Metabolism
Part I: Fermentations
Part II: Respiration
Learning objectives are to gain an
appreciation of:
•
Catabolism and anabolism
•
ATP Generation and energy
conservation
•
Fermentation
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Importance of Metabolism
• Industrial
– Yogurt, cheese
– Bread, wine, beer
• Medical/Health
– Strain identification
– Digestion
• Environmental
– Cycling of elements
– Pollutant transformation
Images: (1) www.bact.wisc.edu (2) en.wikipedia.org
Metabolism: the bigger picture
• What is metabolism?
– Chemical reactions that occur in living
organisms in order to maintain life.
• What does “maintain life” mean?
– Growth and division
– Maintaining cellular structures
– Sense/respond to environment
• Two parts of metabolism:
–
2
nutrients + C-source
Cell components
chemicals or light
Two parts of metabolism
• Anabolism- synthesis of complex molecules
from simpler ones during which energy is
added as input
• Catabolism- the breakdown of larger,
more complex molecules into smaller,
simpler ones, during which energy is
released, trapped, and made available for
work
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Anabolism and Growth
• Appropriate nutrients:
– Carbon source
– N, S, P sources
– Trace metals
• Appropriate environmental conditions:
– pH
– Oxygen
– Temperature.
– Light
Carbons Sources
Autotroph
Heterotroph
Organic Compounds
CO2
Anabolism
(biosynthesis)
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Energy Sources:
Chemotrophs
Phototrophs
Chemical compounds
Organotrophs
Light
Litotrophs
Organic compounds,
i.e., glucose, succinate
Inorganic compounds,
S, Fe2+,CO2, H2, CH4
Catabolism
ATP, pmf
ATP and Energy
• Adenosine triphosphate
• two of the phosphate
bonds are high energy
bonds
• breaking bond to
remove phosphate
releases energy
Figure 5.6
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The Role of ATP in Metabolism
• Reactions in which the terminal phosphate of ATP is
removed results in a:
• Exergonic breakdown of ATP can be coupled with:
• Energy conserving reactions are used to catalyze
the formation of ATP from ADP and Pi, and thus to
restore the energy balance of the cell
ATP is produced by three processes:
• _________________ phosphorylation
– Two reactions are coupled to make ATP.
• __________________ phosphorylation
– Carried out by:
– Requires:
• Photophosphorylation
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Chemical-based ATP synthesis:
Substrate level phosphorylation
Respiration-linked phosphorylation
ATP synthesis by substrate-level
phosphorylation
• Phosphorylated intermediates are generated in:
– Glycolysis (Embden-Meyhof pathway)
– Tricarboxylic Acid Cycle (TCA)
– Fermentation
• Finally, the Pi is transferred from a high energy
phosphorylated intermediate to ADP by a kinase
ADP obtains phosphate from
metabolic intermediate
•
molecule which has a high
energy bond
ATP is formed
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Embden Meyerhof Pathway
a.k.a. glycolysis
• Major pathway for:
• Major pathway of:
• Widespread method of:
• The end result is the release of a small amount of
energy
– conserved as ATP through:
– And fermentation end products.
Glycolysis and Fermentation
• Glycolysis is an anoxic process
• It is divided into two major stages
• Nets two ATPs and two NADHs
• End product of glycolysis is:
• The fate of this metabolite varies:
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Energy Input
Reductions
ATP by SLP
Glucose + 6O2
6CO2 + 6H2O
∆G˚’= -2830 kJ/mol
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Glycolysis and NADH
• Glycolysis also generates NADH
• But, cells need:
• Fermentation can regenerate:
– This can be done through:
– Substrates can be reduced by NADH
Example:
Lactic acid
fermentation
NADH
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Fermentation
ATP production by substrate
level phosphorylation
What is fermentation?
• Widespread method of anaerobic metabolism
• The end result is the release of a small amount of
energy
– conserved as ATP through substrate level
phosphorylation
• Incomplete oxidation of substrates
• Need to have a fermentation balance
– Oxidation-reduction state of products equal the
substrates
– NAD+ recycled
• Fermentation end products are generally secreted
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Why is fermentation important?
• Ecologically important for decomposition
of organic material in anaerobic
environments
• Byproducts are usually energy rich and
used by other microbes as energy/carbon
sources
• Useful in food industry
• Digestion
Lots of
fermentable
carbon sources
Substrate level P
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Lactic Acid Fermentation: I
• Carried out by several groups of
bacteria
– Lactobacillus and Lactococcus
• Gram +
• No cytochromes
• Anaerobes
• Only use sugars
Lactic Acid
Fermentation
Fermentations have
to maintain redox
balance.
Key Reaction
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Lactic acid bacteria and dairy
products
• These bacteria are used to to make
cheese and yogurt from milk.
• Carbon/energy source in milk is lactose
• Lactose is hydrolyzed
• Causes pH to decrease
• Milk proteins coagulate
http://web.mit.edu/esgbio/www/lm/s
ugars/lactose.gif
http://www.reluctantgourmet.com/images/cheese.jpg
What happens to the lactate?
• Transported out by lactate symporter
– Takes a proton out with it.
• Lactate is still a good carbon/energy
source.
Glucose  2 Lactate ∆G˚’= -196 kJ/mol
Made 2 ATPs = +63 kJ/mol
-133 kJ/mol left in lactate
Wasted energy!
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Lactic acid bacteria are wasteful
or are they?
• They live in organic rich environments
without oxygen
• They generate a proton gradient so more
ATP can be made
• They make acid which inhibits competitors
• They can grow in low iron environments
Substrates other than glucose
• Monosaccharides
– Fructose, mannose, galactose get
converted to glucose-6-phosphate or
fructose-6-phosphate
• Disaccharides--get cleaved in
monosaccharides by specific enzymes
– Lactose => galactose and glucose
– Maltose => 2 glucoses
– Sucrose => glucose and fructose
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Large complex polysaccharides
• Starch, cellulose:
found in plant material
• Glycogen: found in
animals
• Bacterium needs
specific enzyme to
break down the sugar
chains into monomers
– The enzymes are often
secreted.
Some Complex Polysaccharides
Cellulose
Aerobic decomposition:
myxobacteria
cytophaga
sporocytophaga
Fermented by:
some clostridia
glucanases
Starch
Amylases in:
Bacillus acidocaldarius
Streptococcus bovis
Bacteroides amylophilus
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Polysaccharides and Catabolic Enzymes
In many cases the sugar monomers are ultimately metabolized
either by glycolysis or another pathway to generate pyruvate.
More energy can be conserved
from glucose by oxidizing it to CO2
Fermentations products are not fully oxidized
∆G˚’
glucose >> 2 lactate + 2H+
= -198 kJ/mol
∆G˚’
glucose + 6O2 >> 6CO2 + 6H2O = -2830 kJ/mol
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Next Lecture
The maximum energy stored in glucose can be
conserved only when its complete oxidation is
coupled to the reduction of an external electron
accepting substrate.
Cells do this by:
• Oxidative phosphorylation
• The tricarboxylic acid cycle
Study Questions
1. How is ATP produced when organisms grow fermentatively?
Does it matter what type of fermentation is occurring?
2. Why is pyruvate reduced in fermentations? To what can it be
reduced? What do the fermentation products have in common?
3. Do fermentations produce a lot of ATP? Why not? What types
of environments might fermentative bacteria (such as lactic
acid bacteria) do well in?
4. What are the roles of ATP and NAD+ in glycolysis?
5. Compare and contrast the metabolism of lactose or maltose
with that of glucose?
6. Can molecules such as cellulose enter gram + cells? Gram –
cells? Why/why not? If not, how are they consumed?
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