Microbial Metabolism: Pathways, Energy, and Diversity

CH 5: Microbial Metabolism

Introduction to Microbial Metabolism

Microbial metabolism encompasses all chemical reactions occurring within a microorganism, enabling growth, energy production, and biosynthesis. These reactions are fundamental to the survival and proliferation of microbes such as Escherichia coli (E. coli) in laboratory growth media.

• Metabolic reactions are divided into catabolism (energy-releasing) and anabolism (energy-requiring).

• Growth in culture is fueled by nutrients in the medium, which provide sources of carbon, nitrogen, and energy.

• Metabolism is regulated and organized into pathways, often catalyzed by enzymes.

CH 5 Learning Objectives

• Define metabolism and distinguish between catabolism and anabolism.

• Explain the role of ATP as an energy intermediate.

• Describe oxidation-reduction (redox) reactions and their role in metabolism.

• Identify and provide examples of three types of phosphorylation reactions that generate ATP.

• Explain the function of metabolic pathways, including glycolysis, pentose phosphate, and Entner-Doudoroff pathways.

• Describe the Krebs cycle and chemiosmotic model for ATP generation.

• Compare aerobic and anaerobic respiration, fermentation, and photosynthesis.

• Summarize energy production in cells and categorize nutritional patterns among organisms.

Metabolism

Overview of Metabolic Reactions

Metabolism is the sum total of all chemical reactions in an organism. These reactions are catalyzed by enzymes and organized into regulated pathways.

• Catabolic reactions break down macromolecules into simpler molecules, releasing energy (exergonic).

• Anabolic reactions build up macromolecules from simpler molecules, requiring energy input (endergonic).

• Pathways are often depicted as a series of enzyme-catalyzed steps: Starting molecule → Enzyme 1 → Reaction 1 → Enzyme 2 → Reaction 2 → ... → Product.

ATP: The Energy Currency

Structure and Function of ATP

Adenosine triphosphate (ATP) is the primary energy carrier in cells, performing much of the cellular "work." ATP consists of adenine, ribose, and three phosphate groups.

• ATP hydrolysis (catabolic): Releases energy for cellular processes.

• ATP formation (anabolic): Requires energy input.

Chemical reactions involving ATP/ADP:

• ATP + H2O → ADP + Pi + H+   (energy-releasing)

• ADP + Pi + H+ → ATP + H2O   (energy-requiring)

Equations:

• $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{H}^+$

• $\text{ADP} + \text{P}_i + \text{H}^+ \rightarrow \text{ATP} + \text{H}_2\text{O}$

Catabolism vs. Anabolism

Fundamental Differences

• Catabolism: Degradation of complex molecules into simpler ones, releasing energy (e.g., breakdown of glucose during glycolysis).

• Anabolism: Synthesis of complex molecules from simpler ones, consuming energy (e.g., synthesis of proteins from amino acids).

• ATP links catabolic and anabolic reactions, serving as an energy shuttle.

Oxidation-Reduction (Redox) Reactions

Role in Metabolism

Redox reactions involve the transfer of electrons between molecules, crucial for energy extraction in metabolic pathways.

• Oxidation: Loss of electrons (often as hydrogen atoms).

• Reduction: Gain of electrons.

• Electron carriers such as NAD+ and FAD capture and transfer electrons during catabolic processes.

Example: $\text{NAD}^+ + 2\text{H} \rightarrow \text{NADH} + \text{H}^+$

Phosphorylation Mechanisms for ATP Generation

Types of Phosphorylation

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate (occurs in glycolysis and fermentation).

• Oxidative phosphorylation: ATP synthesis driven by electron transport chain and chemiosmosis (occurs in respiration).

• Photophosphorylation: ATP synthesis driven by light energy (occurs in photosynthetic organisms).

Carbohydrate Catabolism

Overview

Microorganisms primarily oxidize carbohydrates for cellular energy, using two main processes: respiration and fermentation.

• Respiration: Complete oxidation of glucose using electron transport chain; can be aerobic (O2 as terminal electron acceptor) or anaerobic (other acceptors such as nitrate or sulfate).

• Fermentation: Incomplete oxidation of carbohydrates, producing organic acids or alcohols; occurs in absence of a terminal electron acceptor.

Glycolysis (Embden-Meyerhof Pathway)

Key Steps and Products

Glycolysis is the oxidation of glucose to pyruvic acid, a central pathway in carbohydrate catabolism.

• Occurs in the cytoplasm; does not require oxygen.

• Net gain: 2 ATP (via substrate-level phosphorylation), 2 NADH, and 2 pyruvate per glucose molecule.

• Divided into energy investment and energy harvest phases.

Alternative Pathways of Glucose Catabolism

Pentose Phosphate and Entner-Doudoroff Pathways

• Pentose phosphate pathway: Generates NADPH and pentoses for biosynthesis; important for nucleotide and aromatic amino acid synthesis.

• Entner-Doudoroff pathway: Found in some Gram-negative bacteria; alternative to glycolysis for glucose catabolism.

Krebs Cycle (Citric Acid Cycle)

Function and Products

The Krebs cycle oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and ATP.

• Occurs in the cytoplasm (prokaryotes) or mitochondria (eukaryotes).

• Produces electron carriers for the electron transport chain.

Electron Transport Chain and Chemiosmosis

Mechanism of ATP Generation

Electrons from NADH and FADH2 are transferred through a series of carriers, creating a proton gradient across the membrane.

• Proton motive force: Drives ATP synthesis via ATP synthase.

• Oxidative phosphorylation: Major source of ATP in aerobic respiration.

Aerobic vs. Anaerobic Respiration

Comparison

Feature	Aerobic Respiration	Anaerobic Respiration
Terminal Electron Acceptor	O2	Inorganic (NO3-, SO42-, etc.)
ATP Yield	Up to 38 ATP/glucose	Less than aerobic
End Products	CO2, H2O	Varies (nitrate, sulfate, etc.)

Fermentation

Overview and Products

Fermentation is an anaerobic process that recycles NADH by transferring electrons to organic molecules, producing acids or alcohols.

• Lactic acid fermentation: Produces lactic acid (e.g., Lactobacillus).

• Alcohol fermentation: Produces ethanol and CO2 (e.g., yeast).

Photosynthesis

Light-Dependent and Light-Independent Reactions

Photosynthetic microbes convert light energy to chemical energy, fixing CO2 into organic compounds.

• Light-dependent reactions: Use pigments (chlorophyll, bacteriochlorophylls) to generate ATP and NADPH.

• Light-independent reactions (Calvin cycle): Use ATP and NADPH to fix CO2 into sugars.

• Oxygenic photosynthesis: Produces O2 (plants, algae, cyanobacteria).

• Anoxygenic photosynthesis: Uses other electron donors (e.g., H2S) and does not produce O2 (purple and green sulfur bacteria).

Metabolic Diversity and Nutritional Patterns

Classification by Energy and Carbon Source

Type	Energy Source	Carbon Source
Photoautotroph	Light	CO2
Chemoautotroph	Chemicals	CO2
Photoheterotroph	Light	Organic compounds
Chemoheterotroph	Chemicals	Organic compounds

Summary

• Metabolism consists of enzyme-catalyzed exergonic and endergonic reactions.

• Redox reactions, electron carriers, and ATP synthesis are central to energy production.

• Major metabolic pathways include glycolysis, Krebs cycle, electron transport, fermentation, and photosynthesis.

• Microbial metabolic diversity is classified by energy and carbon sources.