Ch. 5 Flashcards
Define electron transport.
The current of electrons in the cell membranes of prokaryotes, and in mitochondrial and chloroplast membranes
- Electrons flow spontaneously down an energy gradient through a series of electron carriers
List and describe the two types of respiration.
- Aerobic
- Oxygen is the final electron acceptor - Anaerobic
- Non-oxygen final electron acceptor (have lower reduction potential)
Electrons flow through a series of _____.
electron carriers
Electrons are transferred to carriers with _____ electrode potentials.
higher
List the 4 types of electron carriers.
- Flavoproteins (hydrogen and electron carriers)
- Quinones (hydrogen and electron carriers)
- Iron–sulfur proteins (electron carriers)
- Cytochromes (electron carriers)
Which of the electron carriers are lipids (the rest are proteins)?
Quinones
The protein electron carriers exist in multiprotein complexes called _____.
oxidoreductases
The nonprotein portion that carries the electron is called a _____.
prosthetic group
Can some prothetic groups also carry hydrogen (in addition to electrons)?
Yes
What is the prosthetic group of a flavoprotein (Fp) called?
Flavin
What are the flavins FAD and FMN synthesized from?
Riboflavin (Vitamin B2)
Why are quinones mobile in membranes?
Because of their hydrophobic lipid nature
How many isoprenoid units do quinones generally have?
6 to 10
Quinones carry _____ and _____.
- protons
- electrons
What 2 types of quinone do bacteria make?
- Ubiquinone (UQ)
- Menaquinone (MQ or MK)
Compare and contrast quinones and menaquinones.
- Menaquinones are naphthoquinones (additional benzene ring replaces the two methoxy groups present in ubiquinones)
- Menaquinones have a much lower electrode potential than ubiquinones and are used predominantly during anaerobic respiration
What do iron-sulfur proteins carry?
Electrons only
What type of iron and sulfur do iron-sulfur proteins contain?
- Nonheme iron
- Usually contain acid-labile sulfur
- Also contain cysteine sulfur (not acid-labile)
What is the nonheme iron in Fe-S proteins attached to?
- S-residue of cysteine
- Acid-labile sulfur
Cytochromes are electron carriers that have _____ as the prosthetic group.
heme
What are the 5 classes of heme that distinguish cytochromes?
- heme a
- heme b
- heme c
- heme d
- heme o
What wavelength of light do cytochromes absorb?
550 - 560 nm
Which iron in heme is the electron carrier?
The iron in the middle
How many electrons can a cytochrome carry?
1 electron
How are the classes of cytochromes distinguished?
Distinguished based on side groups
Which cytochrome classes are found only in prokaryotes?
- d
- o
Give an overview of how electron transport is organized in bacteria.
- Electron transport chain that transfers electrons from electron donors at a low electrode potential to electron acceptors at a higher electrode potential
- Electrons can enter at the
level of flavoprotein, quinone, or cytochrome (depends on reduction potential of donor)
What are the four complexes of the electron transport chain in mitochondria (same fundamental pattern in bacteria)?
- Complexes I and II: dehydrogenases (NADH-ubiquinone oxidoreducatase and succinate dehydrogenase)
- Complex III: quinones (ubiquinol-cyt c oxidoreductase)
- Complex IV: oxidase (cyt c oxidase)
What do the following abbreviations mean?
- fp
- FeS
- UQ
- b and c
- aa3
- fp = flavoprotein
- FeS = iron-sulfur protein
- UQ = ubiquinone
- b and c = cyt b and cyt c
- aa3 = cyt aa3
What do the E’0 values of the components of the ETC in mitochondria indicate?
- Reduction potentials increase from left to right
- Relates to organization of ETC because it determines the flow of electrons
- Donor must have a lower potential than the acceptor it is donating to
Compare and contrast electron transport in bacteria and mitochondria.
- Routes to oxygen in the bacteria are branched
- Bacteria can alter their ETCs depending on growth conditions
- Both ETCs are organized into dehydrogenase and oxidase complexes connected by quinones
- Bacteria are capable of using non-oxygen electron acceptors during anaerobic respiration (reductases)
- Dehydrogenases may be analogous to NADH dehydrogenase or be variable
What are coupling sites?
Locations in ETC where redox reactions are coupled to proton extrusion and creation of ∆p
How many coupling sites are in mitochondria?
3 (complexes I, III, and IV)
What is the consensus on the ratio of protons translocated to electrons in mitochondria?
10 protons for every 2 electrons donated by NADH
What are the two types of mechanisms for producing proton potentials?
- Scalar translocation (Q loop/cycle)
- Vectorial translocation (proton pump)
What is the Q cycle?
- Quinones are reduced on the inner membrane surface and carry hydrogen across the membrane and become oxidized –> release protons outside the membrane
- Electrons return electrogenically via electron transport carriers to the
inner membrane surface creating a membrane potential
Diagram and explain the Q cycle.
- Ubiquinol (QH2) attaches to complex III –> 2 electrons split into separate paths
- One electron goes to the Rieske center –> cyt c1 –> cyt c
- cyt c gets reduced and detaches from complex III
- Other electron goes to cyt b –> ubiquinone (Q), forming a semi quinone radical ion
- Another ubiquinol attaches to complex III and steps 1-3 repeat
- Other electron goes to cyt b –> semi quinone radical ion, which then picks up two hydrogen ions and forms ubiquinol
- In one turn of the Q cycle, a total of 4 hydrogens are translocated and 2 cytochrome c molecules are reduced
Explain how proton pumps produce proton potentials.
- Complexes I and IV translocate protons
- Protons are expelled by conformational changes in the proteins
Identify the coupling sites in the E. coli electron transport chain under aerobic conditions and compare the two terminal oxidases. How many protons are extruded in each pathway?
- NDH-1, cyt bo, cyt bd
- cyt bo vs. cyt bd
- cyt bo: proton pump (vectorial extrusion); predominant oxidase when oxygen levels are high
- cyt bd: not a proton pump; higher affinity for oxygen (E. coli makes more when oxygen levels are low) - cyt bo: 2 protons per electron
- cyt bd: 1 proton per electron
What NADH dehydrogenases does E. coli encode under aerobic conditions?
- NDH-1
- Proton pump
- Similar to complex I in mitochondria
- Used during fumarate respiration - NDH-2
- Not a proton pump
- Used during aerobic and nitrate respiration
Identify the coupling sites in the E. coli electron transport chain under anaerobic conditions.
- Nitrate is electron acceptor
- NDH-1 (vectorial)
- Quinol (scalar) - Fumarate is electron acceptor
- NDH-1 only
What does E. coli use as an electron acceptor in the absence of oxygen?
Nitrate or fumarate
What is Paracoccus denitrificans? Explain how denitrification is involved in its ETC.
A Gram-negative non-fermenting facultative anaerobe
1.Reduced coenzyme Q provides electrons for nitrate reductase (NaR)
2. NaR reduces NO3 coupled to the oxidation of quinol
3. cyt c donates electrons to nitrite reductase (NiR) to reduce NO2 to nitric oxide
(NO)
4. cyt c donates electrons to nitric oxide reductase (NOR) and nitrous oxide reductase (N2OR)
Explain the aerobic pathway in Paracoccus denitrificans.
- Closely resembles mitochondria
- 2 other terminal oxidases: cyt cbb3 and cyt bb3
- As many as 3 coupling sites from NDH-1 to O2
- As few as 2 coupling sites
Explain the anaerobic pathway in Paracoccus denitrificans.
- Denitrification: reducing nitrate (electron acceptor) to nitrogen gas
- The synthesis and activity of denitrifying enzymes are inhibited by oxygen
- A total of 5 electrons flow in and out of the cell membrane through the electron carriers from UQ to various reductases
