Microbial Metabolism: Electron Flow and Energy Production

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Metabolism: Overview and Key Concepts

Definition and Types of Metabolism

Metabolism is the sum of all chemical reactions occurring within a cell, divided into two main categories: catabolism and anabolism. Catabolism involves the breakdown of molecules to release energy, while anabolism uses energy to synthesize cellular components.

Catabolism: Energy-producing reactions; breakdown of organic or inorganic compounds or light energy to generate ATP and reducing power.
Anabolism: Biosynthetic, energy-consuming reactions; use of ATP and reducing power to build macromolecules and cell structures.

Diagram of anabolism and catabolism in a cell

Redox Reactions in Microbial Metabolism

Oxidation-Reduction (Redox) Reactions

Redox reactions are fundamental to energy production in cells. Oxidation is the loss of electrons, while reduction is the gain of electrons. These reactions are always coupled, with electrons transferred from a donor (which is oxidized) to an acceptor (which is reduced).

Electron donor: Substance that loses electrons (is oxidized).
Electron acceptor: Substance that gains electrons (is reduced).

Example of a redox reaction: glucose oxidation and oxygen reduction

Example: In aerobic respiration, glucose is oxidized to CO2 and O2 is reduced to H2O.

Electron Carriers and NAD+/NADH

Role of Electron Carriers

Electron carriers facilitate the transfer of electrons from donors to acceptors. They can be membrane-bound (e.g., cytochromes) or freely diffusible coenzymes (e.g., NAD+/NADH).

NAD+ (Nicotinamide adenine dinucleotide): Accepts electrons and protons to become NADH.
NADH: Donates electrons in metabolic pathways, especially in respiration.

Structure of NAD+ and its components

NAD+/NADH Cycling

NAD+ and NADH cycle between oxidized and reduced forms, shuttling electrons during metabolic reactions. This cycling is essential for maintaining redox balance in the cell.

NAD+/NADH cycling in enzymatic reactions

Energy Storage and ATP Generation

Short-Term and Long-Term Energy Storage

Cells store energy in various forms for immediate or future use:

Short-term: ATP (adenosine triphosphate) and derivatives of coenzyme A (thioester bonds).
Long-term: Glycogen, poly-β-hydroxybutyrate, and elemental sulfur.

Short and long term energy storage molecules Structure of poly-β-hydroxybutyrate

ATP Synthesis Mechanisms

ATP is the primary energy currency of the cell, generated by three main mechanisms:

Substrate-level phosphorylation: Direct transfer of a phosphate group from a high-energy substrate to ADP.
Oxidative phosphorylation: ATP synthesis driven by the dissipation of the proton motive force across a membrane, catalyzed by ATP synthase.
Photophosphorylation: Light-driven ATP synthesis in phototrophs, similar to oxidative phosphorylation but powered by light energy.

Substrate-level phosphorylation pathway Oxidative phosphorylation and photophosphorylation

Glycolysis and Fermentation

Glycolysis: Pathway and Key Steps

Glycolysis is the central pathway for glucose catabolism, converting glucose to pyruvate and generating ATP and NADH. It occurs in the cytoplasm and is used by many microorganisms.

Net yield per glucose: 2 ATP, 2 NADH, 2 pyruvate.
Key steps: Substrate-level phosphorylation (steps 7 and 10), NAD+ reduction (step 6).

Major pathway of glucose metabolism (glycolysis) Step 6 of glycolysis: NAD+ reduction Phosphorylation steps in glycolysis Key inputs and outputs of glycolysis

Fermentation

Fermentation is an anaerobic process where organic compounds serve as both electron donors and acceptors. It regenerates NAD+ and produces ATP via substrate-level phosphorylation, yielding various end products such as acids, alcohols, and gases.

Homolactic fermentation: Produces lactic acid (e.g., Streptococcus).
Alcohol fermentation: Produces ethanol and CO2 (e.g., yeast).
Heterolactic fermentation: Produces lactic acid, ethanol, and CO2 (e.g., Leuconostoc).

Alcohol fermentation pathway Homolactic fermentation pathway Heterolactic fermentation pathway

Common Fermentation Pathways

Microorganisms utilize diverse fermentation pathways, each with characteristic substrates, products, and energy yields.

Type	Reaction	Energy Yield (kJ/mol)	Organisms
Alcoholic	Hexose → 2 ethanol + 2 CO2	-239	Yeast, Zymomonas
Homolactic	Hexose → 2 lactate	-196	Streptococcus, Lactococcus
Heterolactic	Hexose → lactate + ethanol + CO2 + H2	-119	Leuconostoc, some Lactobacillus
Mixed acid	Hexose → lactate + acetate + succinate + formate + ethanol + CO2 + H2	-202	Escherichia, Salmonella, Shigella
Propionic acid	Hexose → propionate + acetate + CO2 + H2O	-254	Propionibacterium
Butyric acid	Hexose → butyrate + acetate + CO2 + H2	-293	Clostridium butyricum
Butanol	Hexose → butanol + acetone + CO2 + H2	-254	Clostridium acetobutylicum

Table of common fermentations

Respiration: Aerobic and Anaerobic

Overview of Respiration

Respiration is a process where electrons from an electron donor are transferred through an electron transport chain to a terminal electron acceptor, generating a proton motive force used to synthesize ATP via oxidative phosphorylation. Aerobic respiration uses O2 as the terminal acceptor, while anaerobic respiration uses other acceptors such as nitrate or sulfate.

Aerobic respiration: Complete oxidation of glucose to CO2; yields up to 38 ATP per glucose.
Anaerobic respiration: Uses alternative electron acceptors; yields less ATP than aerobic respiration.

Energetics balance sheet for aerobic respiration

Electron Transport Chain and ATP Synthase

Electron Carriers in the Electron Transport Chain

The electron transport chain (ETC) consists of a series of protein and non-protein electron carriers, including NADH dehydrogenases, flavoproteins, iron-sulfur proteins, cytochromes, and quinones. Electrons flow through these carriers, releasing energy used to pump protons across the membrane, creating a proton motive force.

NADH dehydrogenase: Accepts electrons from NADH.
Flavoproteins: Contain FMN or FAD as prosthetic groups.
Iron-sulfur proteins: Transfer electrons via iron atoms.
Cytochromes: Contain heme groups with iron.
Quinones: Lipid-soluble electron carriers.

Structure of cytochrome protein Porphyrin ring in cytochrome Iron-sulfur protein structure Quinone structure

ATP Synthase and Chemiosmosis

ATP synthase (also called ATPase) is a membrane-bound enzyme complex that synthesizes ATP from ADP and inorganic phosphate, powered by the flow of protons down their electrochemical gradient (proton motive force). This process is known as chemiosmosis.

ATP synthase structure and chemiosmosis

Summary Table: Fermentation vs. Respiration

Process	ATP Yield per Glucose	Electron Acceptor	Key Features
Fermentation	2	Organic compound (endogenous)	Substrate-level phosphorylation only; incomplete oxidation
Respiration (Aerobic)	~38	O2	Substrate-level and oxidative phosphorylation; complete oxidation
Respiration (Anaerobic)	Varies (<38)	Nitrate, sulfate, etc.	Substrate-level and oxidative phosphorylation; incomplete oxidation

Additional info: These notes cover core concepts from Chapter 3 (Microbial Metabolism) and related sections on electron flow, energy production, and fermentation, as outlined in standard microbiology curricula.