Microbial Metabolism: Pathways, Enzymes, and Energy Production

Microbial Metabolism

Overview of Metabolism

Microbial metabolism encompasses all chemical reactions occurring within a microbial cell. These reactions are essential for growth, maintenance, and reproduction, and are facilitated by enzymes that lower the activation energy required for reactions to proceed under life-supporting conditions.

• Metabolism: The sum of all chemical reactions in a cell.

• Metabolic Pathway: A specific sequence of chemical reactions catalyzed by enzymes.

• Examples of Microbial Metabolism: Nitrogen cycling, food production (e.g., beer, bread), sewage breakdown, drug production (e.g., penicillin).

Anabolism and Catabolism

Metabolic reactions are classified into two main types: anabolism and catabolism.

• Anabolism: Biosynthetic reactions that build complex molecules from simpler ones, requiring energy input. These are endergonic reactions.

  • Examples: Photosynthesis, synthesis of macromolecules (lipids, carbohydrates, proteins, nucleic acids).

• Catabolism: Degradative reactions that break down complex molecules into simpler ones, releasing energy. These are exergonic reactions.

  • Examples: Fermentation, anaerobic and aerobic cellular respiration.

Enzymes in Microbial Metabolism

Enzyme Structure and Function

Enzymes are biological catalysts that accelerate chemical reactions in cells by lowering the activation energy. Most enzymes are proteins, though some are composed of RNA (ribozymes).

• Enzymes can catalyze thousands of reactions per second.

• Active Site: The region on the enzyme where the substrate binds and the reaction occurs.

• Enzyme activity is influenced by temperature, pH, and substrate concentration.

Enzyme Inhibition and Regulation

• Competitive Inhibitors: Molecules that compete with the substrate for the active site (e.g., sulfanilamide).

• Non-competitive Inhibitors: Molecules that bind elsewhere on the enzyme, altering its shape and function (e.g., ketoconazole).

• Activators: Often vitamins that act as co-enzymes, enhancing enzyme activity.

• Denaturation: Loss of enzyme structure and function due to extreme temperature or pH.

• Feedback Inhibition: End product of a pathway inhibits an earlier step, regulating metabolic flow.

Energy Production in Microbes

Redox Reactions

Energy production in cells involves oxidation-reduction (redox) reactions, where electrons are transferred between molecules.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

ATP Generation by Phosphorylation

ATP (adenosine triphosphate) is generated by adding a phosphate group to ADP (adenosine diphosphate) through phosphorylation.

• Substrate-Level Phosphorylation: Direct transfer of phosphate to ADP during metabolic reactions (e.g., glycolysis, fermentation).

• Oxidative Phosphorylation: ATP synthesis driven by the electron transport chain (ETC) and chemiosmosis in cellular respiration.

Cellular Respiration

Stages of Cellular Respiration

Cellular respiration is a multi-step process that converts glucose into ATP, involving glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation.

• Glycolysis: Occurs in the cytoplasm; converts glucose to pyruvate, producing ATP and NADH.

• Oxidation of Pyruvate: Pyruvate is converted to acetyl CoA, CO2, and NADH.

• Krebs Cycle (Citric Acid Cycle): Acetyl CoA is oxidized, generating NADH, FADH2, ATP, and CO2.

• Oxidative Phosphorylation: Involves the electron transport chain and chemiosmosis, producing the majority of ATP.

Electron Transport Chain (ETC)

• If the final electron acceptor is oxygen, the process is aerobic.

• If the final electron acceptor is inorganic or organic molecules, the process is anaerobic (e.g., Pseudomonas, Bacillus, Desulfovibrio).

• Anaerobic respiration yields less ATP than aerobic respiration.

Glycolytic Pathways

• Embden-Meyerhof Pathway (EMP): $\text{Glucose} \rightarrow 2\ \text{pyruvate} + 2\ \text{Net ATP} + 2\ \text{NADH}$

• Entner-Doudoroff Pathway (EDP): Found in Pseudomonas, Enterococcus faecalis; yields 1 Net ATP and 1 NADPH.

• Pentose Phosphate Pathway (PPP): Operates with or without O2; produces 5-carbon sugars, NADPH, and 1 Net ATP. Important for biosynthesis of nucleic acids and amino acids.

Fermentation

Fermentation Pathways and Products

Fermentation is an anaerobic process that allows cells to regenerate NAD+ and produce ATP in the absence of oxygen. It results in various end products depending on the organism and pathway.

• Lactic Acid Fermentation: Produces lactic acid (e.g., cheese, yogurt).

• Alcoholic Fermentation: Produces ethanol and CO2 (e.g., wine, beer).

• Other Fermentation Types: Produce propionic acid, butyric acid, butanol, and other organic acids.

Lipid and Protein Catabolism

Lipid Catabolism

• Extracellular Lipases: Break down lipids into fatty acids and glycerol.

• Glycerol enters glycolysis; fatty acids undergo beta oxidation to form acetyl CoA, which enters the Krebs cycle.

Protein Catabolism

• Extracellular Proteases/Peptidases: Break down proteins into amino acids.

• Deamination: Removal of amino group from amino acids; the remaining carbon skeleton enters the Krebs cycle.

Biochemical Tests and Bacterial Identification

Enzyme-Based Tests

Biochemical tests are used to identify bacteria based on the presence or absence of specific enzymes.

• Oxidase Test: Detects cytochrome c oxidase in the ETC. Neisseria gonorrhoeae (ox+), Pseudomonas (ox+), E. coli (ox-)

• Fermentation Test: Uses pH indicators and Durham tubes to detect acid and gas production from sugar fermentation. E. coli (lac+ A/G), Shigella (lac-)

• Amino Acid Catabolizing Enzyme Test: Detects decarboxylation (CO2 release) and dehydrogenation (H2 loss).

• Urease Test: Detects conversion of urea to ammonia.

• H2S Test: Detects sulfur removal from amino acids, forming black precipitate with iron. E. coli (H2S-), Salmonella (H2S+)

Metabolic Diversity

Classification by Carbon and Energy Source

Microbes are classified based on their carbon and energy sources.

Type	Carbon Source	Energy Source	Examples
Photoautotrophs	CO2	Light	Cyanobacteria
Photoheterotrophs	Organic Compounds	Light	Green/Purple non-sulfur bacteria
Chemoautotrophs	CO2	Reduced inorganic compounds (H2S, NH3, H2, Fe2+, CO)	Beggiatoa, Nitrobacter, Thiobacillus
Chemoheterotrophs	Organic Compounds (e.g., glucose)	Organic Compounds	Saprotrophs, pathogens

Biosynthesis in Microbes

Macromolecule Synthesis

Biosynthetic pathways require ATP and use intermediates from glycolysis, PPP, and the Krebs cycle to build cellular macromolecules.

• Polysaccharides: $\text{Glucose 6-phosphate} + \text{ATP} \rightarrow \text{Glycogen}$ $\text{Fructose 6-phosphate} \rightarrow \text{Peptidoglycan (PG)}$

• Lipids: Glycerol is generated from G3P (glycolysis intermediate); fatty acids are synthesized from acetyl CoA.

• Amino Acids & Proteins: Some microbes synthesize all amino acids; others require external sources. Amination and transamination reactions modify intermediates to form amino acids.

• Nucleotides: Purines and pyrimidines are synthesized from PPP or EDP intermediates and amino acids.

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