Microbial Metabolism: Catabolism, Anabolism, and Energy Pathways

Microbial Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within a cell, divided into two main processes: catabolism and anabolism. These processes are essential for cellular function, growth, and maintenance.

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

• Anabolism: The assembly of small molecules into larger, complex molecules, requiring energy input.

• Interdependence: Catabolism and anabolism form a continuous cycle, with energy released from catabolism fueling anabolic reactions.

Microbial Nutritional Types

Carbon and Energy Sources

• Heterotrophs: Organisms that rely on complex organic carbon compounds as nutrients (e.g., animals, fungi, many bacteria).

• Autotrophs: Organisms that convert inorganic CO2 into organic carbon compounds (e.g., plants, cyanobacteria).

• Chemotrophs: Obtain energy from breaking chemical bonds. Subdivided into:

  • Organotrophs: Use organic compounds as electron donors (e.g., humans, fungi).

  • Lithotrophs: Use inorganic compounds as electron donors (e.g., H2S, reduced iron).

• Phototrophs: Obtain energy from light (e.g., plants, algae, cyanobacteria).

Major Steps of Cellular Respiration

Glycolysis

Glycolysis is the initial pathway of glucose catabolism, converting one glucose molecule into two pyruvate molecules and generating a small amount of ATP and NADH.

• Location: Cytoplasm (both prokaryotes and eukaryotes)

• Substrates: 1 Glucose, 2 ATP (investment), 2 NAD+

• Products: 2 Pyruvate, 4 ATP (2 net gain), 2 NADH

• Pathways:

  • Embden-Meyerhof-Parnas (EMP): Most common glycolytic pathway.

  • Entner-Doudoroff (ED): Used by some bacteria; less efficient than EMP.

  • Pentose Phosphate Pathway (PPP): Produces 5-carbon sugars for nucleic acids and NADPH for biosynthesis.

Pyruvate Oxidation (Transition/Bridge Reaction)

This step prepares pyruvate for entry into the Krebs cycle by converting it to acetyl-CoA.

• Location: Mitochondrial matrix (eukaryotes), cytoplasm (prokaryotes)

• Substrates: 2 Pyruvate, 2 NAD+, 2 Coenzyme A (CoA)

• Products: 2 Acetyl-CoA, 2 CO2 (waste), 2 NADH

Krebs Cycle (TCA Cycle)

The Krebs cycle completes the oxidation of organic molecules, transferring electrons to carrier molecules.

• Location: Mitochondrial matrix (eukaryotes), cytoplasm (prokaryotes)

• Substrates (per glucose): 2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP

• Products (per glucose): 4 CO2 (waste), 6 NADH, 2 FADH2, 2 ATP

Electron Transport System (ETS)

The ETS uses electrons from NADH and FADH2 to generate a proton gradient, driving ATP synthesis.

• Location: Inner mitochondrial membrane (eukaryotes), cell membrane (prokaryotes)

• Substrates: 10 NADH, 2 FADH2, O2 (final electron acceptor)

• Products: ~34 ATP, H2O

Proton Motive Force (PMF) and Chemiosmosis

• Proton Motive Force (PMF): The electrochemical gradient formed by the accumulation of H+ ions on one side of a membrane.

• Chemiosmosis: The movement of H+ ions across a membrane through ATP synthase, driving the synthesis of ATP.

Comparison of Aerobic Respiration, Anaerobic Respiration, and Fermentation

Feature	Aerobic Respiration	Anaerobic Respiration	Fermentation
Oxygen Requirement	Yes	No	No
Location	Mitochondria & Cytoplasm	Mitochondria & Cytoplasm	Cytoplasm
Electron Transport Chain (ETC)	Yes	Yes (different final acceptors)	No
ATP Yield	36-38	Low (varies)	2
End Products	CO2, H2O	Varies (e.g., methane, nitrogen gas)	Lactic acid, ethanol, CO2

Fermentation

Definition and Products

Fermentation is an anaerobic process that allows cells to generate ATP without oxygen by partially breaking down glucose.

• ATP Yield: 2 ATP per glucose

• Products:

  • Yeast (e.g., Saccharomyces): Ethanol and CO2 (used in beer and bread production)

  • Humans (muscle cells): Lactic acid (produced during intense exercise)

  • Bacteria (e.g., Streptococcus, Lactobacillus): Lactic acid (used in yogurt and cheese production)

Catabolism of Lipids and Proteins

Lipid Catabolism

• Glycerol Separation: Lipids are split into glycerol and fatty acids.

• Glycerol Entry: Glycerol is converted to G3P and enters glycolysis.

• Beta-Oxidation: Fatty acids are broken into 2-carbon units (acetyl-CoA) in the mitochondria.

• Entry into Krebs Cycle: Acetyl-CoA enters the Krebs cycle for further oxidation.

Protein Catabolism

• Digestion: Proteins are hydrolyzed into amino acids.

• Deamination: Removal of the amino group (nitrogen), which is converted to urea for excretion.

• Carbon Skeleton Processing: The remaining carbon skeletons are converted into intermediates such as pyruvate, acetyl-CoA, or Krebs cycle intermediates.

• Energy Yield: Proteins are used as a last resort for energy due to the need for nitrogen removal and their entry at multiple metabolic stages.

Summary Table: Entry Points of Macromolecule Catabolism

Macromolecule	Initial Breakdown	Entry Point	Notes
Carbohydrates	Glycolysis	Glucose → Pyruvate	Main energy source
Lipids	Glycerol & Fatty Acids	G3P (glycolysis), Acetyl-CoA (Krebs)	High energy yield
Proteins	Amino Acids	Pyruvate, Acetyl-CoA, Krebs intermediates	Last resort; nitrogen removal required

Key Equations

• Overall Equation for Aerobic Respiration:

$6\ \mathrm{O}_2 + \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 \rightarrow 6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} + \text{energy (ATP)}$

• ATP Yield from Glucose (Aerobic):

$\text{Glycolysis:}\ 2\ \mathrm{ATP} \\ \text{Krebs Cycle:}\ 2\ \mathrm{ATP} \\ \text{ETS:}\ 34\ \mathrm{ATP} \\ \text{Total:}\ 38\ \mathrm{ATP}$

Additional info: The actual ATP yield may vary depending on the organism and efficiency of the electron transport chain.