Fundamentals of Metabolism

Introduction to Metabolism

Metabolism encompasses all chemical reactions that occur within a cell, enabling energy production, biosynthesis, and growth. Microorganisms display remarkable metabolic diversity, allowing them to thrive in varied environments by utilizing different energy and carbon sources.

• Catabolism: Breakdown of molecules to release energy.

• Anabolism (Biosynthesis): Synthesis of cellular components using energy.

• Chemoorganotrophs: Organisms that obtain energy from organic compounds.

• Phototrophs: Organisms that use light as an energy source.

• Electron Donors and Acceptors: Central to energy generation; donors lose electrons, acceptors gain them.

Example: Escherichia coli can use glucose as both an energy and carbon source, metabolizing it via glycolysis and the citric acid cycle.

Catabolism: Chemoorganotrophs

Glycolysis and the Citric Acid Cycle

Chemoorganotrophs break down organic molecules, such as glucose, to generate ATP. The glycolytic pathway converts glucose to pyruvate, which then enters the citric acid cycle for further oxidation.

• Glycolysis: Converts glucose to pyruvate, generating ATP and NADH.

• Citric Acid Cycle (CAC): Oxidizes pyruvate to CO2, producing NADH, FADH2, and ATP.

• Substrate-level phosphorylation: Direct synthesis of ATP from ADP and a phosphorylated intermediate.

Equation:

Catabolism: Electron Transport and Metabolic Diversity

Electron Transport Chain (ETC)

The electron transport chain is a series of protein complexes that transfer electrons from donors (e.g., NADH) to acceptors (e.g., O2), generating a proton motive force used to synthesize ATP via oxidative phosphorylation.

• Proton Motive Force (PMF): Electrochemical gradient across the membrane.

• ATP Synthase: Enzyme complex that synthesizes ATP using PMF.

• Oxidative phosphorylation: ATP production linked to electron transport.

Equation:

Biosynthesis

Anabolic Pathways and Autotrophy

Biosynthesis involves the construction of cellular components from simple precursors. Autotrophs can fix CO2 via the Calvin cycle, using energy from light or inorganic compounds.

• Calvin Cycle: Main pathway for CO2 fixation in phototrophs.

• RuBisCO: Key enzyme in the Calvin cycle.

• Autotrophy: Ability to use CO2 as the sole carbon source.

Example: Cyanobacteria use the Calvin cycle to convert CO2 into organic matter.

Introduction to Metabolic Diversity

Types of Metabolic Pathways

Microorganisms exhibit diverse metabolic strategies, including phototrophy, chemolithotrophy, and fermentation. These pathways allow adaptation to various ecological niches.

• Phototrophy: Use of light energy to generate ATP.

• Chemolithotrophy: Oxidation of inorganic compounds for energy.

• Fermentation: Anaerobic breakdown of organic compounds, producing ATP via substrate-level phosphorylation.

Phototrophy

Light-Driven Energy Generation

Phototrophic organisms capture light energy to drive ATP synthesis and CO2 reduction. Photosynthesis involves light reactions and carbon fixation.

• Oxygenic Photosynthesis: Produces O2 (e.g., cyanobacteria, plants).

• Anoxygenic Photosynthesis: Does not produce O2 (e.g., purple and green bacteria).

• Photosystems: Protein complexes that harvest light and transfer electrons.

Equation:

Respiratory Processes Defined by Electron Donor

Electron Donors in Respiration

Respiratory processes are classified by the nature of the electron donor. Common donors include H2, S, and organic compounds.

• Hydrogenotrophy: Use of H2 as an electron donor.

• Sulfur oxidation: Use of reduced sulfur compounds as donors.

• Organic compound oxidation: Chemoorganotrophs use organic molecules.

Respiratory Processes Defined by Electron Acceptor

Electron Acceptors in Respiration

Electron acceptors can be oxygen (aerobic respiration) or other compounds (anaerobic respiration), such as nitrate, sulfate, or carbon dioxide.

• Nitrate reduction: NO3- as electron acceptor.

• Sulfate reduction: SO42- as electron acceptor.

• Methanogenesis: CO2 as electron acceptor, producing methane.

One-Carbon (C1) Metabolism

Utilization of Single-Carbon Compounds

Some microorganisms can metabolize single-carbon compounds, such as methane or methanol, for energy and carbon.

• Methanotrophy: Oxidation of methane for energy.

• Methylotrophy: Use of methanol or other C1 compounds.

• Wood-Ljungdahl pathway: CO2 reduction to acetyl-CoA.

Fermentation

Anaerobic Energy Generation

Fermentation allows energy production in the absence of external electron acceptors. ATP is generated by substrate-level phosphorylation.

• Lactic acid fermentation: Pyruvate reduced to lactate.

• Ethanol fermentation: Pyruvate converted to ethanol and CO2.

• Mixed acid fermentation: Multiple products formed.

Equation:

Hydrocarbon Metabolism

Degradation of Hydrocarbons

Some microbes can degrade hydrocarbons, such as alkanes and aromatic compounds, using them as energy and carbon sources. This is important for bioremediation.

• Alkane degradation: Breakdown of saturated hydrocarbons.

• Aromatic compound degradation: Breakdown of benzene, toluene, etc.

• Cometabolism: Degradation of compounds only in the presence of another substrate.

Glossary of Key Terms

Application Questions

Practice Problems

• Identify electron donors and acceptors in given reactions; classify as aerobic or anaerobic.

• Calculate energy yields for metabolic reactions.

• Explain the role of ATP hydrolysis and synthesis in cellular metabolism.

Example: For the reaction , H2 is the electron donor and O2 is the electron acceptor; this is aerobic respiration.

Additional info: Academic context and definitions have been expanded for clarity and completeness. Equations and tables have been inferred and formatted for study purposes.