Lecture 4: Fermentation - Exam 4 Flashcards

1
Q

What is fermentation? Who does it?

A
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2
Q

Where do the reactions occur?

A
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3
Q

How is ATP made?

A
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4
Q

What are the e- donors and acceptors?

A
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5
Q

What is the problem with oxygen? How do aerobes deal with it? Why can’t anaerobes use it?

A

Oxygen is a strong oxidizing agent, making it an excellent electron donor. However, during the stepwise reduction of oxygen in respiration, toxic and highly reactive intermediates are produce:
-Reactive oxygen species (ROS)
-ROS attack iron dependent enzymes.
Aerobic bacteria have evolved systems designed to detoxify ROS
-Catalase
-Peroxidase
-Superoxide dismutase
Anaerobes do not possess these protective systems.
-Depending on aerotolerance, some anaerobes may have some degree of protection
-Microaerophilic bacteria typically possess superoxide dismutase.
-Strict anaerobes have no protection from ROS and cannot tolerate any oxygen.

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6
Q

What are the two types of anaerobic growth?

A

Anaerobic respiration (using nitrate, sulfate, fumarate as the terminal e- acceptor) and fermentation.

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7
Q

What is fermentation?

A

A pathway in which NADH (or other reduced e- carrier) is reoxidized by metabolites produced by the pathway.
-Redox reactions occur in the cytosol, not the membrane
-ATP is produced by substrate-level phosphorylation.
Fermentations are named after the major end products they generate.

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8
Q

What is anaerobic respiration?

A

Similar to aerobic respiration in that electrons extracted from a fuel molecules are passed through an electron transport chain, driving ATP synthesis.
-Sulfate (SO4), nitrate (NO3) carbon dioxide (CO2) or other molecules are terminal electron acceptor of ETC.

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9
Q

Archaea called methanogens use _________ as a terminal electron acceptor, producing _________ as a waste product.

A

Carbon dioxide or acetic acid ; methane
(Methanogens are found in soils, in the digestive systems or ruminants or termites and many other habitats)

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10
Q

With fermentation, what kind of electron acceptor is not involved?

A

NO exogenous terminal electron acceptor is involved

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11
Q

What is the only energy yielding step in fermentation?

A

Glycolysis, with one or two extra reactions tacked on at the end to recharge NAD+.
-Organic molecules serve as both electron donors and acceptors
-The molecule being metabolized is not completely oxidized

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12
Q

Fermentation is not unique to _______.

A

Prokaryotes

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13
Q

What are the benefits of fermentation?

A

Food products (beer, cheese, bread, sauerkrauat, kimchi, yogurt), bioalcohol, organic solvents

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14
Q

Fermentation vs Respiration

A

Fermentation:
-Occurs in cytosol
-Electron movement is not coupled to proton extrusion
-An organic molecules serves as electron donor and acceptor
-ATP generated via substrate-level phosphorylation
Respiration:
-Occurs in cell membranes
-Electron movement is coupled to proton extrusion to generate PMF
-ATP generated using PMF via ATP synthase

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15
Q

Fermentation:
NAD+ is reduced to NADH (glycolysis) then back to NAD+ when…?

A

It donates its electrons to electron sinks

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16
Q

What is a major concern with fermentation?

A

Reoxidation of the reduced NADH.
Reoxidation of NADH is vital. Lack of NAD+/NADH balance can result in stoppage of biological reactions due to lack of electron acceptors.

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17
Q

Fermentation does not involved exogenous electron acceptors (unlike respiration), so…?

A

The fermentation pathways themselves have to produce the electron acceptors that can be used to oxidize NADH.
-These electron acceptors are called electron sinks.

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18
Q

Fermentation: Why are the electron acceptors produced by the fermentation pathways called electron sinks?

A

Because they dispose of the electrons removed during the reoxidation of NADH by being excreted from the cell, into the medium.
-Thus, fermentations are characterized by the excretion of large amounts of reduced organic compounds, like alcohols, lactate, organic acids.

19
Q

Describe ATP generated by substrate-level phosphorylation.

A

-Energy yields are very low compared to respiration.

20
Q

________ is often an intermediate of various fermentations.

A

Pyruvate

21
Q

Fermentation:
PMF is often generated by…?

A

Reversing ATP synthase (remember, ATP Synthase can make or break ATP).

22
Q

Describe the basic metabolic adaptation to anaerobiosis.

A

Many facultative anaerobic bacteria undergo several physiological changes during adaptation to anaerobiosis.
- Replace oxidases with reductases for respiration
-For many organisms, latter half of the TCA cycle becomes a reductive pathway.
-For enterics and lactic acid bacteria, pyruvate dehydrogenase is replaced with pyruvate-formate lyase for oxidation of pyruvate to acetyl CoA.
-Many organisms undergo mixed acid fermentation or butanediol fermentation.

23
Q

What happens in mixed acid fermentation?

A

In mixed fermentation, bacteria utilize two or more different pathways in the terminal step of fermentation.
-Pathways used and products produced are growth condition and species-specific
-The “mixed acid” pathway makes multiple end produced and in variable amounts.

24
Q

What test do we use for mixed acid fermentation?

A

Methyl-Red test

25
Q

Who carries our Butanediol fermentation?

A

Enterobacter and Klebsiella, but can be carried out by a range of anaerobic bacteria.

26
Q

What is 2,3-butanediol?

A

A glycol that acts as a precursor to a multitude of compounds, from rubber to pesticides.

27
Q

Bacterial fermentation of 2,3-butanediol is of great interest because…?

A

Of the varied applications for 2,3-butanediol and its derivative chemical compounds which can be used for synthetic rubber, fuels, solvents, cosmetics, and food additives.

28
Q

What test do we use for butanediol fermentation?

A

Voges-Proskauer Test

29
Q

Bacteria capable of lactic acid fermentation are of significant ______ importance.

A

economic
-Production of a vast array of food, drink, and medicinal products.

30
Q

Lactic Acid fermentation:
What species are commonly used for fermentation of food products? And as what?

A

Lactobacillus species is commonly used for fermentation of food products as Lactic Acid Bacteria (LAB).

31
Q

What are Lactic Acid Bacteria (LAB)?

A

LAB are acid and salt tolerant, and each have optimal pH for growth.
They convert glucose or lactose to lactic acid.

32
Q

Metabolism by cheese-associated microbes greatly impacts…?

A

The sensory attributes of cheese:
-differences in microbial diversity result in tangible changes in flavor, odor, texture, and color.

33
Q

What are Homofermentative (or homolactic) LAB?

A

Only produce lactic acid.
-They prefer temperatures between 86-95F though they grow at lower temps as well.
-They produce flavors characterized by dairy, cream, or yogurt notes.

34
Q

Homofermentative lactate fermentation of lactic acid bacteria (LAB) such as Lactobacillus species have a primary end product of?

A

Primary end product of lactate.
Glycolysis to pyruvate = 2 ATP + 2 NADH + 2H+
These two NADH used to reduce pyruvate to 2 lactate. -Net gain per glucose = 2 lactate + 2ATP

35
Q

Heterofermentative LAB ferment glucose to produce lactic acid, but also…?

A

Acetic acid, ethanol, and carbon dioxide depending on conditions.
-They impart a sharper, more vinegar like-tang to foods. Like pickles.

36
Q

Heterofermentative lactate fermentation typically involves _________ fermentation by multiple LAB species:

A

Successional fermentation
-Leuconostoc spp
-Pediococcus spp
-Lactobacillus spp

37
Q

Heterofermentative lactate fermentation produces lactate via?

A

Via the decarboxylation and isomerase reactions of the PPP (2 NADH are produced instead of NADPH) to produce ribulose monophosphate.

38
Q

Heterofermentative lactate fermentation:
In step 5, there is something unusual. What is it?

A

An unusual reaction splits xyulose-5-P into PGALD and acetyl phosphate.

39
Q

Heterofermentative lactate fermentation:
The 3 NADH produced in the pathway are also _______ in the pathway.

A

Oxidized
-1 NADH reduced pyruvate to lactate
-1 NADH reduces acetyl phosphate to acetaldehyde
-1 NADH reduces acetaldehyde to ethanol

40
Q

The process of heterofermentative lactate fermentation of foods involves several different LAB genera and species, which dominate the culture in _______, based primarily on _______.

A

Succession ; pH

41
Q

Making pickles….
Leuconostoc mesenteroides gets to work, initiating fermentation….

A

Initiating fermentation, metabolizing sugars in the cucumber to produce lactic acid, carbon dioxide, ethanol, and acetic acid.
-This initial fermentation lowers pH from 7.0 to 4.5 the optimal pH for L. mesenteroides.
-As acids accumulate, pH continues to decrease, and more acid-tolerant bacteria take over.
-Pediococcus cerevisiae, Lactobacillus brevis, and Lactobacillus plantarum begin to proliferate outcompeting L. mesenteroides.
-These three species lower the pH further still, until the system reaches as low as pH 3.
-L. brevis and L. plantarum are left to complete the final stage of fermentation.

42
Q

What do LAB do in sourdough?

A

LAB not only act as natural leavening agents in sourdough and impart distinctive taste as result of fermentation products, but also have peptidase systems that specifically target gluten.

43
Q

Describe ethanol fermentation.

A

Glycolysis converts glucose into pyruvate, just like in respiration.
Going from pyruvate to ethanol is a two-step process.
- In the first step, a carboxyl group is removed from pyruvate and released as carbon dioxide, producing a two carbon molecule called acetaldehyde.
-In the second step, catalyzed by alcohol (ethanol) dehydrogenase, NADH passes its electrons to acetaldehyde, regenerating NAD+ and forming ethanol.

44
Q

It’s not the ethanol that kills you….

A

Acetaldehyde is a carcinogen that causes DNA and protein mutation.