🦠Microbiology Unit 8 – Microbial Metabolism

Key Concepts and Definitions

• Metabolism encompasses all chemical reactions involved in maintaining the living state of cells and organisms
• Anabolism constructs molecules from smaller units (requires energy input)
• Catabolism breaks down complex molecules into simpler ones (releases energy)
• Metabolic pathways are series of enzymatic reactions that convert a starting molecule into a product
  • Pathways can be linear, cyclic, or branched
• Enzymes catalyze metabolic reactions by lowering activation energy barriers
• Coenzymes (organic) and cofactors (inorganic) assist enzymes in catalyzing reactions
• Adenosine triphosphate (ATP) is the primary energy currency in cells
  • Hydrolysis of ATP to ADP + Pi releases energy for cellular processes

Metabolic Pathways Overview

• Glycolysis breaks down glucose into pyruvate (occurs in cytoplasm)
  • Produces net gain of 2 ATP and 2 NADH molecules per glucose molecule
• Citric acid cycle (Krebs cycle) oxidizes acetyl-CoA to CO2 (occurs in mitochondrial matrix)
  • Generates 3 NADH, 1 FADH2, and 1 GTP per acetyl-CoA molecule
• Electron transport chain (ETC) transfers electrons from NADH and FADH2 to oxygen (occurs in inner mitochondrial membrane)
  • Creates proton gradient that drives ATP synthesis via chemiosmosis
• Pentose phosphate pathway produces NADPH and ribose-5-phosphate (precursor for nucleotides and amino acids)
• Fatty acid synthesis and beta-oxidation regulate lipid metabolism
• Amino acid synthesis and degradation pathways are diverse and interconnected

Energy Production in Microbes

• Microbes obtain energy through various metabolic processes
  • Chemotrophy utilizes chemical compounds as energy sources
  • Phototrophy captures light energy for ATP production
• Aerobic respiration requires oxygen as the terminal electron acceptor
  • Yields the highest amount of ATP per molecule of glucose (up to 38 ATP)
• Anaerobic respiration uses alternative electron acceptors (nitrate, sulfate, etc.)
  • Produces less ATP than aerobic respiration but more than fermentation
• Substrate-level phosphorylation directly generates ATP from high-energy intermediates (e.g., phosphoenolpyruvate in glycolysis)
• Oxidative phosphorylation couples electron transport to ATP synthesis via proton gradient
• Photophosphorylation produces ATP using light energy (occurs in photosynthetic microbes)

Nutrient Uptake and Utilization

• Microbes require essential nutrients for growth and metabolism
  • Carbon, nitrogen, phosphorus, sulfur, and trace elements
• Macronutrients are needed in larger quantities (carbon, nitrogen, phosphorus)
• Micronutrients (trace elements) are required in smaller amounts (iron, zinc, manganese)
• Passive transport moves molecules down concentration gradients without energy input
  • Includes simple diffusion and facilitated diffusion (via channels or carriers)
• Active transport moves molecules against concentration gradients using energy (usually ATP)
  • Examples include ion pumps and ABC transporters
• Group translocation couples transport with chemical modification (e.g., phosphotransferase system for sugar uptake)
• Microbes can secrete enzymes to break down complex nutrients in the environment before uptake

Fermentation vs. Respiration

• Fermentation is an anaerobic process that generates ATP through substrate-level phosphorylation
  • Does not involve an electron transport chain or oxidative phosphorylation
• Respiration (aerobic or anaerobic) couples electron transport to ATP synthesis via a proton gradient
• Lactic acid fermentation converts pyruvate to lactate (occurs in lactic acid bacteria and muscle cells)
• Alcohol fermentation converts pyruvate to ethanol and CO2 (occurs in yeast and some bacteria)
• Mixed acid fermentation produces a mixture of acids (acetic, lactic, succinic) and ethanol (occurs in Enterobacteriaceae)
• Aerobic respiration uses oxygen as the terminal electron acceptor and yields the most ATP
• Anaerobic respiration utilizes alternative electron acceptors (nitrate, sulfate, etc.) and produces less ATP than aerobic respiration

Biosynthesis and Growth

• Microbes synthesize a wide range of biomolecules for growth and reproduction
  • Proteins, nucleic acids, lipids, carbohydrates, and cell wall components
• Anabolic pathways construct complex molecules from simpler precursors
  • Requires energy input (usually in the form of ATP)
• Amino acid biosynthesis pathways produce the 20 standard amino acids
  • Some microbes can synthesize all amino acids, while others require external sources
• Nucleotide biosynthesis generates purines and pyrimidines for DNA and RNA synthesis
• Fatty acid and phospholipid synthesis are essential for cell membrane formation
• Peptidoglycan biosynthesis is crucial for bacterial cell wall integrity
• Microbial growth is influenced by nutrient availability, temperature, pH, and other environmental factors

Regulation of Microbial Metabolism

• Microbes adapt their metabolism in response to environmental changes
• Transcriptional regulation controls gene expression at the mRNA level
  • Repressors and activators bind to DNA and modulate transcription
• Post-transcriptional regulation modifies mRNA stability or translation efficiency
  • Includes RNA degradation and riboswitch-mediated control
• Allosteric regulation involves the binding of effectors to enzymes, altering their activity
  • Feedback inhibition is a common mechanism to prevent excessive product accumulation
• Covalent modification (e.g., phosphorylation) can reversibly activate or inactivate enzymes
• Catabolite repression prioritizes the utilization of preferred carbon sources (e.g., glucose) over others
• Two-component systems allow microbes to sense and respond to environmental signals

Real-World Applications and Importance

• Microbial metabolism is harnessed in various biotechnological processes
  • Production of antibiotics, enzymes, and other valuable compounds
• Fermentation is used in the food and beverage industry (yogurt, cheese, beer, wine)
• Bioremediation employs microbes to degrade pollutants and clean up contaminated sites
• Microbial fuel cells convert chemical energy from organic matter into electrical energy
• Metabolic engineering modifies microbial pathways to optimize product formation
  • Enables the production of biofuels, pharmaceuticals, and biomaterials
• Understanding microbial metabolism is crucial for combating antibiotic resistance
  • Identifying new drug targets and developing alternative therapies
• Microbiome research reveals the importance of microbial metabolism in human health
  • Dysbiosis is linked to various diseases (obesity, inflammatory bowel disease, etc.)