Download FERMENTATION: an anaerobic biological reaction process in which

5/22/06
FERMENTATION: an anaerobic biological
reaction process in which a reduced organic
compound (like glucose) acts as an electron
donor and another organic compound acts as
an electron acceptor
What are the products of fermentation in yeast?
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Commercially Valuable fermentation reactions:
Alcoholic fermentation by yeast used in brewing
and winemaking
Bacteria also can carry out alcoholic
fermentation under anaerobic conditions
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The three common metabolic fates of pyruvate generated by
glycolysis:
1. Under aerobic conditions, the pyruvate is completely
oxidized via the citric acid cycle to CO2 and H2O [NADH
acts as a high energy compound]
2. Under anaerobic conditions, alcoholic fermentation
occurs in yeast: pyruvate is converted to ethanol + CO2
[free energy of NADH oxidation is dissipated as heat]
3. Under transient anaerobic conditions in muscle cells,
pyruvate is reduced to lactate [free energy of NADH
oxidation is dissipated as heat]
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fermentation: catabolic reactions producing ATP in which
organic compounds serve as both the primary electron donor
and ultimate electron acceptor and ATP is produced by
substrate level phosophorylation
Note: fermentation is extremely inefficient
compared to aerobic respiration.
From first principles: Why?
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The potential energy drop between glucose and an
electron acceptor like pyruvate is a fraction of the
potential energy drop that occurs during cellular
respiration where O2 is the electron acceptor
See ATP accounting table in previous lecture....
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Lactic acid fermentation: this type of fermentation occurs in
some fungi and bacteria (used to make yogurt and cheese) and in
human muscle cells when oxygen is limiting
[Note, no CO2 is produced in this type of fermentation]
What does your textbook say about lactic acid and
muscle function?
What did the New York Time's article say about
lactic acid and muscle function?
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* From a UC Berkeley web site -- caption is obviously written
by someone not terribly familiar with basic chemistry
Seems like there is some problems with the
presentation and interpretation
Think of parallels wih pyruvate dehydrogenase...
This rat muscle cell has been labeled with a red dye
(left) that highlights an enzyme, lactate
dehydrogenase, that catalyzes the conversion of
lactate to energy*, and a green dye (center)
highlighting an enzyme, cytochrome oxidase, where
oxygen is burned*. The colors overlap to glow yellow
exactly at the membrane of the mitochondria,
demonstrating that lactate is burned for energy on
the inside of the mitochondrial membrane of muscle
cells. (Brooks lab, UC Berkeley)
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Control of Metabolic Processes in the Cell
The cell has an elaborate interlocking system of
feedback controls that coordinate the rates of
glycolysis, fatty acid breakdown, the Krebs (citric
acid cycle) and electron transport
As a result of many control mechanisms, the body
oxidizes fats and sugars 5-10 times more rapidly
during a period of strenuous exercise than during
a period of rest
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How do cells regulate these complex reaction
sequences?
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The cell controls its metabolic activities by
controling the
• The activity of enzymes catalyzing key reactions
• the transport of molecules from one cellular
compartment to another
How could the activity of an enzyme be
modulated upwards or downwards?
Many possibilites here
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Allosteric Interaction:
a regulatory mechanim in which a small molecule
binds noncovalently to a protein and alters its
activity by altering its shape
Important ponts:
• the small molecule is called an effector or a
modulator
• if the allosteric interaction involves an enzyme
the modulator binds at a site other than the
active site
• the modulator can activate or inactivate a
protein
allo = other stere = solid or shape
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End product inhibition is one example of allostery
(see also pgs. 156-157 of Campbell)
• Feedback inhibition regulates the flow through this
metabolic pathway. The end product Z binds to and
inhibits (lowers the activity of) the enzyme that catalyses
the B to X reaction. In this way Z controls its own
concentration.
• Thus when quantities of the final product of a metabolic
pathway begin to accumulate, the product binds to the first
enzyme (unique to its synthesis) and slows down its
catalytic action.
• This is called negative regulation
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Enzymes are also subject to positive regulation whereby the
enzyme's catalytic rate is increased rather than decreased by the
regulatory molecule
Examine the Krebs cycle at this web site—which is labelled
Detailed info on each Krebs cycle Reaction on 205 home page:
http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/krebscyc/krebscyc.htm
Which reactions are regulated?
Look at isocitrate dehydrogenase and α ketoglutarate
dehydrogenase
What compounds act as negative regulators?
What compounds act as positive regulators?
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How else could the activity of an enzyme be regulated?
• In bacteria, the trp repressor protein inhibits the transcription of a
suite of genes coding for enyzmes required for the synthesis of the
amino acid tryptophan
• In the absence of tryptophan, the recognition helices are not in the
proper orientation to contact the promoter DNA; no repressor
binds and the genes coding for the enzymes are transcribed
• If tryptophan levels are high, it binds to the repressor protein and
causes a subtle shift in the orientation of the recognition helices;
now the protein binds the promoter DNA with high affinity
NOTE: this regulatory circuitry involves allosteric regulation of a
regulatory protein (trp repressor) that controls tryptophan biosynthesis
by controlling the level of production of the enzymes required for the
biosynthesis of this amino acid
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