Microbial Metabolism: Catabolism, Electron Carriers, and Energy Production

Microbial Metabolism and Energy Production

Catabolic Pathways and Energy Release

Catabolism refers to metabolic pathways that break down macromolecules (complex molecules) into simpler molecules, releasing energy in the process. This energy is essential for cellular activities and is often stored in the form of ATP.

• Catabolic Pathway: A sequence of reactions that decompose complex molecules, releasing energy.

• Exergonic Reaction: A chemical reaction that releases energy.

• Examples: Glycolysis (breakdown of glucose), breakdown of glycogen, and the citric acid cycle.

Additional info: Catabolic reactions are crucial for providing the energy and building blocks required for anabolic (biosynthetic) processes.

Electron Carriers and ATP Generation

Electron carriers are molecules that transport electrons during cellular respiration, facilitating the production of ATP. The main electron carriers in microbial metabolism are NAD+, NADH, FAD, FADH2, and NADPH.

• NAD+ (Nicotinamide adenine dinucleotide): Accepts 2 electrons and 1 hydrogen ion to become NADH.

• FAD (Flavin adenine dinucleotide): Accepts 2 electrons and 2 hydrogen ions to become FADH2.

• NADPH: Similar to NADH, but primarily used in anabolic reactions.

• Role: These carriers shuttle electrons to the electron transport chain (ETC), where ATP is generated.

Key Equations:

• $\text{NAD}^+ + 2e^- + H^+ \rightarrow \text{NADH}$

• $\text{FAD} + 2e^- + 2H^+ \rightarrow \text{FADH}_2$

Oxidation-Reduction (Redox) Reactions

Redox reactions are fundamental to energy production in cells. Oxidation involves the loss of electrons, while reduction involves the gain of electrons.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Electron Transport Chain (ETC): A series of protein complexes in the mitochondrial membrane that transfer electrons from carriers to oxygen, generating a proton gradient used to produce ATP.

Example: During aerobic respiration, electrons from NADH and FADH2 are transferred to oxygen via the ETC, producing water and ATP.

Major Metabolic Pathways

Glycolysis

Glycolysis is the process by which glucose is broken down in the cytoplasm to produce pyruvate, ATP, and NADH. It is the first step in both aerobic and anaerobic respiration.

• Location: Cytoplasm

• Initial Substrate: Glucose

• Phases:

  • Energy Investment Phase: 2 ATP are used to phosphorylate glucose and its intermediates.

  • Energy Payoff Phase: 4 ATP and 2 NADH are produced, resulting in a net gain of 2 ATP and 2 NADH per glucose molecule.

• Net Products (per glucose): 2 pyruvate, 2 ATP, 2 NADH

Key Equation:

• $\text{Glucose} + 2\text{NAD}^+ + 2\text{ADP} + 2\text{P}_i \rightarrow 2\text{Pyruvate} + 2\text{NADH} + 2\text{ATP} + 2\text{H}_2\text{O}$

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle is a metabolic pathway in mitochondria that oxidizes acetyl-CoA, producing ATP, NADH, and FADH2. It is central to aerobic respiration.

• Location: Mitochondrial matrix (in eukaryotes)

• Initial Substrate: Acetyl-CoA

• Products (per acetyl-CoA): 3 NADH, 1 FADH2, 1 ATP (or GTP), 2 CO2

• Role: Supplies electrons to the ETC via NADH and FADH2

Key Equation:

• $\text{Acetyl-CoA} + 3\text{NAD}^+ + \text{FAD} + \text{ADP} + \text{P}_i \rightarrow 2\text{CO}_2 + 3\text{NADH} + \text{FADH}_2 + \text{ATP}$

Example: For each glucose molecule, the Krebs cycle turns twice (once for each acetyl-CoA), producing a total of 6 NADH, 2 FADH2, and 2 ATP.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

The ETC is a series of protein complexes embedded in the inner mitochondrial membrane (or plasma membrane in prokaryotes) that transfer electrons from NADH and FADH2 to oxygen, generating a proton gradient used to synthesize ATP via oxidative phosphorylation.

• Location: Inner mitochondrial membrane (eukaryotes), plasma membrane (prokaryotes)

• Final Electron Acceptor: Oxygen (aerobic respiration)

• ATP Yield: Approximately 34 ATP per glucose molecule (from ETC and oxidative phosphorylation)

Key Equation:

• $\text{NADH} + \text{FADH}_2 + O_2 + ADP + P_i \rightarrow NAD^+ + FAD + H_2O + ATP$

Fermentation

Fermentation Pathways

Fermentation is an anaerobic process that allows cells to generate ATP without oxygen by regenerating NAD+ from NADH. It results in the production of organic end products such as lactic acid or ethanol.

• Location: Cytoplasm

• Products: Lactic acid (in animals), ethanol and CO2 (in yeast)

• ATP Yield: 2 ATP per glucose (from glycolysis only)

• Electron Acceptor: Organic molecules (e.g., pyruvate)

Example: In lactic acid fermentation, pyruvate is reduced to lactic acid, regenerating NAD+ for glycolysis.

Summary Table: Major Metabolic Pathways

Pathway	Location	Main Substrate	Main Products	ATP Yield (per glucose)	Electron Carriers Produced
Glycolysis	Cytoplasm	Glucose	2 Pyruvate, 2 ATP, 2 NADH	2 (net)	2 NADH
Krebs Cycle	Mitochondrial matrix	Acetyl-CoA	4 CO2, 2 ATP, 6 NADH, 2 FADH2	2	6 NADH, 2 FADH2
ETC & Oxidative Phosphorylation	Inner mitochondrial membrane	NADH, FADH2	ATP, H2O	~34	Uses NADH, FADH2
Fermentation	Cytoplasm	Pyruvate	Lactic acid or ethanol, CO2	2	Regenerates NAD+

Key Terms and Definitions

• ATP (Adenosine Triphosphate): The primary energy currency of the cell.

• ADP (Adenosine Diphosphate): The product of ATP hydrolysis; can be converted back to ATP.

• Phosphorylation: The addition of a phosphate group to a molecule, often to ADP to form ATP.

• Substrate-level Phosphorylation: Direct transfer of a phosphate group to ADP from a substrate.

• Oxidative Phosphorylation: ATP synthesis driven by the electron transport chain and chemiosmosis.

• Electron Acceptor: A molecule that receives electrons during redox reactions (e.g., oxygen in aerobic respiration).

Summary of ATP Yield from Glucose (Aerobic Respiration)

• Glycolysis: 2 ATP (net), 2 NADH

• Krebs Cycle: 2 ATP, 6 NADH, 2 FADH2

• ETC & Oxidative Phosphorylation: ~34 ATP (from NADH and FADH2)

• Total ATP per glucose: ~38 ATP (prokaryotes), ~36 ATP (eukaryotes)

Additional info: The actual ATP yield may vary depending on cell type and conditions.