5-3_-_Catabolism_in_chemoorganotrophs

Catabolism in Chemoorganotrophs

• Lecture Overview: How chemoorganotrophic microbes obtain energy from organic molecules.

  • Key Processes: Glycolysis, citric acid cycle, fermentation, aerobic and anaerobic respiration.

  • Important Concepts: Proton motive force and ATP synthase.

  • Textbook Reference: Chapter 3.6-3.10

Energy Acquisition in Chemoorganotrophs

• Energy Sources:

  • Chemicals:

    • Organic: glucose, acetate

    • Inorganic: Fe2+, NH4+

  • Types of Chemoorganotrophs:

    • Chemoorganotrophs

    • Chemolithotrophs

    • Phototrophs

  • Reactions:

    • Glucose + 6 O2 -> 6 CO2 + 6 H2O

    • H2S + 2 O2 -> S° + H2O

ATP Production Mechanisms

• How Biology Generates ATP:

  • Substrate-level phosphorylation:

    • ATP produced directly in metabolic reactions from exergonic reactions.

  • Oxidative phosphorylation:

    • Energy from electron transfer creates proton motive force to synthesize ATP.

  • Photophosphorylation:

    • Light energy captures protons to generate ATP.

Catabolism of Glucose

• Chemoorganotrophs prefer glucose as an energy source, undergoing a sequence of oxidation reactions.

• Other organic compounds, including various sugars, can also be used to generate energy.

Glycolysis

• Process Overview:

  • Glucose is broken down into two pyruvate molecules through multiple steps.

  • Important metabolic pathway found in all life domains; provides energy quickly and feeds into the citric acid cycle.

  • Oxygen Requirement:

    • Does not require O2, can proceed to respiration or fermentation.

Glycolysis Details

• Overall Reaction:

  • Input: Glucose + 2 NAD+ + 2 Pi + 2 ADP

  • Output: 2 pyruvate + 2 NADH + 2 ATP + 2 H+ + 2 H2O

• Substrate-level phosphorylation: Produces 2 ATP per glucose.

• Redox Balance:

  • Generates NADH with no electron acceptor available to regenerate NAD+; resolved via fermentation or respiration.

Citric Acid Cycle (CAC)

• Also known as Kreb’s cycle; begins with conversion of pyruvate to acetyl-CoA.

• Utilizes a variety of organic compounds (e.g., lipids, amino acids) alongside sugars.

• Provides key intermediates for anabolic reactions and serves as a metabolic hub.

• Occurs in mitochondria in eukaryotes.

Citric Acid Cycle Efficiency

• Overall Reaction:

  • Input: Acetyl-CoA + 2 NAD+ + NADP+ + FAD + Pi + ADP + 2 H2O

  • Output: 2 CO2 + CoA + 2 NADH + NADPH + FADH2 + ATP + 2 H+

• Generates ATP and reducing power (NADH/FADH2) for electron transport chain.

Electron Transport Chain (ETC)

• Located in cytoplasmic membrane (inner mitochondrial membrane for eukaryotes).

• Restores redox balance and generates proton motive force for ATP production.

• Electrons pass through a series of carriers towards terminal acceptors (O2 for aerobic respiration).

Key Electron Carriers in ETC

• Types:

  • Iron-sulfur proteins: Metal cofactors facilitating electron transfer.

  • Quinones: Non-protein molecules transport electrons between carriers.

  • Cytochromes: Proteins with heme groups facilitating high-potential electron transfers.

ATP Synthase Function

• Converts proton motive force into mechanical energy to synthesize ATP from ADP.

• Approximately 3.3 protons translocated generates 1 ATP.

• Can function reversibly to maintain PMF in certain organisms.

Chemoorganotrophic Flexibility

• Preferential use of glucose (catabolite repression) but can utilize different organic molecules (fatty acids, amino acids).

• Multiple terminal electron acceptors available for respiration (nitrate, sulfate).

E. coli: Respiratory Versatility

• E. coli as a facultative anaerobe:

  • Capable of aerobic respiration, anaerobic respiration, and fermentation.

  • Adapts electron transport chain based on oxygen availability.

Fermentation Mechanisms

• Definition: Chemotrophic metabolism without external electron acceptors (anaerobic).

• Uses substrate-level phosphorylation to generate ATP; maintains redox balance through fermentation products.

Lactic Acid Fermentation

• Glucose converted to 2 pyruvate, then to 2 lactate with net gain of 2 ATP.

• Regenerates NAD+ allowing continued glucose breakdown.

Ethanol Fermentation

• Produces 2 pyruvate, decarboxylated to 2 acetaldehyde and then to ethanol, with CO2 released.

• Widely used by certain yeast (e.g., Saccharomyces cerevisiae) in food/beverage fermentation.

Fermentation Diversity

• Microbes can ferment various organic compounds, not just glucose.

• Generates energy-rich compounds for ATP, allowing redox balance and diverse metabolic adaptations.

Comparison: Fermentation vs. Aerobic Respiration

• Lactic Acid Fermentation:

  • Net: 2 ATP from glucose.

• Aerobic Respiration:

  • Net: Up to 38 ATP from complete oxidation of glucose.