Lecture 4: Fermentation - Exam 4 Flashcards
What is fermentation? Who does it?
Where do the reactions occur?
How is ATP made?
What are the e- donors and acceptors?
What is the problem with oxygen? How do aerobes deal with it? Why can’t anaerobes use it?
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.
What are the two types of anaerobic growth?
Anaerobic respiration (using nitrate, sulfate, fumarate as the terminal e- acceptor) and fermentation.
What is fermentation?
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.
What is anaerobic respiration?
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.
Archaea called methanogens use _________ as a terminal electron acceptor, producing _________ as a waste product.
Carbon dioxide or acetic acid ; methane
(Methanogens are found in soils, in the digestive systems or ruminants or termites and many other habitats)
With fermentation, what kind of electron acceptor is not involved?
NO exogenous terminal electron acceptor is involved
What is the only energy yielding step in fermentation?
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
Fermentation is not unique to _______.
Prokaryotes
What are the benefits of fermentation?
Food products (beer, cheese, bread, sauerkrauat, kimchi, yogurt), bioalcohol, organic solvents
Fermentation vs Respiration
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
Fermentation:
NAD+ is reduced to NADH (glycolysis) then back to NAD+ when…?
It donates its electrons to electron sinks
What is a major concern with fermentation?
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.
Fermentation does not involved exogenous electron acceptors (unlike respiration), so…?
The fermentation pathways themselves have to produce the electron acceptors that can be used to oxidize NADH.
-These electron acceptors are called electron sinks.
Fermentation: Why are the electron acceptors produced by the fermentation pathways called electron sinks?
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.
Describe ATP generated by substrate-level phosphorylation.
-Energy yields are very low compared to respiration.
________ is often an intermediate of various fermentations.
Pyruvate
Fermentation:
PMF is often generated by…?
Reversing ATP synthase (remember, ATP Synthase can make or break ATP).
Describe the basic metabolic adaptation to anaerobiosis.
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.
What happens in mixed acid fermentation?
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.
What test do we use for mixed acid fermentation?
Methyl-Red test
