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Bacterial Physiology (BIOL 471) > Ch. 5 > Flashcards

Ch. 5 Flashcards

1
Q

Define electron transport.

A

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

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2
Q

List and describe the two types of respiration.

A
  1. Aerobic
    - Oxygen is the final electron acceptor
  2. Anaerobic
    - Non-oxygen final electron acceptor (have lower reduction potential)
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3
Q

Electrons flow through a series of _____.

A

electron carriers

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4
Q

Electrons are transferred to carriers with _____ electrode potentials.

A

higher

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5
Q

List the 4 types of electron carriers.

A
  1. Flavoproteins (hydrogen and electron carriers)
  2. Quinones (hydrogen and electron carriers)
  3. Iron–sulfur proteins (electron carriers)
  4. Cytochromes (electron carriers)
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6
Q

Which of the electron carriers are lipids (the rest are proteins)?

A

Quinones

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7
Q

The protein electron carriers exist in multiprotein complexes called _____.

A

oxidoreductases

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8
Q

The nonprotein portion that carries the electron is called a _____.

A

prosthetic group

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9
Q

Can some prothetic groups also carry hydrogen (in addition to electrons)?

A

Yes

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10
Q

What is the prosthetic group of a flavoprotein (Fp) called?

A

Flavin

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11
Q

What are the flavins FAD and FMN synthesized from?

A

Riboflavin (Vitamin B2)

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12
Q

Why are quinones mobile in membranes?

A

Because of their hydrophobic lipid nature

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13
Q

How many isoprenoid units do quinones generally have?

A

6 to 10

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14
Q

Quinones carry _____ and _____.

A
  1. protons
  2. electrons
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15
Q

What 2 types of quinone do bacteria make?

A
  1. Ubiquinone (UQ)
  2. Menaquinone (MQ or MK)
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16
Q

Compare and contrast quinones and menaquinones.

A
  • 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
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17
Q

What do iron-sulfur proteins carry?

A

Electrons only

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18
Q

What type of iron and sulfur do iron-sulfur proteins contain?

A
  • Nonheme iron
  • Usually contain acid-labile sulfur
  • Also contain cysteine sulfur (not acid-labile)
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19
Q

What is the nonheme iron in Fe-S proteins attached to?

A
  • S-residue of cysteine
  • Acid-labile sulfur
20
Q

Cytochromes are electron carriers that have _____ as the prosthetic group.

A

heme

21
Q

What are the 5 classes of heme that distinguish cytochromes?

A
  1. heme a
  2. heme b
  3. heme c
  4. heme d
  5. heme o
22
Q

What wavelength of light do cytochromes absorb?

A

550 - 560 nm

23
Q

Which iron in heme is the electron carrier?

A

The iron in the middle

24
Q

How many electrons can a cytochrome carry?

A

1 electron

25
Q

How are the classes of cytochromes distinguished?

A

Distinguished based on side groups

26
Q

Which cytochrome classes are found only in prokaryotes?

A
  • d
  • o
27
Q

Give an overview of how electron transport is organized in bacteria.

A
  • 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)
28
Q

What are the four complexes of the electron transport chain in mitochondria (same fundamental pattern in bacteria)?

A
  • Complexes I and II: dehydrogenases (NADH-ubiquinone oxidoreducatase and succinate dehydrogenase)
  • Complex III: quinones (ubiquinol-cyt c oxidoreductase)
  • Complex IV: oxidase (cyt c oxidase)
29
Q

What do the following abbreviations mean?

  1. fp
  2. FeS
  3. UQ
  4. b and c
  5. aa3
A
  1. fp = flavoprotein
  2. FeS = iron-sulfur protein
  3. UQ = ubiquinone
  4. b and c = cyt b and cyt c
  5. aa3 = cyt aa3
30
Q

What do the E’0 values of the components of the ETC in mitochondria indicate?

A
  • 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
31
Q

Compare and contrast electron transport in bacteria and mitochondria.

A
  • 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
32
Q

What are coupling sites?

A

Locations in ETC where redox reactions are coupled to proton extrusion and creation of ∆p

33
Q

How many coupling sites are in mitochondria?

A

3 (complexes I, III, and IV)

34
Q

What is the consensus on the ratio of protons translocated to electrons in mitochondria?

A

10 protons for every 2 electrons donated by NADH

35
Q

What are the two types of mechanisms for producing proton potentials?

A
  1. Scalar translocation (Q loop/cycle)
  2. Vectorial translocation (proton pump)
36
Q

What is the Q cycle?

A
  • 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
37
Q

Diagram and explain the Q cycle.

A
  1. Ubiquinol (QH2) attaches to complex III –> 2 electrons split into separate paths
  2. One electron goes to the Rieske center –> cyt c1 –> cyt c
  3. cyt c gets reduced and detaches from complex III
  4. Other electron goes to cyt b –> ubiquinone (Q), forming a semi quinone radical ion
  5. Another ubiquinol attaches to complex III and steps 1-3 repeat
  6. 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
38
Q

Explain how proton pumps produce proton potentials.

A
  • Complexes I and IV translocate protons
  • Protons are expelled by conformational changes in the proteins
39
Q

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?

A
  1. NDH-1, cyt bo, cyt bd
  2. 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)
  3. cyt bo: 2 protons per electron
    - cyt bd: 1 proton per electron
40
Q

What NADH dehydrogenases does E. coli encode under aerobic conditions?

A
  1. NDH-1
    - Proton pump
    - Similar to complex I in mitochondria
    - Used during fumarate respiration
  2. NDH-2
    - Not a proton pump
    - Used during aerobic and nitrate respiration
41
Q

Identify the coupling sites in the E. coli electron transport chain under anaerobic conditions.

A
  1. Nitrate is electron acceptor
    - NDH-1 (vectorial)
    - Quinol (scalar)
  2. Fumarate is electron acceptor
    - NDH-1 only
42
Q

What does E. coli use as an electron acceptor in the absence of oxygen?

A

Nitrate or fumarate

43
Q

What is Paracoccus denitrificans? Explain how denitrification is involved in its ETC.

A

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)

44
Q

Explain the aerobic pathway in Paracoccus denitrificans.

A
  • 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
45
Q

Explain the anaerobic pathway in Paracoccus denitrificans.

A
  • 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

Bacterial Physiology (BIOL 471) (15 decks)

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