 2 e-
- Cleave the peroxide bridge
- 2 H+ from matrix added to the oxygens —> Fe binds one, Cu binds the other, 1 proton attached to O in each case
- 2 more H+ from matrix
- Release of H2O
How many chemical protons are from pumped in matrix and what is their purpose?
Reduce O2 —> H2O
Complex IV is the site of _______poisoning
CN and CO
Totals of H+ at each complex
Complex I = 4
Complex II = 0
Complex III = 2
Complex IV = 4
_____ H+ per 2 e- being passed
10
Significance of respiratory inhibitors (2)
- Blocks the transfer of electrons (therefore ATP synthase activity)
- Allowed the order of complexes to be determined
Rotenone and Amytal
- Inhibit complex I
- Electrons don’t move to CoQ —> NADH builds up —> TCA cycle slows down because NADH is negative allosteric modifier of isocitrate dehydrogenase
Antimycin A
Inhibits complex III
Cyanide, Azide, Carbon Monoxide
- Inhibit Complex IV
- Prevents the Oxidation- reduction because if the iron becomes reduced it eventually has to be reset to be oxidized if it is going to accept more electron to keep the reaction going.
- Prevent the release of electrons to oxygen
Where do CN and N3 bind?
Fe3+ of heme a3
Where does CO bind?
Affects?
Fe2+ of heme a3
2 things: oxygen delivery and mitochondrial respiration and the ability to make ATP —> only survive by glycolysis
What is downfall of aerobic respiration?
Reactive oxygen species are made
3 types of Reactive Oxygen Species
- O2- superoxide
- H2O2 - hydrogen peroxide
- OH* - hydroxyl radical
O2- superoxide
Properties?
Forms when?
Sources?
- highly reactive
- limited lipid solubility in PM
- forms when O2 accepts a single electron
- Sources: Complex I (quinone structure), Complex II (Flavin group of FAD), CoQ
H202 hydrogen peroxide
Technically not a radical but can generate an OH radical by accepting an e- and 2 H+
OH* hydroxyl radical
Forms when?
Properties?
- Most reactive
- Formed when H2O2 accepts an e- and 2H+
- Lipid peroxides
- Particularly damaging to lipids in PM, esp. unsaturated fatty acids
ROS can be generated how?
Enzymatically and nonenzymatically
Non-enzymatic reactions
- Haber-Weiss
2. Fenton
Haber-Weiss reaction
O2- + H2O2 —> OH*
Fenton reaction
H2O2 in presence of Fe or Cu —> OH* + OH-
What are free radicals?
- An independent, free species
- Have unpaired electrons in orbital —> search for electrons and take from other species.
- This initiates chain propagating reaction where radical after radical is formed.
- Most serious when happens in PM
What is a radical?
NOT free
Doesn’t go anywhere but can be formed and enzyme will continue on with reaction
Non enzymatic mechanisms to protect against ROS (2)
- Vitamin E
2. Vitamin C
Vitamin E (alpha-tocopherol)
- Lipid soluble
- Stored in adipose tissue therefore can be damaging at high concentrations
- Protects against lipid peroxides / any PM type damage
Vitamin C (ascorbic acid)
- Water soluble
- Excreted in urine
- Needed for regenerating reduced/protective from of Vitamin E
Enzymatic mechanisms for protecting against ROS
- Superoxide dismutase
- Catalase
- Glutathione**
Catalase
- Converts hydrogen peroxide into water and oxygen
- Found in peroxisomes
Superoxide dismutase
Converts superoxide into hydrogen peroxide
- Found in mitochondria, cytosol, extracellular (kidney, liver, heart)
Glutathione protective function
GSH (reduced form) is more protective
Structure: Glu-Cys-Gly
SH group in Cys acts as nucleophile that can interact with electrophilic compounds. When SH group is oxidized —> form S-S bond with another glutathione molecule
** One of the most important mechanisms against ROS**
Enzyme: Glutathione perioxidase
2 GSH (reduced) + peroxide —> GSSH (oxidized) + H2O + ROH
Enzyme: glutathione reductase (GSH reductase)
Takes GSSG and gives GSH
