Microbial Metabolism
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Microbial Metabolism
Microbiology 274
March 22, 2007
Free energy of reactions
Temperature
∆G = ∆H - T∆S
Free Energy:
How much energy
during a reaction is
available to do
work
Entropy:
Enthalpy:
How much heat is
lost or gained
during the reaction
How much
randomness is lost
or gained during
the reaction
Page 152 in your text
Free energy of reactions
• Exergonic reaction:
A+B
• Endergonic reaction:
A+B
∆G°’ is negative
C+D
∆G°’ is positive
C+D
Figure 8.5
Metabolism
• Organisms need to synthesize many complex
organic molecules
• Synthesis from monomers to polymers -- these
reactions are endergonic and require a source of
energy
ATP
ATP is used as a source of energy to change
endergonic reactions to exergonic reactions
Energy Coupling
A+B
ATP
C+D
ADP + Pi
A+B
C+D
enzyme
Figure 8.6
Enzymes as catalysts for reactions
• Enzymes are proteins
• Highly specific for the reaction that they
catalyze
• Reduce the energy of activ ation (Ea ) of
a reaction
C6H12O6 + O2
CO2 + H2O
Ea
∆G°’
Figure 8.14 is similar
Back to ATP…
ATP is constantly depleted by cells
ADP + Pi = ATP
∆G = 7.3 kcal/mole
In order to make more ATP, cells need to
input energy…
Oxidation-Reduction Reactions
• Loss of Electrons = Oxidation (LEO)
• Gain of Electrons = Reduction (GER)
• Electron Donors and Electron A cceptors
Acceptor + ne-
Donor
Page 153 in your text
Standard Reduction Potential
• Measure of the tendency of a donor to lose
electrons (E’0)
2H+ + 2e- H2
NAD+ + 2H+ + 2e1
/2O2 + 2H+ + 2e-
E’0 = -420 mV
NADH + H2 E’0 = -320 mV
H2O
E’0 = +820 mV
Figure 8-7
Mechanisms of Energy Release
• Fermentation -- oxidation of an organic
compound in the absence of external electron
acceptor
• Respiration -- oxidation of an organic
compound where oxygen is the final electron
acceptor
• Anaerobic Respiration -- oxidation of organic
compounds where an external substrate other than
oxygen serves as final electron acceptor
The Three Ways to Make ATP
• Substrate Level Phosphorylation
– Glycolysis
– Fermentation
• Respiration
– Electron transport systems
• Photophosphorylation
– Photosynthesis
Chapter 9 in your text
Substrate-Level Phosphorylation
• Glycolysis
Glucose + 2ADP + 2Pi + 2NAD+
2 Pyruvate + 2ATP + 2NADH + 2H+
Glycolysis (continued)
Glucose
ATP
hexokinase
ADP
Glucose 6-phosphate
6 Carbon
Stage
phosphoglucoisomerase
ATP
Fructose 6-phosphate
phosphofructokinase
ADP
Fructose 1,6-bisphosphate
aldolase
Glyceraldehyde 3- P
Dihydroxyacetone- P
Glycolysis (continued)
Glyceraldehyde 3- P
Pi
Dihydroxyacetone- P
NAD+
dehydrogenase
NADH
+ H+
1,3-Bisphosphoglycerate
ADP
ATP
phosphoglycerate
kinase
3-Phosphoclycerate
phosphoglycerate
mutase
2-Phosphoglycerate
3 Carbon
Stage
Glycolysis (continued)
2-Phosphoglycerate
enolase
Phosphoenolpyuvate
ADP
pyruvate
kinase
ATP
Pyruvate
3 Carbon
Stage
• Conversion of glucose to pyruvate results in the
formation of NADH
• To maintain homeostasis, cells must reoxidize
NADH to NAD+ or glycolysis will stop!
• Cells use electron acceptors to oxidize NADH
– Fermentation -- reduction of organic compounds
– Electron transport chains -- reduction of O2
Fermentation
NADH
NAD+
pyruvate
lactate
lactate
dehydrogenase
e.g. Bacillus
Lactobacillus
Streptococcus
Fermentation (continued)
CO2
pyruvate
pyruvate
decarboxylase
NADH
acetaldehyde
NAD+
ethanol
alcohol
dehydrogenase
Eg. Yeast
Diversity of microbial fermentation
Pathway
End Products
Examples
Lactic Acid
Lactic acid (2 molecules)
Lactobacillus, Enterococcus,
Streptococcus spp.
Heterolactic
Lactic acid, ethanol and CO2
Leuconostoc
Alcohol
Ethanol and CO2
Saccharomyces (yeast)
Proprionic acid
Proprionic acid and CO2
Proprionibacterium spp.
Butyric acid
Butyric acid, butanol, acetone,
isopropyl alcohol and CO2
Clostridium spp.
Butanediol
Butanediol and CO2
Enterobacter, Serratia, Erwinia,
and Klebsiella
Mixed acid
Ethanol, acetic acid, lactic acid,
E. coli, Salmonella, and Shigella
succinic acid, formic acid and CO2
Methanogenesis Methane and CO2
Archaea
Respiration
• NADH can be reoxidized by donating
electrons to an external electron acceptor
such as oxygen.
• The lower the redox potential of the
acceptor, the more energy can be obtained
in the form of ATP.
Tricarboxylic Acid Cycle
Lipid and protein
metabolites
pyruvate
NAD+
acetyl CoA
NADH + H+
oxaloacetate
malate
dehydrogenase
citrate
synthase
citrate
L-malate
isocitrate
NADH
fumarate
isocitrate
dehydrogenase
FADH2
α-ketoglutarate
succinate
succinate
dehydrogenase
succinyl CoA
α-ketoglutarate
dehydrogenase
Tricarboxylic Acid Cycle
pyruvate
ATP, acetyl CoA, NADH
- acetyl CoA
oxaloacetate
malate
dehydrogenase
citrate
synthase
NAD+
NADH + H+
-
ATP, succinyl CoA, citrate, NAD+
citrate
L-malate
isocitrate
NADH
fumarate
-
ATP
isocitrate
dehydrogenase
FADH2
α-ketoglutarate
succinate
succinate
dehydrogenase
succinyl CoA
-
α-ketoglutarate
dehydrogenase
succinyl CoA
NADH
Tricarboxylic Acid Cycle
AMP, CoA, NAD+
+
pyruvate
NAD+
acetyl CoA
NADH + H+
+
oxaloacetate
malate
dehydrogenase
citrate
synthase
ADP
citrate
L-malate
isocitrate
NADH
fumarate
+
isocitrate
dehydrogenase
FADH2
α-ketoglutarate
succinate
succinate
dehydrogenase
succinyl CoA
ADP
α-ketoglutarate
dehydrogenase
