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. Catabolic reactions break down molecules to release energy, while anabolic reactions use energy to build cellular components.

Catabolism: Energy-producing processes that degrade organic or inorganic compounds, or use light, to generate energy for cellular activities. Waste products such as acids, alcohols, gases, and reduced electron acceptors are produced.
Anabolism: Biosynthetic, energy-consuming processes that use nutrients and energy to synthesize macromolecules and other cell components.

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: an electron donor transfers electrons to an electron acceptor, resulting in the donor being oxidized and the acceptor being reduced.

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

Example of a redox reaction: glucose oxidation and oxygen reduction

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): A key electron carrier that cycles between oxidized (NAD+) and reduced (NADH) forms.
NADH: The reduced form, which carries electrons to the electron transport chain or other acceptors.

Structure of NAD+ and NADH

NAD+/NADH Cycling

NAD+ accepts electrons during catabolic reactions, becoming NADH. NADH can then donate electrons in other reactions, regenerating NAD+ and allowing the cycle to continue. This cycling is essential for maintaining redox balance in the cell.

NAD+/NADH cycling in enzymatic reactions

Energy Storage and ATP Production

Short-Term and Long-Term Energy Storage

Cells store energy in various forms to ensure a constant supply for cellular processes.

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

Short and long term energy stores in cells Structure of poly-β-hydroxybutyrate

ATP Synthesis: Types of Phosphorylation

ATP is synthesized by three main mechanisms:

Substrate-level phosphorylation: A phosphate group is transferred from a high-energy substrate directly to ADP, forming ATP.
Oxidative phosphorylation: ATP is produced by ATP synthase using energy from a proton motive force generated by electron transport chains.
Photophosphorylation: Light energy drives the formation of ATP via a proton motive force in phototrophic organisms.

Substrate-level phosphorylation pathway Oxidative phosphorylation and photophosphorylation

Glycolysis and Fermentation

Glycolysis: Pathway and Key Steps

Glycolysis is the central pathway for glucose catabolism, occurring in the cytoplasm. It converts one glucose molecule into two pyruvate molecules, generating ATP and NADH.

Net yield: 2 ATP and 2 NADH per glucose.
Key steps: Substrate-level phosphorylation occurs at steps 7 and 10; NAD+ is reduced to NADH at step 6.

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

Fermentation

Fermentation is an anaerobic process where organic compounds serve as both electron donors and acceptors. ATP is generated solely by substrate-level phosphorylation, and fermentation products (e.g., acids, alcohols, gases) are excreted.

Homolactic fermentation: Produces lactic acid (e.g., Streptococcus, Lactococcus).
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 Types

Fermentation pathways vary among microorganisms, producing different end products and energy yields.

Type	Reaction	Energy Yield (kJ/mol)	Organisms
Alcoholic	Glucose → 2 ethanol + 2 CO2	-218	Yeast, Zymomonas
Homolactic	Glucose → 2 lactate	-196	Lactococcus, Streptococcus
Heterolactic	Glucose → lactate + ethanol + CO2 + H2	-119	Leuconostoc, some Lactobacillus
Propionic acid	Glucose → propionate + acetate + CO2 + H2	-254	Propionibacterium
Mixed acid	Glucose → ethanol + acetate + formate + CO2 + H2	-202	Escherichia, Salmonella, Shigella
Butyric acid	Glucose → butyrate + acetate + CO2 + H2	-254	Clostridium butyricum
Butanol	Glucose → butanol + acetone + CO2 + H2	-254	Clostridium acetobutylicum

Table of common fermentations

Respiration: Aerobic and Anaerobic

Overview of Respiration

Respiration involves the complete oxidation of substrates (e.g., glucose) to CO2, with electrons transferred to a terminal electron acceptor via an electron transport chain. This process generates a proton motive force used by ATP synthase to produce ATP (oxidative phosphorylation).

Aerobic respiration: Oxygen is the terminal electron acceptor.
Anaerobic respiration: Other inorganic molecules (e.g., nitrate, sulfate) serve as terminal electron acceptors.

Energetics balance sheet for aerobic respiration

Electron Transport Chain (ETC)

The ETC consists of a series of protein and non-protein electron carriers that transfer electrons from NADH and FADH2 to the terminal electron acceptor, generating a proton gradient across the membrane.

NADH dehydrogenases: Accept electrons from NADH and transfer them to flavoproteins.
Flavoproteins (FMN, FAD): Accept and donate electrons and protons.
Iron-sulfur proteins: Transfer electrons via iron atoms.
Cytochromes: Contain heme groups for electron transfer.
Quinones: Non-protein carriers that shuttle electrons and protons.

Structure of cytochrome protein Structure of coenzyme Q (quinone)

Proton Motive Force and ATP Synthase

The ETC generates a proton motive force (PMF) by pumping protons across the membrane. ATP synthase uses the PMF to synthesize ATP from ADP and inorganic phosphate.

ATP synthase and chemiosmosis

Summary Table: Fermentation vs. Respiration

Feature	Fermentation	Respiration
ATP Yield	Low (2 ATP/glucose)	High (up to 38 ATP/glucose)
Electron Acceptor	Endogenous (organic)	Exogenous (O2 or other inorganic)
Pathway	Substrate-level phosphorylation	Oxidative phosphorylation
End Products	Acids, alcohols, gases	CO2, H2O (aerobic)

Key Terms and Concepts

Catabolism: Breakdown of molecules to release energy.
Anabolism: Synthesis of cellular components using energy.
Redox reaction: Coupled oxidation and reduction reactions.
Electron carrier: Molecule that transfers electrons (e.g., NAD+/NADH, FAD/FADH2).
ATP: Main energy currency of the cell.
Substrate-level phosphorylation: Direct transfer of phosphate to ADP.
Oxidative phosphorylation: ATP synthesis using energy from electron transport and PMF.
Fermentation: Anaerobic process generating ATP and fermentation products.
Respiration: Complete oxidation of substrates with an external electron acceptor.