Microbial Metabolism and Enzymatic Reactions

Contact Information

  • Instructor: Dr. MP Di Bonaventura

  • E-mail: mdibonaventura@york.cuny.edu

  • Course: Bio 265 - Clinical Microbiology Lecture

Overview of Microbial Metabolism

  • Definition: Microbial metabolism is the sum of controlled chemical reactions that occur within cells.

  • It encompasses two main processes:

    • Catabolism: Breakdown of nutrients, providing energy and precursor metabolites for anabolism and other cellular functions.

    • Anabolism: Biosynthesis that uses energy and precursor metabolites from catabolism to build macromolecules and cellular structures.

Energy Concepts

  • Energy distribution in microbial metabolism:

    • Energy is lost as heat, stored as ATP, and involved in various metabolic processes.

    • CATABOLISM:

      • Energy stored as ATP.

      • Larger building block molecules derive from the breakdown of nutrients.

      • Processes involved: Glycolysis, Krebs cycle, Electron Transport Chain.

    • ANABOLISM:

      • Uses stored ATP for biosynthetic reactions to build macromolecules.

Redox Reactions

  • Definition: Redox reactions, or oxidation-reduction reactions, involve the transfer of electrons from an electron donor to an electron acceptor.

  • Importance in cellular energy:

    • Cells use redox reactions to extract energy from nutrient molecules like glucose.

    • Biological oxidations primarily involve the transfer of hydrogen atoms (dehydrogenation reactions).

    • Key coenzymes involved: NADH and FADH2, which provide energy for ATP synthesis.

ATP Production Methods

  • ATP is synthesized via three main mechanisms:

    1. Substrate-level phosphorylation

    2. Oxidative phosphorylation

    3. Photophosphorylation

  • ATP is utilized for:

    • Biosynthetic reactions

    • Processes such as active transport across the plasma membrane

Metabolic Pathways and Enzymes

  • Definition: Metabolic pathways are sequences of chemical reactions that are organized into steps, each catalyzed by an enzyme.

  • Enzymes:

    • Enzymes are biological catalysts that are specific for a given substrate or chemical reaction.

    • Active site: The distinct region where substrate fits into the enzyme.

    • Activation Energy: Enzymes lower the activation energy required for reactions by forming an enzyme-substrate complex.

    • After the reaction, the enzyme is recovered unchanged.

Factors Affecting Enzyme Activity

  • Enzymatic activity is influenced by:

    • Temperature

    • pH

    • Concentration of the enzyme and substrate

    • Presence or absence of inhibitors

  • Temperature and pH can lead to denaturation of proteins, losing functionality.

Enzyme Inhibition

  • Competitive Inhibition:

    • Competitive inhibitors bind to the active site, competing with the substrate. Example: Sulfa drugs which inhibit folic acid synthesis by competing with PABA.

  • Non-competitive Inhibition:

    • These inhibitors bind somewhere other than the active site and disrupt the function. Example: Poisons like fluoride.

    • Regulation of allosteric enzymes occurs through binding at allosteric sites.

Carbohydrate Catabolism

  • Processes for energy extraction from carbohydrates:

    • Cellular Respiration: Involves complete oxidation of glucose in the presence of O2.

    • Fermentation: An alternative process that does not fully oxidize glucose and does not use the Krebes cycle or ETC.

    • Both processes utilize glycolysis as a shared pathway.

Steps in Aerobic Cellular Respiration

  1. Glycolysis: Glucose ($C6H{12}O_6$) oxidized to two pyruvate molecules; produces ATP and NADH.

  2. Transition Step: Converts pyruvate to Acetyl-CoA.

  3. Krebs Cycle: Acetyl group of Acetyl-CoA is oxidized to CO2, while reducing NAD+ to NADH and FAD to FADH2, and producing ATP.

  4. Electron Transport Chain: NADH and FADH2 carry electrons to produce ATP (up to 32 ATP total).

ATP Formation Mechanisms

  • Substrate-Level Phosphorylation:

    • Direct transfer of a phosphate group from a substrate molecule like phosphoenolpyruvate (PEP) to ADP.

  • Oxidative Phosphorylation:

    • Involves a proton gradient across the membrane, which powers ATP synthesis via ATP synthase.

    • Concurrent processes include active transport and flagellar movement.

Ratio of ATP Production

  • Each NADH generates approximately 2.5 ATP, while each FADH2 generates roughly 1.5 ATP.

  • In total: approx 32 ATP are produced during aerobic respiration through various pathways.

Fermentation Processes

  • Definition: Fermentation harvests energy from the oxidation of organic molecules (e.g., sugars) without utilizing Krebs cycle or ETC.

  • Only glycolysis occurs, with pyruvate or derivatives serving as electron acceptors to regenerate NAD+ for glycolysis.

  • Examples include:

    • Lactic acid fermentation (Streptococcus, Lactobacillus)

    • Alcoholic fermentation (Saccharomyces cerevisiae - yeast)

Identification of Microbial Species Through Fermentation End-Products

  • Chemical analysis of fermentation end-products assists in identifying microbes and potential pathogens. Some end products include butyric acid, mixed acid, and butanediol.

Test for Fermentation

  • Durham tube method: Utilizes phenol red lactose broths to gauge gas production post-incubation, indicating presence or absence of fermentation.

Microbial Enzymes and Hydrolysis

  • Microbes secrete enzymes to hydrolyze complex macromolecules into simpler forms for metabolism:

    • Proteases: Breakdown proteins into amino acids

    • Amylases & Cellulases: Degrade starch and cellulose

    • Lipases: Hydrolyze lipids into glycerol and fatty acids

Anabolic Pathways and Their Connection to Catabolic Pathways

  • Most ATP produced during catabolism fuels the synthesis of new cellular components via anabolic pathways.

  • Precursor metabolites derived from glycolysis and Krebs cycle are essential for the synthesis of macromolecules:

    • Amino acids: Used for protein synthesis

    • Carbohydrates: Used for polysaccharides, peptidoglycan

    • Glycerol/Fatty acids: Essential for lipid creation in cell membranes

    • Purines/Pyrimidines: Building blocks for nucleotides of DNA/RNA

Amphibolic Pathways

  • Amphibolic pathways can function in both anabolism and catabolism, with glycolysis and Krebs cycle exemplifying this dual role.

Catabolic Versatility and Energy Yields

  • Distinction between aerobic (using oxygen) and anaerobic (using other molecules like nitrate) respiration.

  • Fermentation: Utilizes pyruvate or derivatives as terminal electron acceptors, primarily regenerating NAD+.

  • Comparison of energy yields:

    • Aerobic respiration yields maximum energy.

    • Anaerobic respiration and fermentation yield less energy but are crucial for survival in specific environments.