Microbial Metabolism 1

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:

∆G°’ is negative

A+B


C+D

Endergonic reaction: ∆G°’ is positive

A+B

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

C+D
ATP

A+B

ADP + Pi

C +enzyme
D

Figure 8.6


Enzymes as catalysts for
reactions


Enzymes are proteins



Highly specific for the reaction that they catalyze



Reduce the energy of activation (E ) of a reaction
a

C6H12O6 + O2

CO2 + H2O

Ea

∆G°’

Figure 8.14 is similar


Back to ATP…
ATP is constantly depleted by cells
ADP + P = ATP
i

∆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 Acceptors

Acceptor + ne-

Donor

Page 153 in your text


Standard Reduction Potential


Measure of the tendency of a donor to lose electrons (E’ )
0

2H+ + 2eNAD+ + 2H+ + 2e-

H2

E’0 = -420 mV
NADH + H2 E’0 =

-320 mV
1

/2O2 + 2H+ + 2emV

H 2O

E’0 = +820


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
ADP
6 Carbon
Stage

hexokinase
Glucose 6-phosphate
phosphoglucoisomerase
Fructose 6-phosphate

ATP

Fructose 1,6-bisphosphate
phosphofructokinase

ADP
aldolase
Glyceraldehyde 3- P

Dihydroxyacetone- P


Glycolysis (continued)
Glyceraldehyde 3- P

Pi

1,3-Bisphosphoglycerate

Dihydroxyacetone- P

NAD+
NADH
+ H+
ADP

3-Phosphoclycerate
2-Phosphoglycerate

dehydrogenase

ATP

phosphoglycerate
kinase

phosphoglycerate
mutase

3 Carbon
Stage


Glycolysis (continued)
2-Phosphoglycerate

enolase

Phosphoenolpyuvate

ADP
ATP

Pyruvate

pyruvate
kinase

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

malate
dehydrogenase

pyruvate

NAD+

acetyl CoA

NADH + H+

oxaloacetatecitrate

citrate

synthase

L-malate

isocitrate
NADH

fumarate

isocitrate
dehydrogenase

FADH2

αketoglutarat
e α-ketoglutarate

succinate
succinate
dehydrogenase

succinyl CoA

dehydrogenase


Tricarboxylic Acid Cycle
pyruvate

- acetyl CoA
-

NAD+

ATP, acetyl CoA, NADH

malate
dehydrogenase

oxaloacetatecitrate

NADH + H+
ATP, succinyl CoA, citrate, NAD+

citrate

synthase

L-malate

isocitrate

NADH

fumarate

-

ATP

isocitrate
dehydrogenase

FADH2

αsuccinyl CoA
ketoglutarat
NADH
e α-ketoglutarate

-

succinate
succinate
dehydrogenase

succinyl CoA

dehydrogenase


Tricarboxylic Acid Cycle

AMP, CoA, NAD+

pyruvate

NAD+

acetyl CoA

NADH + H+

+

+ ADP

malate
dehydrogenase

oxaloacetatecitrate

citrate

synthase

L-malate

isocitrate
NADH

fumarate

+ ADP

isocitrate
dehydrogenase

FADH2

αketoglutarat
e α-ketoglutarate

succinate
succinate
dehydrogenase

succinyl CoA

dehydrogenase