Download Microbial Metabolism

Microbial Metabolism
•
•
•
•
•
•
•
•
Overview of metabolism
Overview of nutrition
Culture media
Energetics
Enzyme catalysis
Oxidation and reduction
Electron carriers
Energy conservation
Metabolism
Energy classes of microbes
• Bacteria need three things to grow:
– Energy source
– Nutrients
– Suitable environmental conditions
• Energy source
– Phototroph (light)
– Chemotroph (chemicals)
• Chemoorganotroph (organic chemicals)
• Chemolithotroph (inorganic chemicals)
Macronutrients
•
•
•
•
•
•
•
•
•
•
•
Carbon (CO2 or organic compounds)
Hydrogen (H2O or organic compounds)
Oxygen (H2O or organic compounds)
Nitrogen (NH3, NO3-, organic N-compounds)
Phosphorus (PO43-)
Sulfur (H2S, SO42-, organic compounds)
Potassium (K+)
Magnesium (Mg2+, salts)
Sodium (Na+)
Calcium (Ca2+, salts)
Iron (Fe3+, Fe2+, or salts)
Iron as a nutrient
• Needed for aerobic metabolism
(cytochromes, iron-sulfur proteins)
• Insoluble under aerobic conditions
– Fe(OH)3, FeOOH
– Solubilized by siderophores
Siderophore
Iron uptake
Micronutrients and growth
factors
• Micronutrients: Metals and metalloids
– Generally not necessary to add to medium
– Deficiencies can arise when medium
constituents are very pure
• Growth factors: organic requirements
– Vitamins, amino acids, purines, pyrimidines,
acetate
Culture media
• Defined: all chemicals are ostensibly known
• Complex (undefined): contains substances
with unknown chemistries, such as
peptones, yeast extract, lake water, soil
extract, etc.
Energetics
• Gibbs Free-Energy (G)
• Reaction has a free-energy change
– Negative: exergonic
– Positive: endergonic
– Zero: equilibrium
• Standard concentrations—tables of ΔGf°’
Calculation of reaction energetics
• First, must write balanced equation
– E.g., 2H2 + O2 → 2H2O
• Calculation of ΔG°’ for a reaction
– ΔG°’ = ΔGf°’products - ΔGf°’reactants
– ΔG°’ = 2 x (-237.2 kJ/mol) – (2 x 0 + 0)
• Calculation of ΔG for a reaction
– ΔG = ΔG°’ + RT x ln(k)
Chemical kinetics and enzyme
catalysis
Enzymes as catalysts
Redox Reactions
• Reactions can be written as half-reactions
– Oxidation: removal of electrons
• S → P + e- or
H2 → 2H+ + 2e-
– Reduction: addition of electrons
• S + e- → P or
O2 + 4H+ + 4e- → 2H2O
• Energetics of redox reactions can be
considered as electrical potentials (see
electron tower)
Electron Tower
• A redox reaction needs a reducing and oxidizing
half-reaction
• Reactions with stronger tendency to give up
electrons are higher (more negative) on the tower
• To determine which direction the reactions go, see
which is “higher” on the electron tower
• Note the position of important electron carriers
(NAD, FAD, cytochrome a) and external electron
donors/acceptors (H2, organic compounds, O2)
Electron carriers: NAD
+
NAD
as co-enzyme
NADH as co-enzyme
NAD as electron carrier
• NAD+ + ED → EDox + NADH
• NADH + EA → EAred + NAD+
• Overall reaction:
– ED +EA → EDox + EAred
High-energy compounds
• ATP is the energy currency of the cell
– High energy released when phosphate is
hydrolyzed (ATP, ADP, AMP)
• Acetyl phosphate
• Acetyl coenzyme A
• Phospho-enol pyruvate