Microbial Metabolism
Metabolic Reactions
Enzymology
Catabolism
Litho/Phototrophy
Anabolism
Chemoorganoheterotroph
Metabolism Overview:
Reduction;
e- gain
from donor
Oxidation;
e- loss to
acceptor
Laws of Thermodynamics
• First: energy is neither created not destroyed; it simply
changes state.
– E.g. light to chemical & heat;
– E.g. chemical to work and heat
• Second: the order of a system always decreases; it
becomes more random (i.e. entropy increases).
– E.g. gases expand
– E.g. concentration gradients
• Free Energy of Chemical Reactions:
– Enthalpy (H) = reaction heat
– Entropy (S) = degree of randomness; disorderliness
– Change in Free Energy during a chemical reaction:
(ΔG = ΔH - T·ΔS)
Standard Free Energy Change (ΔGo’) and
Reaction Equilibrium
• Negative ΔGo’ = spontaneous (exergonic) reaction.
– Negative ΔH; heat release during reaction
– Positive ΔS; increase in randomness
– Reaction equilibrium shifts to product.
• Positive ΔGo’ = endergonic reaction.
–
–
–
–
Positive ΔH = heat absorbed during reaction
Negative ΔS = reaction becomes more ordered
Reaction equilibrium shifts to reactants
Requires coupling with strong exergonic reaction to shift reaction
equilibrium to the product. The role of ATP!
• ΔGo’ = 0
– Direction of reaction is from high to low concentrations.
– Reversible reactions.
Oxidation-Reduction (Redox)
ΔEo’ = Change in standard reduction potential (Eo’)
ΔE’o = (E’o acceptor - E’o donor)
ΔGo’ = -nF·ΔE’o
Coupled ½ reactions:
D + e- → DA+ + e- → A
+
D- = donor;
More negative E’o
A+ = acceptor;
More positive E’o
D oxidized
A reduced
Spontaneous (-ΔGo’ or +ΔE’o):
½ O2 + NADH → H2O + NAD+
ΔE’o = 0.815V – (-0.42V) = 1.235V
Endergonic (+ΔGo’ or -ΔE’o):
H2O + NADP+ → ½ O2 + NADPH
ΔE’o = -0.42V – 0.815V = -1.235V
Better
Donors
Better
Acceptors
Electron
Carriers:
• Electrons like to flow from
negative to positive E’o;
releasing energy in the
transfer.
•
NADH
reduced
Electron Carrier Functions:
–
Control the release of energy for
work.
– Supply “reducing power” in anabolic
reaction.
•
Electron Carrier Types:
– NADH / NADPH (free)
– FAD / FMN (free or as flavoprotein)
– Coenzyme Q (= Ubiquinone)
(“dissolved” in lipid membranes)
– Cytochromes (Iron-porphyrin ring +
protein)
– Non-heme Iron Protein (e.g.
ferredoxin
FAD
oxidized
Electron Carriers:
Ubiquinone (CoQ)
Heme (iron-porphyrin ring)
A cell must get fuel to do work.
• Fueling reactions:
– Control electron flow (donors to acceptors)
• Electron carriers
• Fueling or work (reductive biosynthesis)
– Capture energy in a standard form (ATP)
• Substrate level phosphorylation
• Electron Transport System (ETS) involvement
– Photophosphorylation
– Oxidative phosphorylation
• Work (mechanical, transport, biosynthesis)
• Enzymes are needed to catalyze and control the
rate of both fueling and work reaction.