Microbial Metabolism: Energy, Enzymes, and Pathways

Microbial Metabolism

Defining the Requirements for Life

Microbial metabolism encompasses all biochemical reactions necessary for life, including both energy-releasing and energy-synthesizing processes. These reactions are divided into catabolism (breaking down molecules to release energy) and anabolism (building cellular material using energy). Metabolic processes rely on the transfer of electrons from electron donors to electron acceptors, and cells conserve energy by converting it into forms that can perform cellular work, primarily through the generation of adenosine triphosphate (ATP).

• Catabolism: Energy-releasing reactions that break down molecules.

• Anabolism: Energy-consuming reactions that build cellular components.

• ATP: The universal energy currency in cells, generated and used to fuel cellular processes.

Energy Classes of Microorganisms

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

• Chemoorganotrophs: Conserve energy from organic chemicals.

• Chemolithotrophs: Oxidize inorganic compounds (e.g., H2, H2S, NH4+).

• Phototrophs: Convert light energy into ATP.

• Heterotrophs: Obtain carbon from organic compounds.

• Autotrophs: Obtain carbon from CO2.

Catalysis and Enzymes

Activation Energy and Catalysis

For a chemical reaction to occur, reactant molecules must overcome an energy barrier known as activation energy. Catalysts, including biological catalysts called enzymes, lower this barrier, increasing the rate of reaction without being consumed or altering the reaction's equilibrium.

• Catalyst: Substance that facilitates a reaction, lowers activation energy, and increases reaction rate.

• Enzyme: Biological catalyst, usually a protein, highly specific to its substrate.

Enzyme Structure and Function

Enzymes possess an active site where substrates bind. The specificity of this site ensures that only particular substrates are converted to products. Enzyme activity can be affected by temperature and pH, which may denature the protein and alter its function.

• Active Site: Region of the enzyme where substrate binding and catalysis occur.

• Denaturation: Loss of enzyme structure and function due to extreme conditions.

Enzyme Cofactors: Prosthetic Groups and Coenzymes

Many enzymes require additional non-protein molecules for catalysis. Prosthetic groups are tightly and permanently bound, while coenzymes are loosely bound and often derived from vitamins.

• Prosthetic Group: Covalently attached, permanent (e.g., heme in cytochromes).

• Coenzyme: Loosely attached, often vitamin-derived (e.g., NAD).

Models of Enzyme-Substrate Interaction

Enzyme catalysis depends on substrate binding and positioning within the active site. Two models describe this interaction: the lock-and-key model and the induced fit model.

• Lock-and-Key Model: Substrate fits perfectly into the active site.

• Induced Fit Model: Active site changes shape to accommodate the substrate.

Energy Conservation and Storage

Energy-Rich Compounds

Microorganisms store energy long-term in insoluble polymers that can be oxidized to generate ATP. Examples include glycogen, poly-β-hydroxybutyrate, elemental sulfur in prokaryotes, and starch and lipids in eukaryotes.

Glycolysis and Fermentation

Pathways of Energy Conservation

Chemoorganotrophs conserve energy through two main pathways: fermentation and respiration. Fermentation is anaerobic, using organic compounds as both electron donors and acceptors. Respiration can be aerobic (using O2) or anaerobic (using other compounds as electron acceptors).

• Fermentation: Anaerobic catabolism; organic compounds donate and accept electrons.

• Respiration: Aerobic or anaerobic; donor is oxidized with O2 or another acceptor.

Comparison: Respiration vs. Fermentation

Respiration yields more ATP than fermentation due to the complete oxidation of substrates and the use of an electron transport chain (ETC).

Electron Donors and Acceptors

Eukaryotic Electron Transport Chain (ETC)

In eukaryotes, NADH is the primary electron donor to the ETC, which is located in the mitochondrial membrane. The final electron acceptor is molecular oxygen, resulting in the production of water and ATP.

Prokaryotic Electron Transport Chain

Prokaryotes (bacteria and archaea) utilize a variety of electron donors and acceptors, allowing for metabolic diversity in different environments.

Glycolysis (Embden–Meyerhof–Parnas Pathway)

Overview of Glycolysis

Glycolysis is a universal pathway for the catabolism of glucose, producing two ATP molecules per glucose via substrate-level phosphorylation. Glucose can be fermented (without O2) or respired (with O2).

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP from a high-energy compound.

Fermentation Pathways

Types of Fermentation

Fermentation produces various end products depending on the organism and substrate. Common types include alcoholic, homolactic, heterolactic, mixed acid, butyric acid, butanol, caproate/butyrate, and acetogenic fermentations.

Type	Reaction	Organisms
Alcoholic	Hexose → 2 ethanol + 2 CO2	Yeast, Zymomonas
Homolactic	Hexose → 2 lactate	Streptococcus, some Lactobacillus
Heterolactic	Hexose → lactate + ethanol + CO2	Leuconostoc, Oenococcus
Mixed acid	Hexose → lactate + ethanol + acetate + succinate + formate + CO2 + H2	Enteric bacteria (Escherichia, Salmonella, Shigella, Klebsiella, Enterobacter)
Butyric acid	Hexose → butyrate + 2 CO2 + 2 H2	Clostridium butyricum
Butanol	Hexose → butanol + acetate + CO2 + H2	Clostridium acetobutylicum
Caproate/butyrate	Hexose → caproate + butyrate + CO2 + H2	Clostridium kluyveri
Acetogenic	Fructose → 2 acetate + 2 H2	Clostridium aceticum

Respiration: Electron Carriers and the Proton Motive Force

Electron Transport Systems

Electron transport systems are membrane-associated and mediate the transfer of electrons, conserving energy to synthesize ATP. Key components include NADH dehydrogenases, flavoproteins, iron–sulfur proteins, cytochromes, and quinones.

• Cytochromes: Proteins with heme groups that transfer electrons.

• Quinones: Nonprotein molecules that shuttle electrons and protons within the membrane.

Proton Motive Force

During electron transport, protons are released outside the membrane, generating a pH gradient and an electrochemical potential known as the proton motive force. This force drives ATP synthesis via ATPase.

Options for Energy Conservation

Anaerobic Respiration

Some microorganisms use electron acceptors other than oxygen, such as nitrate, ferric iron, sulfate, carbon dioxide, or organic compounds. Anaerobic respiration conserves less energy than aerobic respiration but still generates a proton motive force and uses ATPase.

Chemolithotrophy

Chemolithotrophs use inorganic chemicals as electron donors (e.g., H2S, H2, Fe2+, NH4+). These organisms are typically autotrophic, using CO2 as a carbon source.

Phototrophy

Phototrophs use light as an energy source. Photophosphorylation is the light-mediated synthesis of ATP. Photoautotrophs use ATP and CO2 for biosynthesis, while photoheterotrophs use ATP and organic carbon.

Biosynthesis of Cellular Components

Sugars and Polysaccharides

Prokaryotic polysaccharides are synthesized from glucose derivatives such as uridine diphosphoglucose (UDPG). Gluconeogenesis allows synthesis of glucose from non-carbohydrate precursors, and the pentose pathway produces 5-carbon sugars for nucleic acids.

Amino Acids

Amino acid biosynthesis involves multistep pathways, with carbon skeletons derived from glycolysis or the citric acid cycle. The amine group is incorporated from ammonia and transferred by specific enzymes.

Fatty Acids and Lipids

Fatty acid biosynthesis varies with species and temperature. In Bacteria and Eukarya, fatty acids are added to glycerol to form lipids, while Archaea use phytanyl side chains. All membranes require polar groups for proper architecture.