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Glucose to Pyruvate Sp. 25

Glucose to Pyruvate Overview

  • Chapters: 7.2-7.4 of Prescott's Principles of Microbiology

  • Authors: Joanne Willey, Kathleen Sandman

Learning Objectives

  • Familiarize with energy carriers and their role in electron transport during respiration.

    • Focus on: ATP and substrate-level phosphorylation.

  • Differentiate between heterotrophs and chemotrophs based on energy and electron sources.

  • Understand aerobic respiration pathways for glucose breakdown:

    • Glycolysis: Inputs, outputs, key enzymes discussed in class.

    • Entner Doudoroff pathway: General understanding and outputs.

    • Pentose Phosphate Pathway: General understanding and outputs.

Review of Metabolism

  • Catabolic pathways break down large molecules to store energy in small carriers like ATP and NADH.

  • ATP and nucleotide triphosphates: Store energy in phosphodiester bonds.

  • NADH, NADPH, FADH2: Each stores energy as electron pairs, contributing to reducing power.

  • Enzymes lower the ΔG to facilitate reactions by coupling energy transfer to biosynthesis and cell function.

Microbial Metabolism

  • Energy is essential for movement and growth in all living cells.

  • Catabolism: Breakdown of complex molecules into simpler ones; provides energy for:

    • Anabolism: Constructing new products.

  • Energy from catabolism is primarily in the form of ATP.

Electron Acceptors

  • Many microbes can grow in the presence of molecular oxygen (O2), using it as a terminal electron acceptor in aerobic respiration.

Energy Carriers and Electron Transfer

  • Energy transfer reactions in cells involve carriers:

    • Molecules that capture and release small amounts of energy reversibly.

    • Notable examples: NADH, FADH2, and ATP.

  • NADH: Acts as an electron donor; NAD+: Acts as an electron acceptor.

Formation of NADH

  • NADH can carry 2-3 times as much energy as ATP.

    • NADH: Reduced form.

    • NAD+: Oxidized form.

    • Reaction: NAD+ + 2H+ + 2e– → NADH + H+

    • The reduction involves the gain of two electrons and two hydrogen atoms.

Flavin Adenine Dinucleotide (FAD)

  • FAD: Another coenzyme involved in electron transfer.

  • FADH2: Reduced form, distilled from FAD via two electrons and protons.

Adenosine Triphosphate (ATP)

  • Structure: Composed of a base, a sugar, and three phosphates.

  • Formation: ADP plus inorganic phosphate (Pi) creates ATP, representing the cell's energy currency.

  • Substrate-Level Phosphorylation: Direct transfer of phosphate to another molecule.

ATP Energy Transfer

  • ATP hydrolysis releases energy in three ways:

    1. Hydrolysis releasing inorganic phosphate (Pi).

    2. Hydrolysis releasing pyrophosphate (PPi).

    3. Phosphorylation of an organic molecule.

  • ATP is fully renewable energy source as it facilitates cellular functions from muscle contractions to biosynthesis.

Catabolism in Microbes

  • Microbes can exploit numerous substrates:

    • Polysaccharides broken down to pyruvate.

    • Pyruvate can undergo fermentation or further catabolism to CO2 and H2O in the TCA cycle.

    • Lipids and amino acids can catabolize to glycerol and acetate.

Nutritional Types of Organisms

  • Heterotrophs: Use organic molecules as carbon and energy sources.

  • Autotrophs: Use CO2 as their primary carbon source.

  • Phototrophs: Derive energy from light.

  • Chemotrophs: Obtain energy from chemical compounds.'

  • Lithotrophs: Use reduced inorganic substances as electron sources; Organotrophs: Use electrons from organic compounds.

Chemoorganotrophic Pathways

  • Chemoorganotrophs also termed chemoheterotrophs, employ pathways including:

    • Aerobic respiration

    • Anaerobic respiration

    • Fermentation

Respiration Mechanisms

  • Main respiration types:

    • Aerobic respiration: With oxygen as the final electron acceptor.

    • Anaerobic respiration: Different oxidized molecules serve as terminal electron acceptors.

  • Electron transport chain plays a pivotal role in respiration for ATP synthesis via proton motive force (PMF).

Fermentation Process

  • Utilizes endogenous electron acceptors, typically intermediates from the catabolism of the organic energy source (e.g., pyruvate).

  • No electron transport chain involved; ATP is synthesized solely through substrate-level phosphorylation.

Major Catabolism Pathways for Glucose

  • Prokaryotes metabolize glucose utilizing three primary pathways:

    1. Glycolysis: Produces 2 ATP and 2 NADH.

    2. Entner-Doudoroff pathway: Yields 1 ATP, 1 NADH, and 1 NADPH.

    3. Pentose phosphate pathway: Generates 1 ATP and 2 NADPH.

  • Fermentation completes glucose catabolism without an electron transport chain.

Pathway Summaries

  • Embden-Meyerhof Pathway: Most common for glucose degradation to pyruvate in aerobic respiration.

  • Energy Investment Phase: Requires 2 ATP to activate glucose.

  • Energy Yield Phase: Produces NADH and ATP, net gain of 2 ATP.

Individual Pathway Details

  • Entner-Doudoroff Pathway:

    • Key for some Gram-negative bacteria; replaces 6-carbon phase in glycolysis.

    • Yields: 1 ATP, 1 NADPH, 1 NADH per glucose molecule.

  • Pentose Phosphate Pathway:

    • Functioning alongside glycolysis or Entner-Doudoroff.

    • Produces NADPH and sugars essential for biosynthesis.

Summary of Glycolysis

  • Converts one glucose molecule to two pyruvate molecules.

  • Net gain: 2 ATP, 2 NADH; 2 ATP expended, 4 ATP harvested.

Final Notes on Metabolism

  • All organisms seek ATP as an energy currency, reducing power, and precursor metabolites.

  • Essential pathways contribute to metabolic efficiency by integrating varied nutrient utilizations.