Microbial Metabolism: Structured Study Notes

Microbial Metabolism

Overview of Metabolism

Microbial metabolism encompasses all chemical reactions occurring within a cell, divided into two main types: catabolism and anabolism. These processes are interconnected, with catabolic reactions providing energy and building blocks for anabolic reactions.

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

• Catabolism: Breakdown of complex molecules into simpler ones, releasing energy (exergonic).

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy (endergonic).

• ATP: Adenosine triphosphate, the main energy currency of the cell, produced by catabolic reactions and consumed in anabolic reactions.

• Metabolic Pathway: A series of enzyme-controlled steps where the product of one reaction becomes the substrate for the next.

• Example: Catabolic: Glucose breakdown during cellular respiration. Anabolic: Synthesis of peptidoglycan for bacterial cell walls.

Enzyme Structure, Activity, and Regulation

Enzymes are biological catalysts that accelerate chemical reactions by lowering activation energy. Their activity is regulated by various mechanisms to ensure efficient cellular function.

• Enzyme: A protein (or RNA) catalyst that is not consumed in the reaction.

• Collision Theory: Chemical reactions occur when particles collide with sufficient energy; enzymes increase reaction rates by facilitating these collisions.

• Enzyme Action: Substrate binds to the active site, forming an enzyme-substrate complex; products are released, and the enzyme is reused.

• Specificity: The active site’s shape and chemistry favor particular substrates.

• Apoenzyme: Protein portion of an enzyme.

• Cofactor: Nonprotein helper required for enzyme activity (can be metal ions).

• Coenzyme: Organic cofactor (e.g., NAD+).

• Holoenzyme: Complete, active enzyme (apoenzyme + cofactor/coenzyme).

• Reaction Rate: Increases with substrate concentration until active sites are saturated.

• Denaturation: High temperature or extreme pH can alter protein shape, causing loss of function.

Enzyme Regulation

• Competitive Inhibitor: Resembles substrate and blocks the active site; effect can be reduced by increasing substrate concentration.

• Noncompetitive Inhibitor: Binds to an allosteric site, changing enzyme shape and function; effect is not overcome by more substrate.

• Allosteric Site: Regulatory site separate from the active site.

• Feedback Inhibition: End product of a pathway inhibits an early step, preventing waste of energy and materials.

• Gene Regulation: Genes encode enzymes; some are constitutive (always produced), others are inducible (produced only when substrate is present).

Oxidation-Reduction Reactions and Electron Carriers

Redox reactions are fundamental to energy transfer in cells, involving the movement of electrons and hydrogen atoms between molecules.

• Oxidation: Loss of electrons or hydrogen.

• Reduction: Gain of electrons or hydrogen.

• Redox Pair: One molecule is oxidized while another is reduced.

• NAD+: Electron carrier; reduced to NADH.

• FAD: Electron carrier; reduced to FADH2.

• Electron Transport Chain: NADH and FADH2 deliver electrons to the chain, facilitating ATP production.

ATP Production Mechanisms

Cells generate ATP through three main mechanisms, each with distinct features and requirements.

• Substrate-Level Phosphorylation: Enzyme directly transfers phosphate from a substrate to ADP. Occurs during glycolysis and Krebs cycle.

• Oxidative Phosphorylation: Electron transport chain creates a proton gradient across a membrane; ATP synthase uses this gradient to produce ATP. Requires a final electron acceptor (e.g., O2).

• Photophosphorylation: Light-excited electrons power an electron transport system; ATP synthase produces ATP. Occurs in photosynthetic organisms.

Cellular Respiration Pathways

Cellular respiration is a multi-step process that converts glucose into ATP, involving glycolysis, the transition reaction, the Krebs cycle, and the electron transport chain.

• Glycolysis: Occurs in the cytoplasm; converts glucose to 2 pyruvate, net 2 ATP, and 2 NADH.

• Transition Reaction: Pyruvate is converted to acetyl-CoA, CO2, and NADH.

• Krebs Cycle: Acetyl-CoA enters the cycle, producing CO2, NADH, FADH2, and a small amount of ATP.

• Electron Transport Chain: NADH and FADH2 donate electrons, creating a proton gradient; ATP synthase produces most ATP via oxidative phosphorylation.

• Aerobic Respiration: O2 is the final electron acceptor.

• Anaerobic Respiration: Uses inorganic acceptors other than O2 (e.g., nitrate, sulfate).

Summary Equation for Aerobic Respiration

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

Fermentation

Fermentation allows cells to regenerate NAD+ when respiration cannot occur, enabling glycolysis to continue. It does not use an electron transport chain and yields low ATP.

• Purpose: Regenerate NAD+ for glycolysis.

• Organic Electron Acceptor: Used instead of inorganic acceptors.

• ATP Yield: Only from substrate-level phosphorylation.

• Lactic Acid Fermentation: Pyruvate is converted to lactic acid.

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2.

• Microbial Identification: Fermentation end products (acid, gas, indole, etc.) are used in biochemical tests to identify microbes.

Connections to Other Metabolic Pathways

Respiration and fermentation are integrated with other cellular pathways, allowing flexibility in energy and biosynthesis.

• Fat Metabolism: Fatty acids undergo beta-oxidation to acetyl-CoA; glycerol enters glycolysis.

• Protein Metabolism: Amino acids are deaminated; carbon skeletons enter glycolysis or Krebs cycle.

• Pentose Phosphate Pathway: Produces NADPH and five-carbon sugars for nucleic acid biosynthesis.

• Krebs Cycle: Amphibolic; participates in both catabolic and anabolic pathways.

• Integrated Metabolism: Shared intermediates can be used for energy or biosynthesis of carbohydrates, lipids, amino acids, nucleotides, and peptidoglycan.

Photosynthesis and Nutritional Patterns

Photosynthesis is divided into light and dark reactions, and microbes exhibit diverse nutritional patterns based on energy and carbon sources.

• Light Reactions: Produce ATP and NADPH using oxidative phosphorylation; require membrane, chlorophyll, and water.

• Oxygenic Photosynthesis: Uses H2O as electron donor; produces O2 (e.g., cyanobacteria).

• Anoxygenic Photosynthesis: Uses H2S as electron donor; produces elemental sulfur.

• Dark Reactions (Calvin-Benson Cycle): Use ATP and NADPH to fix CO2 into organic molecules.

Nutritional Patterns

Type	Energy Source	Carbon Source	Examples
Photoautotroph	Light	CO2	Cyanobacteria, plants
Photoheterotroph	Light	Organic compounds	Some bacteria
Chemoautotroph	Inorganic chemicals	CO2	Nitrifying bacteria
Chemoheterotroph	Organic chemicals	Organic compounds	Most medically important microbes

Key Concept Distinctions

Concept Pair	Essential Difference
Catabolism vs. Anabolism	Catabolism releases energy; anabolism requires energy.
Oxidation vs. Reduction	Oxidation loses electrons; reduction gains electrons.
Respiration vs. Fermentation	Respiration uses an electron transport chain; fermentation does not.
Aerobic vs. Anaerobic Respiration	Aerobic uses O2; anaerobic uses another inorganic final acceptor.
Competitive vs. Noncompetitive Inhibitors	Competitive block the active site; noncompetitive bind elsewhere.

Example Applications

• Microbial Identification: Biochemical tests (e.g., O/F test) use fermentation and respiration end products to distinguish microbes.

• Antibiotics: Some act as competitive inhibitors of microbial enzymes.

• Metabolic Engineering: Manipulating pathways for industrial production of metabolites (e.g., ethanol, lactic acid).

Additional info: Ribosomal RNA genes are used to classify organisms into the three domains of life: Archaea, Bacteria, and Eukaryotes. Most enzymes are proteins, but some are RNA-based (ribozymes).