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Bacterial respiration and fermentation Flashcards

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1
Q

what is glycolysis?

A
  • Series of reactions that extract energy from glucose by splitting it into 2x 3-carbon pyruvates
  • Occurs in the cytoplasm
  • Doesn’t require oxygen
  • Generates ATP by substrate-level phosphorylation
  • 2 phases: upper/energy-requiring phase, lower/energy-releasing phase
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2
Q

what is the process of glycolysis?

A

upper phase:
- glucose phosphorylated twice by hexokinase to form glucose-6 phosphate, and PFK to form fructose-1,6 phosphate
- fructose-1,6-phosphate is unstable and forms 2x G3P
- uses 2ATP

lower phase:
- 2x G3P is oxidised and phosphorylated and eventually forms 2x pyruvate
- 2NAD is reduced to 2NADH + 2H+
- 2 dephosphorylation steps result in 2Pi donated to 2ADP to form 2ATP (this occurs twice so overall 4ATP made)

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3
Q

what are the key products of glycolysis?

A

upper phase: 2ATP used
lower phase: 4ATP and 2NADH produced

overall yield per mol glucose:
- net 2ATP
- 2NADH
- 2x pyruvate

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4
Q

what is the link reaction?

A
  • occurs in the mitochondrial matrix
  • 2x pyruvate (3C) is oxidised and decarboxylated by pyruvate dehydrogenase into 2x acetyl CoA (2C) and 2x CO2 (1C)
  • one mol NADH formed per pyruvate, so 2NADH per glucose
  • no ATP produced
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5
Q

what is the krebs cycle?

A
  • occurs in mitochondrial matrix
  • one turn of cycle produces 3x NADH, 1x FADH2 and 1x ATP
  • cycle goes around twice for each mol of glycose as there are 2x pyruvate and thus 2x acetyl CoA
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6
Q

what is the process of the Krebs cycle?

A
  1. acetyl CoA combines with oxaloacetate to form citrate by citrate synthase
  2. citrate converted to alpha-ketoglutarate, releasing molecule of CO2 and reducing NAD to NADH
  3. a-ketoglutarate oxidised by a-ketoglutarate dehydrogenase, reducing NAD to NADH and releasing CO2, forming succinyl CoA
  4. phosphate released from succinyl CoA to ADP to form ATP and produce succinate
  5. succinate oxidised to fumarate by SDH, and 2H+ and 2e- are transferred to FAD to form FADH2
  6. fumarate is hydrated to malate, and malate oxidised to reform oxaloacetate, and another NAD is reduced to NADH in this process
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7
Q

what are the key products of the Krebs cycle?

A

yield per mol of glucose (2 turns of Krebs):
- 2x ATP
- 6x NADH
- 2x FADH2
- 4x CO2

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8
Q

what are the key products of glycolysis, link reaction and Krebs cycle?

A
  • 4x ATP -> stored as energy
  • 10x NADH -> to ETC
  • 2x FADH2 -> to ETC
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9
Q

what is oxidative phosphorylation?

A
  • made up of two components: ETC and chemiosmosis
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10
Q

what is the electron transport chain?

A
  • collection of membrane-embedded proteins and organic molecules organised into 4 large complexes (I-IV)
  • as electrons travel through the chain, they go from a higher to a lower energy level
  • complexes use the energy released to pump protons from mitochondrial matrix into the intermembrane space to form a proton gradient for ATP synthesis
  • complexes are found in inner membrane of mitochondria
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11
Q

where do electrons come from in the ETC?

A

come from NADH and FADH2:
- NADH donates electrons in redox reactions, and transfers its electrons directly to complex I (NADH dehydrogenase)
- complex I pumps protons across the membrane from the energy released
- NADH becomes oxidised back to NAD+
- FADH2 is less efficient at donating electrons than NADH, so it transfers them to complex II (succinate dehydrogenase) which does not pump protons across the membrane

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12
Q

what is the step-by-step process of the ETC?

A
  1. complex I (NADH dehydrogenase) relieves NADH of 2H+ and 2e- to convert back to NAD+.
    - energy received is used by complex I to pump 4H+ from mitochondrial matrix to the intermembrane space
    - e- is passed to coenzyme Q which transfers e- to complex III
  2. complex II (succinate dehydrogenase) oxidises succinate to fumarate (Krebs cycle) to produce FADH2
    - FADH2 is oxidised to FAD+ and donates 2e- and to CoQ which transfers them to complex III
  3. CoQ is ubiquinone (Q) which accepts 2e- and 2H+ each from complex I/complex II to form the reduced ubiquinol (QH2)
  4. complex III (cytochrome reductase) accepts 2e- from ubiquinol and transports them to cytochrome C
    - ubiquinol is oxidised back to ubiquinone
    - cytochrome C transports electrons to complex IV
    - pumps 2H+ from ubiquinol to intermembrane space
  5. complex IV (cytochrome C oxidase) oxidises cytochrome C and receives its electrons
    - pumps 4H+ to intermembrane space
    - the electrons are used to reduce oxygen to water
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13
Q

what are cytochromes?

A
  • a group of proteins with heme prosthetic groups
  • they contain an iron core in which the iron can be oxidised (Fe3+/ferric) or reduced (Fe2+/ferrous)
  • cytochrome C is water soluble
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14
Q

what is the Q cycle?

A
  • 2 ubiquinols (QH2) are oxidised into ubiquinones (Q), releasing 4H+
  • 1 Q is reduced to QH2 (recycling step)
  • 2 cytochrome C molecules are reduced
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15
Q

how is ATP produced in chemiosmosis?

A
  • ATP uses the electrochemical gradient of H+ from intermembrane space to mitochondrial matrix (PMF)
  • energy from the flow of H+ is used to phosphorylate ADP to ATP
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16
Q

what are the key products from oxidative phosphorylation?

A
  • 10H+ are translocated across membrane per NADH oxidised (H+/e- ratio of 5)
  • 6H+ are translocated across membrane per FADH2 oxidised (H+/e- ratio of 3)
  • costs 2.7 H+/ATP synthesised, so can produce 3.7ATP per NADH, and 2.2 ATP per FADH2
  • cost of actively transporting NADH, pyruvate, ADP and Pi increases H+/ATP ratio to >4

therefore, ATP yield per mol of glucose is ~30, including glycolysis and Krebs

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17
Q

what type of metabolism does E. coli display?

A

E. coli is a facultative anaerobe:
- it experiences many different environmental conditions so needs to adapt
- if oxygen is present -> aerobic respiration (max potential to conserve energy)
- if oxygen is absent but alternative electron acceptors available = anaerobic respiration
- if oxygen and electron acceptors are absent = fermentation

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18
Q

where does E. coli perform glycolysis and Krebs?

A

they occur in the same way as eukaryotes, but both processes occur in cytoplasm

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19
Q

where is the ETC localised in E. coli?

A
  • localised in the inner cytoplasmic membrane
  • inner membrane is where the PMF/proton gradient is set up
  • protons are moved from the cytoplasm (n-) to the periplasm (p+)
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20
Q

what are the main components of the E. coli ETC?

A
  • NADH and FADH2 donate electrons to the electron donor complexes
  • 2x NADH dehydrogenases (complex I) called Nuo and Ndh
  • 1x succinate dehydrogenase (complex II) called SDH
  • no equivalent complex III/cytochrome reductase
  • 2x terminal quinol oxidases called Cyo and Cyd which directly oxidise quinols to quinones (instead of complex IV/cytochrome C)
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21
Q

what is the role of the dehydrogenase electron donor complexes in the ETC of E.coli?

A
  • Electrons are donated from NADH or FADH2 via dehydrogenase enzymes and reduce quinones to quinols much like the mitochondria in the ETC
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22
Q

what is the structure and function of Nuo?

A
  • 13-14 subunits (NuoA-N)
  • ~550 kDa, 64 TM helices
  • 1 FMN cofactor (where electrons from NADH enter) and 9 Fe-S clusters which allow electrons to transfer to site of quinone reduction
  • Large membrane domain with 4 proton channels
  • 4 Proton pumped from N side to P side
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23
Q

what is the structure and function of Ndh?

A
  • Single subunit (Ndh)
  • ~45 kDa, monotopic membrane-associated (interacts with only the cytoplasmic face)
  • 1 FAD
  • Oxidises NADH and converts quinone to quinol
  • Not a proton pump as it does not extend into periplasm
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24
Q

what is the structure and function of SDH?

A
  • 4 subunits (SdhABCD) – 2 form 6 TM helix region and 4 extend into the cytoplasm
  • ~150 kDa
  • 1 FAD, 3 Fe-S, 1 heme b
  • Succinate oxidised to fumarate which produces protons (Krebs cycle)
  • Protons and electrons are used to reduce quinones to quinols
  • Not a proton pump
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25
Q

what are the terminal oxidases in E. coli?

A
  • E. coli lacks cytochrome reductase (complex III) and a cytochrome c oxidase (complex IV)
  • Instead it has two different respiratory terminal oxidases, Cyo and Cyd, which directly oxidise the quinols produced by the NADH-dehydrogenases and SDH
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26
Q

what is the difference between Cyo and Cyd?

A

they have different H+/e- ratios:
- Cyd conserves less energy but allows oxygen reduction at low oxygen tensions and is more resistant to some toxic compounds encountered in the host

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27
Q

what is the structure and function of Cyo?

A
  • Cytochrome bo3 heme-copper oxidase (heme b, heme o3 and Cu centre), four subunits (CyoABCD)
  • Accepts electrons directly from quinol
  • Releases two H+ from quinol oxidation to the p-side
  • Also pumps 2H+ from n-side to p-side - higher H+/e- ratio than Cyd
  • overall pumps 4H+
  • Lower affinity for oxygen – works under hyperoxic conditions
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28
Q

what is the structure and function of Cyd?

A
  • Cytochrome bd oxidase - contains 3 hemes (two b hemes and one heme d), 4 subunits (CydABHX)
  • Releases two H+ from quinol oxidation to the p-side
  • Does not pump protons – lower H+/e- ratio than Cyo
  • High affinity for oxygen – works under microoxic conditions
  • More resistant to sulphide, hydrogen peroxide, nitric oxide which are found in the gut
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29
Q

which combination of electron donor and electron acceptor gives the highest H+/e- ratio?

A

Nuo and Cyo:
- when oxidising NADH by Nuo and converting quinol back to quinone by Cyo, 8H+ are translocated across membrane per NADH oxidised
- produces H+/e- ratio of 4 and largest PMF
- costs 3.33H+/ATP synthesised (10 protons per 3ATP in E. coli)

therefore can produce 2.4ATP per NADH oxidised via Nuo and Cyo

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30
Q

which electron donor and electron acceptor combination is the major pathway under high oxygen conditions?

A

E. coli typically uses Ndh and Cyo, even though it generates less PMF:
- 4H+ are translocated across the membrane per NADH oxidised
- H+/e- ratio of 2
- only 4 protons are moved per 2 electrons, so half the PMF produced as Nuo+Cyo
- costs 3.33H+/ATP synthesised (10 protons per 3ATP)

therefore can produce 1.2ATP per NADH oxidised via Ndh and Cyo

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31
Q

why does E. coli preferentially use Ndh under aerobic conditions?

A
  • differing H+/e- ratios allow bacteria to optimise efficiency of energy generation under varying conditions to adapt
  • Ndh has a higher turnover number and favours increased metabolic flux over maximal energy efficiency of respiratory chain

Nuo is used under microaerobic (low oxygen) conditions with Cyd, or in absence of oxygen entirely with alternative electron acceptors

32
Q

which E. coli ETC complexes are good targets for antimicrobials?

A

Ndh and/or Cyd are present in many pathogenic bacteria (e.g. salmonella, mycobacterium tuberulosis) and protozoa (toxoplasma gondii)
- they are important during host infection
- they are not present in mammalian mitochondria so human cells are not affected

33
Q

what is the ATP yield of aerobic respiration in E. coli?

A

20ATP per molecule of glucose under vigorous aeration

34
Q

what are the main pairings of the electron donor and acceptor complexes?

A

Ndh and Cyo work together under high aeration

Nuo works with Cyd under low oxygen tension

35
Q

what is the definition of anaerobic respiration?

A

the use of a membrane-embedded ETC to generate PMF, but with a terminal electron acceptor other than oxygen

  • ATP is still produced by oxidative phosphorylation via PMF, but the ETC uses an exogenous terminal acceptor other than oxygen
36
Q

what are examples of inorganic alternative electron acceptors used by E. coli?

A

nitrate (nitrate/nitrite: midpoint potential (Em) = +420mV)

nitrite (nitrite/ammonia: Em = +360mV)

37
Q

what are examples of organic alternative electron acceptors used by E. coli?

A
  • Fumarate (fumarate/succinate: Em = + 30 mV)
  • Trimethylamine N-oxide (TMAO/TMA: Em = +130 mV) -> Osmolyte used in marine organisms
  • Dimethyl sulfoxide (DMSO/DMS: Em = +160 mV)
38
Q

in what direction does redox energy travel in the ETC? what does this mean is required from electron donors/acceptors?

A

energy moves from negative to positive, therefore:
- good electron donors have a more negative midpoint potential
- good electron acceptors have a more positive midpoint potential

39
Q

what is the midpoint potential of the main donors/acceptors in mitochondria?

A

donor: NADH is at -340mV
acceptor: oxygen is at +820mV

40
Q

what are the possible donors and acceptors in E. coli?

A
  • formate and hydrogen are better donors than NADH as they have midpoint potentials of -430mV and -420mV respectively (more negative)
  • succinate and fumarate couple has little energy difference with quinone so provides less PMF (+30mV to +110mV) - not good donor
  • fumarate (+30mV) can also be an acceptor is menaquinone (-75mV) is used
  • nitrate is more favourable for an acceptor as it has the most positive midpoint potential (+420V), second to oxygen
41
Q

what are the donors, quinones and acceptors used under aerobic conditions in E. coli?

A
  • NADH is donor
  • Ubiquinone is reduced to ubiquinol and re-oxidised to ubiquinone
  • Oxygen is electron acceptor
42
Q

what are the donors, quinones and acceptors used under anaerobic conditions in E. coli?

A
  • can use any electron donors e.g. formate, hydrogen, NADH, lactate, G3P
    menaquinone (MK) has a more negative midpoint potential so is a better electon donor and a worse electron acceptor
  • this allows energy difference to be in favourable direction from donor to quinone to acceptor
  • acceptors may be fumarate, nitrate, nitrite
43
Q

which quinone does E. coli use under aerobic condtions?

A

benzoquinone:
- midpoint potential (Em) = +110mV
- ubiquinone (UQ) is reduced by 2e- and 2H+ to ubiquinol (UQH2)

44
Q

which quinone does E. coli use under anaerobic conditions?

A

Naphthoquinone:
- Em = -75mV
- menaquinone (MK) can be reduced by 2e- and 2H+ to menaquinol (MKH2)

45
Q

how is the Krebs cycle amphibolic?

A

it provides substrates for biosynthesis by the sequential oxidation of acetyl CoA, fomring:
- aspartate and other amino acids
- purines and pyrimidines
- fatty acids
- glutamate
- hemes

46
Q

what is the process of the link reaction under anaerobic conditions in E. coli?

A
  • in absence of oxygen, pyruvate dehydrogenase (PDH) is inhibited
  • pyruvate is converted to formate and acetyl CoA by enzyme Pyruvate Formate Lyase (PFL)
  • acetyl CoA is converted to acetate by phosphotransacetylase and acetate kinase, generating ATP via substrate-level phosphorylation
  • acetate is excreted
  • formate acts as electron donor to ETC via formate dehydrogenase (FDH)
47
Q

how does the link reaction switch from anaerobic to aerobic respiration?

A

PFL is inhibited by oxygen
- there is now a switch back to pyruvate decarboxylation by PDH under aerobic conditions

48
Q

what is the process of the Krebs cycle under anaerobic conditions?

A

forms oxidative and reductive branches:

  1. reductive branch produces succinate
    - PEP carboxykinase forms oxaloacetate from pyruvate
    - oxaloacetate is sequentially reduced to malate, then fumarate by DH enzymes
    - fumarate is used by ETC as terminal electron acceptor
    - fumarate reductase converts fumarate to succinate to form FAD
  2. oxidative branch forms alpha-ketoglutarate
    - oxaloacetate and acetyl CoA combine to form citrate
    - citrate forms isocitrate which is oxidised to form a-ketoglutarate
    - NADH is formed
    - a-ketoglutarate dehydrogenase is inhibited to cycle cannot conttinue
49
Q

what products from the glycolysis and and anaerobic Link can act as electron donors in ETC?

A
  • NADH from glycolysis
  • formate from PFL in Link reaction

only if suitable electron acceptor is present

50
Q

what is the process of anaerobic respiration in E. coli with fumarate as electron acceptor?

A
  • Nuo is favoured under anaerobic conditions to pump protons
  • fumarate reductase (Frd) is a multi-subunit enzyme with flavin cofactor and 3FeS centres, and reduces fumarate to succinate
  • no redox loop: protons liberated upon quinol oxidation are released back into cytoplasm

Nuo coupled to Frd = H+/e- of 2
1.2ATP produced per NADH oxidised

51
Q

what is formate-dependent nitrate reduction in E. coli anaerobic respiration?

A

formate dehydrogenase (FdnGHI) is a multi-subunit enzyme with 2 MO[MGD] cofactors, 5 Fe-S clusters and 2 heme-b groups:
- it is periplasmic facing
- FdnGHI converts formate to CO2 and 2H+
- electrons released reduce MK to MKH2 and 2H+s are released to periplasm

Nitrate reductase (NarGHI) is multi-subunit enzyme containing two Mo[MGD] cofactors, five [Fe-S] clusters and 2 heme b groups:
- membrane-anchored and faces cytoplasm
- when MKH2 is oxidised back to MK, redox loop is formed and 2H+ are pumped to periplasm

52
Q

what is the H+/e- ratio and amount of ATP produced under formate-dependent nitrate reduction (Fdn and Nar)?

A

Fdn coupled to Nar = H+/e- of 2
1.2ATP formed per formate oxidised

53
Q

what is the alternative nitrate reductase to Nar?

A

NapABC is a multi-subunit enzyme containing two Mo[MGD] cofactors, an [Fe-S] cluster and 6 c-type cytochromes:
- its catalytic domains are in the periplasm, so protons moved by the redox loop are used to reduce nitrate to nitrite
- Nap has a higher affinity for nitrate than Nar, so can use nitrate under low concs
- protons are consumed upon nitrate reduction in the periplasm

54
Q

what is the H+/e- ratio and no. ATP produced when Fdn is coupled to Nap (formate-dependent nitrate reduction)?

A

Fdn coupled to Nap = H+/e- of 1
0.6ATP per formate oxidised

55
Q

what determines the bioenergetics of the ETC?

A
  • the donor/acceptor couples and their redox potentials
  • the organisation of enzymes in the membrane can alter PMF generation

gives organism flexibility

56
Q

what is the ETC of Paracoccus denitrificans?

A

P. denitrificans is a metabolically versatile gram-negative soil bacterium which can grow under aerobic and anaerobic conditions
- contains SDH, NADH dehydrogenase and soluble cytochrome oxidases
- under aerobic conditions, similar to mitochondrial ETC
- has 2 terminal oxidases
- can use carbon compounds as electron donors
- denitrification (nitrate, nitrite, nitric oxide, nitrous oxide, nitrogen)
- can use hydrogen peroxide as electron acceptor

57
Q

what is the ETC of Helicobacter pylori?

A

H. pylori is a gram-negative, microaerophilic, helical bacterium found in human stomach (causes gastric ulcers):
- uses MK and MKH2 for quinone pool
- has 1 high affinity oxidase
- fumarate and hydrogen peroxide are alternative electron acceptors
- cytochrome bc1 complex oxidises MKH2 to MK
- lacks SDH (Frd may act in reverse)
- flavodoxin acts as electron donor via complex 1
- hydrogenase is essential for pathogenesis

58
Q

what is the ETC in Campylobactor jejuni?

A

C. jejuni is a gram-negative, microaerophile, spiral shaped and motile bacterium, and causes food-borne bacterial gastroenteritis:
- highly branched set of respiratory chains
- wide range of electron donors and alternative electron acceptors in addition to oxygen

59
Q

what is the definition of microbial fermentations?

A

fermentations use endogenous organic molecules as electron acceptors in the absence of oxygen and in the absence of a respiratory ETC
- there is no chemiosmosis of ATP via ATP-synthase
- ATP production is limited to substrate-level phosphorylation in cytoplasm
- pyruvate acts as electron acceptor
- glycolysis is an incomplete catabolic pathway, and NADH generated must be reoxidised in order to regenerate NAD+ and restore redox balance to keep glycolysis/ATP production going

60
Q

why must NAD+ be restored in fermentation?

A

glycolysis is an incomplete catabolic pathway, and NADH generated must be reoxidised in order to regenerate NAD+ and restore redox balance to keep glycolysis/ATP production going
- in absence of ETC, NADH is reoxidised to form fermentation products that are more reduced than the organic starting substrate
- can produce byproducts such as CO2, hydrogen and water

61
Q

why is less biomass produced in fermentation compared to aerobic respiration?

A
  • most carbon substrate is routed to fermentation products which are ultimately excreted as waste products
  • 5x less biomass is produced compared to aerobic
62
Q

what are the ATP yields of fermentation?

A

only 1-3ATP per mol of substrate e.g. glucose
- fermentation is the least favoured way to produce ATP

63
Q

what is the process of fermentation in eukaryotic muscle tissue?

A

in absence of oxygen:
- 2x pyruvate remains in cytoplasm and is reduced to 2x lactate by lactate dehydrogenase
- this uses 2NADH as electron donor, so 2NADH is oxidised to 2NAD+
- this regenerates NAD+ and restores redox balance to allow glycolysis to continue making ATP by substrate-level phosphorylation

overall yield per mol of glucose = 2ATP

64
Q

what is the process of homolactic fermentation in bacteria?

A
  1. pyruvate s reduced to lactate by LDH, using NADH as electron donor to regenerate NAD+ and restore redox balance
  2. glycolysis can continue to make ATP via substrate-level

overall yield per mol glucose = 2ATP

65
Q

which bacteria use homolactic fermentation?

A

carried out by gram-positive lactic acid bacteria e.g. species of Lactobacillus
- they are acid-tolerant, typically anaerobes
- lack cytochromes
- do not respire

66
Q

what is key about fermentation products and the starting substrate?

A

Fermentation products have the same redox level as the starting substrate
- Glucose is not fully oxidised, only forms pyruvate which is then reduced

67
Q

what is the process of heterolactic fermentation?

A
  • Catabolise sugars by pentose phosphoketolase pathway
  • Generates pentose sugar that is cleaved into G-3-P & Acetyl-P, by phosphoketolase.
  • G3P is metabolised to pyruvate and then lactate by LDH (restores NAD+)
  • Acetyl-P is converted to Acetyl-CoA and then ethanol (restores NAD+)
  • Overall yield per mol. glucose is 1 ATP

this is the catabolism of glucose by the pentose phosphoketolase pathway:
1. glucose is converted to ribulose-5-phosphate and 1 carbon is lost via CO2 release
- this uses 1ATP
2. Phosphoketolase splits R5P isomer xylulose-5-phosphate to G3P (3C) and acetyl-P (2C)
3. G3P is metabolised to pyruvate and then lactate by LDH
- 1NAD+ is used and 1NAD+ is produced
- 2ATP produced
4. acetyl-P is reduced to ethanol via acetyl CoA and acetaldehyde intermediates
- 2NAD+ is produced

overall yield per mol glucose = 1ATP

68
Q

what is the process of alcoholic fermentation?

A
  1. glucose is converted to 2x pyruvate, producing 2ATP and using 2NAD+
  2. 2x pyruvate is decarboxylated by pyruvate decarboxylase to 2x acetaldehyde, releasing 2CO2
  3. 2x acetaldehyde is reduced to 2x ethanol via alcohol dehydrogenase
    - NADH is used as electron donor to restore NAD+ and redox balance

overall yield per mol glucose = 2ATP

69
Q

which organisms carry out alcoholic fermentation?

A

*Carried out by yeast (e.g. Saccharomyces cerevisiae) and some bacteria (e.g. Zymomonas mobilis, but here pyruvate is generated by ED glycolytic pathway which has a net yield of only 1 ATP per glucose)

70
Q

what is the process of mixed acid fermentation?

A
  1. 2x pyruvate formed from glycolysis, using 2NAD+ and producing 2ATP
  2. PFL splits pyruvate to 2x acetyl CoA and formate
  3. 1 acetyl CoA is converted to acetate which is excreted, forming 1ATP
  4. the other acetyl CoA is converted to ethanol via alcohol dehydrogenase, oxidising 2NADH to restore 2NAD+
  5. formate-hydrogen-lyase (FHL) converts formate to hydrogen and CO2 which diffuse from cell
  6. pyruvate is reduced by lactate dehydrogenase to lactate, with NADH donating an electron (NAD+ restored)
  7. precursor PEP of pyruvate is converted to oxaloacetate which is reduced to succinate by electrons from NADH
  8. fumarate acts as terminal electron acceptor, and 2NADH is oxidised to 2NAD+ (small amount of anaerobic respiration)

overall yield per mol glucose = 2.3ATP

71
Q

what are the key products of mixed acid fermentation?

A

pyruvate is converted into a mixture of end products:
- ethanol
- formate
- acetate
- H2
- CO2
- lactate
- succinate
- 3ATP per mol of glucose

these can all be in varied amounts:
- end product ratio is not fixed, unlike the other fermentations

72
Q

why is there a variation of the amounts of end products produced in mixed acid fermentation? what is the approximate amount of ATP produced?

A

proportions of end products vary depending on the growth conditions to balance ATP production and redox balance
- product stoichiometry is therefore variable, not fixed, unlike the other fermentations

around 2.3ATP is produced

73
Q

key things to know for mixed acid fermentation

A
  • know the end products
  • know that the end product amounts vary based on the cell’s activity and environment, and whether it wants to make more ATP or balance redox
  • also know that formate uses formate hydrogen lyase (FHL) to be converted to H2 + CO2, not FDH as seen in anaerobic respiration
  • overall, this is a mixture of ethanol fermentation, lactate fermentation, formation of acetate (like anaerobic link reaction) and succinate from reductive branch of anaerobic Krebs
74
Q

what organisms use mixed acid fermentation?

A

carried out by gram-negative Enterobacteriaceae such as E. coli

75
Q

what is Acetone-Butanol-Ethanol (ABE) fermentation?

A
  • carried out by gram-positive Clostridium species
  • produces solvents A:B:E in ratio 3:6:1
  • originally conceived to produce acetone but now generates butanol as renewable biofuel
  • generates ATP and re-assimilates acetic and butyric acid products to generate acetone
  • re-generates acetyl CoA and butyryl-CoA which can be alternatively converted to ethanol and butanol to reform NAD+
76
Q

what is malolactic fermentation (MLF)?

A
  • a secondary fermentation in wine production, performed by lactic acid bacteria such as Oenococcus oeni
  • malic acid is decarboxylated to form lactic acid, which is excreted to increase pH and deacidify the wine
  • antiport of malate and lactic acid by MleP generates a membrane potential
  • decarboxylation of malate by MleA consumes a proton in the cytoplasm to contribute to the PMF
  • ATP synthesis is chemiosmotic, so no redox balancing is needed

this fermenation breaks the usual fermentation rules

Microbiology 2 (10 decks)

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