Tech Georgia School of Biology
An evolutionary approach to learning
energy metabolism
How do cells obtain energy from food?
How do cells make ATP?
Redox reactions
Chemiosmosis and oxidative phosphorylation
What metabolic pathways were present in the
LUCA?
Biology 1510
Pathways common to all 3 domains
Pathways in organisms that live in
environments resembling early Earth
Fall 2012
Georgia Tech School of Biology
Oxidation-reduction is the core of energy
metabolism
Oxidation is the loss of electrons
Organic molecules (food) and inorganic chemical electron
donors are oxidized
Reduction is the gain of electrons
The amount of free energy released depends on the
reduction potential difference of the redox pair
Catabolic pathways feature a series of redox reactions
(electron-transfer reactions)
NAD+/NADH is the primary electron carrier
Biology 1510
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Georgia Tech School of Biology
becomes oxidized
becomes reduced
Free energy, G
port
trans
tron ain
ch
Explosive
release of
heat and light
energy
+
2H
(from food via NADH)
Controlled
release of
+
2H + 2e
energy for
synthesis of
ATP
ATP
/2 O2
1
Elec
Free energy, G
H2 + 1/2 O2
ATP
ATP
2 e
/2 O2
1
2H
+
H2O
(a) Uncontrolled reaction
Biology 1510
H2O
(b) Cellular respiration
Fall 2012
Georgia Tech School of Biology
Oxidized and reduced forms of NAD
NAD = nicotinamide adenine dinucleotide.
For NADH + H+ +1/2 O2 NAD+ + H2O, Go = -52.4 kcal/mol.
Fall 2012
Georgia Tech School of Biology
Respiration: transfer of electrons from electron donors to
electron acceptors to charge a membrane proton gradient
H+
membrane
NADH
NAD+
H+ electrochemical gradient
Electron transport chain
Terminal electron acceptors
O2, NO3-, SO42-, Mn4+, Fe3+,
CO2, etc.
See also:
http://www.microbelibrary.org/images/Tterry/anim/
ETSbact.html
Electron donors {[CH2O], H2, H2S, CH4, Fe2+, etc.}
Biology 1510
Fall 2012
Georgia Tech School of Biology
Q: An organism that gets electrons from H2 and
makes its own organic carbon from carbon
dioxide is classified as a:
a)
b)
c)
d)
Chemoheterotroph
Chemoautotroph
Photoheterotroph
Photoautotroph
Biology 1510
Fall 2012
Georgia Tech School of Biology
Terminal Electron Acceptors
Different e- acceptors are used sequentially in microbial
ecosystems, reflecting the energy yields of different
pathways (redox stratification).
O2
G = -479 kJ mol-1
NO3- G = -453 kJ mol-1
Mn4+ G = -349 kJ mol-1
Fe3+ G = -114 kJ mol-1
SO42- G = -77 kJ mol-1
Biology 2012
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Fall Tech School of Biology
Anaerobic respiration in marine sediments
-479 kJ/mol
-454 kJ/mol
-114 kJ/mol
Biology 1510
(Jorgensen 2000, Fig. 5.11)
Fall 2012
Georgia Tech School of Biology
Chemiosmosis in prokaryotes
Electron transport chain
generates proton gradient
across membrane.
Resulting proton motive
force drives ATP synthesis
and active transport.
Fenchel, Origin & Early Evolution
of Life, Oxford U Press 2002, Fig 6.2
Biology 1510
Fall 2012
Georgia Tech School of Biology
Periplasmic space
Proton gradient powers ATP
synthase during respiration
(oxidative phosphorylation)
F0 portion in membrane
-resembles flagellar motor
F1 portion (ATP synthase)
-resembles DNA helicase
http://www.youtube.com/watch?v=uOoHKCMAUMc
See also:
http://www.microbelibrary.org/images/Tterry/an
im/ATPsynthbact.html
Biology 1510
H+
Stator
Rotor
Internal
rod
Catalytic
knob
ADP
+
P
i
Cytoplasm
ATP
Fall 2012
Georgia Tech School of Biology
Q: If cytoplasmic ATP concentration is high,
and the extracellular H+ concentration is low,
then
ATP synthase will synthesize ATP at a slower rate
ATP synthase will synthesize ATP at a faster rate
ATP synthase will hydrolyze ATP and pump
protons out of the cell
ATP synthase will hydrolyze ATP and pump
protons into the cell
Biology 1510
Fall 2012
Georgia Tech School of Biology
Q: Which of the following would NOT have been a
possible energy-harvesting pathway for the LUCA?
a. Organic molecules as electron donor, sulfate as
electron acceptor
b. Organic molecules as electron donor, oxygen as
electron acceptor
c. H2 as electron donor, Fe+3 as electron acceptor
Biology 1510
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Georgia Tech School of Biology
Electron transport chain in mitochondria reduces
oxygen as the terminal electron acceptor
Biology 1510
Fall 2012
Georgia Tech School of Biology
Q: Oxidative phosphorylation occurs in
a) All cells in the presence of oxygen
b) Only in mitochondria in the presence of oxygen
c) Only in mitochondria, using either oxygen or
alternative electron acceptors
d) All respiring cells, using either oxygen or
alternative electron acceptors
e) All respiring cells, except Archaea
Biology 1510
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Georgia Tech School of Biology
Pathways for oxidation of glucose
H+ electrochemical gradient
ETC
ADP
ATP NADH
Glycolysis
NADH
Pyruvate
oxidation
NAD+ CO2
Glucose, NAD+, ADP
Biology 1510
NADH + FADH2
ATP
Citric acid cycle
NAD+
FAD
CO2
ADP
Fall 2012