Microbial Metabolism and Bioenergetics: Study Notes

Microbial Metabolism and Bioenergetics

Introduction to Metabolism

Metabolism encompasses all the chemical reactions that occur within a microbial cell, divided into two main processes: catabolism and anabolism. These processes are essential for energy production and the synthesis of cellular components.

• Catabolism: The breakdown of large molecules into smaller ones, releasing energy.

• Anabolism: The synthesis of larger molecules from smaller ones, requiring energy input.

Classification of Microorganisms by Energy and Carbon Source

Microorganisms are classified based on their energy and carbon sources. This classification helps in understanding their ecological roles and metabolic capabilities.

Classification	Energy Source	Carbon Source	Examples
Chemolithoautotrophs	Chemical	Inorganic: CO2	Only some bacteria and archaea
Chemoheterotrophs	Chemical	Organic Carbon Compounds	All animals, fungi, most protozoa and bacteria
Photoautotrophs	Light	Inorganic: CO2	Only bacteria and eukaryotes with chloroplasts (most plants)
Photoheterotrophs	Light	Organic Compounds	Includes some bacteria and algae

Redox Reactions and Electron Carriers

Redox (reduction-oxidation) reactions are fundamental to microbial metabolism, involving the transfer of electrons between molecules. These reactions are essential for energy generation.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Reducing agent: Donates electrons and becomes oxidized.

• Oxidizing agent: Accepts electrons and becomes reduced.

Key Electron Carriers:

• Nicotinamide adenine dinucleotide (NAD+)

• Nicotinamide adenine dinucleotide phosphate (NADP+)

• Flavin adenine dinucleotide (FAD)

These carriers shuttle high-energy electrons between compounds in metabolic pathways. Their reduced forms (NADH, NADPH, FADH2) can donate electrons to other molecules.

ATP: The Energy Currency

Adenosine triphosphate (ATP) is the primary energy carrier in cells. ATP is generated by the addition of a phosphate group to ADP (adenosine diphosphate), a process called phosphorylation.

• Substrate-level phosphorylation: Direct transfer of a phosphate group from a high-energy substrate to ADP to form ATP.

• Oxidative phosphorylation: Energy released from the transfer of electrons (oxidation) is used to generate ATP via the electron transport chain (ETC).

• Photophosphorylation: Light energy is used to generate ATP during photosynthesis, as electrons move through a system of carrier molecules.

General ATP Formation Equation:

$\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

Enzymes and Catalysis

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required. They are essential for metabolic processes.

• Activation energy: The minimum energy required to initiate a chemical reaction.

• Enzymes do not alter the overall change in free energy ($\Delta H$) of a reaction.

Enzyme Inhibition:

• Competitive inhibition: Inhibitor binds to the active site, blocking substrate binding.

• Noncompetitive inhibition: Inhibitor binds to an allosteric site, changing the enzyme's shape and reducing activity.

Factors Affecting Enzyme Activity

• pH: Extreme pH can cause enzyme denaturation.

• Temperature: High temperatures can denature enzymes.

• Substrate concentration: Enzyme activity increases with substrate concentration until saturation is reached.

Glycolysis

Glycolysis is a central metabolic pathway that breaks down glucose into pyruvate, generating ATP and NADH. It does not require oxygen and can function under aerobic or anaerobic conditions.

• Energy investment phase: Uses ATP to phosphorylate glucose and split it into two three-carbon molecules (glyceraldehyde-3-phosphate, G3P).

• Energy payoff phase: Oxidizes G3P to pyruvate, producing ATP and NADH.

Overall Glycolysis Equation:

$\text{Glucose} + 2\ \text{NAD}^+ + 2\ \text{ADP} + 2\ \text{P}_i \rightarrow 2\ \text{Pyruvate} + 2\ \text{NADH} + 2\ \text{ATP} + 2\ \text{H}_2\text{O}$

Reactants: Glucose, 2 ATP, 4 ADP, 2 NAD+

Products: 2 Pyruvate, 2 ADP, 4 ATP, 2 NADH

Respiration Pathways

• Aerobic respiration: Final electron acceptor is O2.

• Anaerobic respiration: Final electron acceptor is an inorganic molecule other than O2.

• Fermentation: Organic molecules serve as both electron donors and acceptors; does not use an electron transport chain.

Pyruvate Decarboxylation and Krebs Cycle

Pyruvate produced in glycolysis is converted to acetyl-CoA, which enters the Krebs cycle (citric acid cycle). The cycle completes the oxidation of glucose derivatives, generating NADH, FADH2, and ATP (or GTP).

• Pyruvate Decarboxylation: Pyruvate is converted to acetyl-CoA, producing NADH and CO2.

• Krebs Cycle: Acetyl-CoA is oxidized, producing NADH, FADH2, ATP (or GTP), and CO2.

Key Products per Glucose (2 cycles):

• 6 NADH

• 2 FADH2

• 2 ATP (or GTP)

• 4 CO2

Electron Transport Chain (ETC) and Oxidative Phosphorylation

The ETC is located in the cytoplasmic membrane (prokaryotes) or mitochondrial inner membrane (eukaryotes). It transfers electrons from NADH and FADH2 to O2 (aerobic) or other acceptors (anaerobic), generating a proton motive force used to synthesize ATP.

• Electrons move through carriers with increasingly positive reduction potentials.

• Proton motive force is generated by pumping H+ outside the membrane.

• ATP synthase uses this gradient to produce ATP from ADP and Pi.

General ETC Equation (Aerobic):

$\text{NADH} + H^+ + 1/2\ \text{O}_2 \rightarrow \text{NAD}^+ + \text{H}_2\text{O}$

Summary Table: Major Metabolic Pathways

Pathway	Main Purpose	Key Products
Glycolysis	Breakdown of glucose to pyruvate	2 ATP, 2 NADH, 2 Pyruvate
Krebs Cycle	Oxidation of acetyl-CoA	6 NADH, 2 FADH2, 2 ATP (per glucose)
Electron Transport Chain	ATP synthesis via oxidative phosphorylation	~34 ATP (per glucose, aerobic)

Additional info: These notes provide a concise overview of microbial metabolism, focusing on energy generation and the major biochemical pathways. For further study, review the regulation of these pathways and their integration in different microbial lifestyles.