MCROBIO Module 6_ Microbial Metabolism
Page 2: Introduction
A. What is metabolism?
Metabolism is the sum of all chemical reactions in an organism.
Catabolic reactions provide energy and building blocks for anabolism.
Anabolic reactions use energy to build large molecules.
Glucose is used as an energy source for ATP synthesis and precursor for amino acids.
B. Energy classes of microorganisms
Heterotrophs obtain carbon from organic matter.
Autotrophs use carbon dioxide as their carbon source.
Phototrophs convert light energy into ATP.
Chemoorganotrophs use organic chemicals as the energy source.
Chemolithotrophs use inorganic chemicals as the energy source.
Page 3: Principle of Energy Conservation — Bioenergetics
A. Basic bioenergetics
Free energy (G) is the energy released during a chemical reaction available to do work.
∆𝐺 represents the change in free energy during a reaction at standard conditions.
Exergonic reactions release free energy.
Endergonic reactions require free energy.
Page 4: Catalysis and Enzymes
A. Overview
Activation energy is the energy required to bring all molecules in a chemical reaction into a reactive state.
Catalysts, like enzymes, lower the activation energy of reactions.
Enzymes are biological catalysts, typically proteins, that are highly specific and have an active site for substrate binding.
Microbial Metabolism
Enzyme Catalysis (Page 5)
Enzymes bind substrates to form enzyme-substrate complexes.
Prosthetic groups bind tightly to enzymes.
Coenzymes are loosely bound and are often vitamin derivatives.
Enzymes position substrates in their active sites for reactions.
Lysozyme breaks specific glycosidic linkages.
Some reactions require energy input from ATP hydrolysis or proton motive force.
Oxidation-Reduction (Page 5)
Redox reactions involve electron and proton transfers.
Oxidation is electron removal, while reduction is electron addition.
Redox reactions occur in pairs with electron donors and acceptors.
Energy from redox reactions is used to synthesize energy-rich compounds like ATP.
Oxidation-Reduction Couples (Page 6)
Molecules act as electron donors or acceptors based on circumstances.
Reduction potential determines electron donation tendency.
Redox tower shows the range of reduction potentials.
Glucose acts as a reductant, and oxygen as an oxidant.
Electron Carriers and NAD+/NADH Cycling (Page 7)
Redox reactions in cells involve electron carriers like prosthetic groups and coenzymes.
NAD+/NADH is directly involved in energy-generating reactions.
NADH is a good electron donor, while NAD+ is a weak electron acceptor.
NADP+/NADPH is primarily involved in biosynthetic reactions.
Coenzymes like NAD+/NADH increase the diversity of redox reactions.
NAD+/NADH and NADP+/NADPH (Page 8)
NAD+/NADH and NADP+/NADPH are hydrogen carriers in redox reactions.
NAD/NADH cycling helps maintain a balance of coenzymes in microbial cells.
Electron shuttling mediated by NAD+/NADH is common in microbial catabolism.
NAD+ and NADH facilitate redox reactions without being consumed and are recycled.
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Process of NAD+/NADH cycling:
Enzyme I binds substrate and NAD+
Substrate reaches activation energy, forms product
NAD+ becomes NADPH by receiving proton from substrate
Enzyme II binds NADH and another substrate
NADH donates electrons to substrate, gets reduced
Electrons often associated with hydrogen atoms in biological systems
Energy-rich compounds store chemical energy in phosphorylated compounds like ATP
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Conservation of energy in cells:
Chemical energy from redox reactions conserved in phosphorylated compounds, primarily ATP
Cells use compounds with free energy of hydrolysis > -30 kJ/mol as energy currency
ATP and CoA are energy-rich compounds, G6P is energy-low
ATP has two anhydride P bonds yielding 32kJ/mol energy each
Coenzyme A derivatives have thioester bonds yielding energy
Long-term energy storage involves insoluble polymers like glycogen in prokaryotes
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Coupling redox reactions to ATP synthesis:
Fermentation and respiration linked to ATP synthesis in chemoorganotrophs
Fermentation: substrate-level phosphorylation
Respiration: oxidative phosphorylation using proton motive force
ATP synthesis in fermentation by phosphorylating a substrate
ATP synthesis in respiration driven by proton motive force activating ATPase
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Glycolysis and Fermentation:
Fermentation: anaerobic catabolism where organic compounds donate and accept electrons
Respiration: aerobic or anaerobic catabolism with electron donor oxidized with oxygen
Glycolysis: glucose oxidized to pyruvate, first pathway of respiration
Pyruvate fate depends on microorganism's type of metabolism
In respiration, pyruvate converted to Acetyl CoA, enters Krebs cycle
Electron carriers enter ETC, transferring electrons until oxygen is final acceptor
Microbial Metabolism
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Microbial Metabolism Overview
Microbes use various metabolic pathways to generate ATP.
Glycolysis and its pathways are crucial for energy production.
Glycolysis Process
Three stages: preparatory reactions, redox reactions, and fermentation.
Each stage involves specific enzymatic reactions to convert glucose to pyruvate.
Importance of Glycolysis
Glycolysis does not require oxygen.
Glucose is converted to pyruvate through a series of enzymatic steps.
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Continuation of Glycolysis
Further breakdown of intermediates to produce ATP and NADH.
NADH is essential for maintaining redox balance in cells.
Detailed Glycolysis Steps
Conversion of G3P to 1,3-biphosphoglycerate and subsequent ATP synthesis.
Oxidation of NADH and reduction of pyruvate in the final step.
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Fermentation and Redox Balance
Fermentation helps regenerate NAD+ to maintain redox balance.
Summary of the EMP pathway and ATP generation in different conditions.
Respiration: Citric Acid Cycle
Overview of the citric acid cycle and its importance in energy production.
Steps involved in the citric acid cycle for the conversion of pyruvate to ATP.
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Citric Acid Cycle Steps
Detailed breakdown of the citric acid cycle reactions.
Importance of oxaloacetate and acetate in the cycle for energy conservation and biosynthesis.
Role of Citric Acid Cycle
The cycle is essential for glucose respiration and biosynthesis of key metabolites.
Continuous regeneration of oxaloacetate is crucial for the cycle to proceed.
Microbial Metabolism
Glyoxylate Cycle (Page 17)
Variation of the Citric Acid Cycle (CAC) when acetate is used as an electron donor
C4-C6 citric acid cycle intermediates are common products
Catabolism of C2 compounds through the glyoxylate cycle
Shortage of C4 CAC intermediates compensated by synthesizing oxaloacetate from pyruvate or phosphoenolpyruvate
Reactions involve enzymes like isocitrate lyase and malate synthase
Malate produced can enter a new round of acetyl-CoA oxidation in the CAC
Energetics Balance Sheet for Aerobic Respiration (Page 18)
ATP synthesis in bacteria occurs in the cytoplasmic membrane
ATP generation via oxidative phosphorylation in the respiratory chain
Overview of respiration and electron carriers
Electron Transport and Proton Motive Force (Page 20)
Proton motive force generated through electron transport
Electrochemical gradient and PMF essential for various cellular mechanisms
Complexes involved in the electron transport chain: I, II, III, and IV
Proton extrusion and electron transfer in each complex
ATP synthase synthesizes ATP driven by the proton motive force
ATP Synthase (Page 21)
ATP synthase consists of F1 and F0 components
F1 complex sticks into the cytoplasm and acts as a rotor
ATPase activity driven by the proton motive force
Formation of ATP from respiratory electron flow is called oxidative phosphorylation
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Microbial Metabolism General Microbiology
ATP Synthesis Process
F0 subunit carries out proton-translocation across the membrane
Protons entering F0 subunit activate the rotor
Activation of F1 subunit leads to oxidative phosphorylation process
Synthesis of ATP in beta F1 subunit using ADP and inorganic phosphate
ATP synthesis occurs at the expense of proton motive force
3 protons translocated to ATPase to generate 1 ATP
Roughly 3 ATP generated (3 protons needed for every ATP)
Options for Energy Conservation
Alcohol and lactic acid fermentation
Fermentation not respiration; electron donor and acceptor are organic compounds
No electron transport chain involved
Energy source is the electron donor
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Microbial Metabolism General Microbiology
Alcohol and Lactic Acid Fermentation
Alcohol fermentation
Yeasts example
Glycolysis results in 2 pyruvates
Pyruvate decarboxylated to Acetaldehyde, then to Ethanol
NADH oxidized back to NAD+; pyruvate is the electron acceptor
Lactic acid fermentation (homolactic)
Primary metabolite is lactic acid
Pyruvate converted to lactate/lactic acid
NADH oxidized back to NAD+; pyruvate is the electron acceptor
Catabolic Diversity
Microbes can respire aerobically and anaerobically
Oxygen is the final electron acceptor in aerobic respiration
Chemolithotrophic metabolisms typically aerobic
Photosynthesis by phototrophs uses light energy for electron flow and ATP generation
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Microbial Metabolism General Microbiology
Proton Motive Force and Catabolic Diversity
Mechanisms of microbial energy conservation linked to the proton motive force
Respiration and anaerobic respiration variations based on electron acceptors
Chemoorganotrophy and chemolithotrophy variations based on electron donors