Fermentations
NADH must be oxidized to NAD+
in order to oxidize
glyceraldehyde-3-P
In the absence of an electron
transport chain pyruvate or a
derivative serves as the electron
acceptor for NADH
Can lead to the production of
some ATP
Commonalities of fermentations
NADH is oxidized to NAD+
The electron acceptor is often
pyruvate or a pyruvate derivative
The substrate is partially oxidized
Commonalities of fermentations
ATP is produced by substratelevel phosphorylation
Oxygen is not needed
Fermentations
Different fermentations are often
characteristic of particular
microbial groups
Alcoholic fermentations
Many fungi and some bacteria,
algae and protozoa ferment sugars
to ethanol and CO2
Pyruvate is decarboxylated to
form acetaldehyde
Acetaldehyde reduced to form
ethanol
Lactic acid fermentation
Carried out by bacteria, fungi,
algae, protozoa and animal muscle
cells
Pyruvate is reduced to lactate
Homolactic fermenters
Reduce almost all their pyruvate
to lactate using lactate
dehydrogenase
Heterolactic fermenters
Form substantial amounts of
products other than lactate
Products include lactate, ethanol
and CO2
Formic acid fermentation
Pyruvate converted to formic acid
and other products
2 types of formic acid
fermentations:
1. Mixed acid fermentation
2. Butanediol fermentation
Mixed acid fermentation
Results in the production of
ethanol and a mixture of acids
including acetic, lactic, succinic
and formic acids
Formic hydrogenlyase will
degrade formic acid to H2 and
CO2
Occurs in Escherichia,
Salmonella, Proteus and other
genera
Butanediol fermentation
The second type of formic acid
fermentation
Pyruvate converted to acetoin
NADH reduces acetoin to 2,3butanediol
Large amounts of ethanol and
small amount of mixed acid
fermentation acids also produced
Butanediol fermentation
Characteristic of Enterobacter,
Serratia, Erwinia and some
species of Bacillus
Voges-Proskauer test
Differentiates between mixed acid
fermenters and butanediol
fermenters
Detects acetoin
Positive for butanediol fermenters
and negative for mixed acid
fermenters
Methyl red test
Mixed acid fermenters acidify
media to a greater extent than
butanediol fermenters
Change in color from red to
yellow indicates pH has dropped
below 4.4 and is read as positive
Mixed acid fermenters are
positive in the methyl red test
Stickland Reaction
Some bacteria obtain energy from
the fermentation of amino acids
One amino acid is oxidized and
another is reduced to regenerate
NAD+
Acetate, CO2, NH3 and ATP are
generated
Stickland Reaction
Many amino acids can be
fermented by this reaction
Amino acids can be fermented by
other mechanisms besides the
Stickland reaction
Fermentations
Many commercial products are the result of fermentation
reactions
Alcoholic beverages and bread (alcoholic fermentation)
Yogurt, Sauerkraut, Pickles (lactic acid fermentation)
Fermentations
The tricarboxylic acid cycle
Represents stage 3 of catabolism
Most of the energy from the
complete oxidation of glucose is
released in the TCA cycle
Also known as citric acid cycle or
Kreb’s cycle
The tricarboxylic acid cycle
Pyruvate is first oxidized,
decarboxylated and joined to CoA
to form acetyl-CoA
Acetyl-CoA combines with
oxaloacetate to form citrate
The tricarboxylic acid cycle
Cycle broken down into three
stages:
6 carbon stage
5 carbon stage
4 carbon stage
The 6 carbon stage
6 carbon citrate decarboxylated
and oxidized to form
-ketoglutarate
The 5 carbon stage
5 carbon -ketoglutarate is
decarboxylated, oxidized and
joined to CoA to form succinylCoA
The 4 carbon stage
4 carbon succinyl-CoA produces
GTP by substrate-level
phosphorylation and forms
succinate
Succinate oxidized by FAD to
form fumarate
The 4 carbon stage
Fumarate  malate oxidized by
NAD+ to form oxaloacetate
Oxaloacetate starts cycle again
The tricarboxylic acid cycle
Catabolism of carbohydrates,
lipids and amino acids results in
the production of acetyl-CoA
which can be oxidized in the TCA
cycle
One molecule of acetyl-CoA
yields 3 NADH, 1 FADH and
GTP
1
The three stages of catabolism (organic molecules)
Little ATP synthesized
Oxidation of glucose to 6 CO2
 4 ATP
Most ATP comes from oxidation
of NADH and FADH2 in the
electron transport chain
Fermentation, aerobic and anaerobic respiration
Differ regarding the final electron acceptors
Electron transport chains
Eukaryotic and prokaryotic electron transport chains differ
regarding their electron carriers and the details of construction
Both operate according to the same basic principles
Electron transport chains
Electrons move from a carrier
with a lower standard reduction
potentials (EO) to a carrier with a
higher EO
Electron transport chains
Electrons move from a carrier
with a lower standard reduction
potentials (EO) to a carrier with a
higher EO
Mitochondrial electron transport chain
Electrons pass from NADH to FMN in complex I
Electrons from succinate can be passed to FAD in complex II
Both complexes pass electrons to Coenzyme Q (ubiquinone)
Mitochondrial electron transport chain
Coenzyme Q passes electrons to complex III
Electrons passed to cytochrome c then to complex IV
Mitochondrial electron transport chain
Electrons eventually combine with 1/2 O2 and 2 H+ to form H2O
Protons pumped across the membrane at various points during
electron transport