Microbial Metabolism: Oxidation-Reduction, ATP Generation, and Carbohydrate Catabolism

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Microbial Metabolism

Oxidation-Reduction Reactions

Oxidation-reduction (redox) reactions are fundamental to microbial metabolism, involving the transfer of electrons between molecules. These reactions are paired: one molecule is oxidized (loses electrons), while another is reduced (gains electrons). In biological systems, electrons are often transferred along with hydrogen atoms, and biological oxidations are frequently dehydrogenations.

Oxidation: Removal of electrons from a molecule.
Reduction: Addition of electrons to a molecule.
Redox Reaction: An oxidation reaction paired with a reduction reaction.
Dehydrogenation: Removal of hydrogen atoms (and their electrons) from a molecule.

Diagram of electron transfer from molecule A to molecule B Diagram showing biological oxidation and reduction involving hydrogen atoms and NAD+

Electron Carriers and NAD+/NADH

Electron carriers such as NAD+ (nicotinamide adenine dinucleotide) play a crucial role in redox reactions. NAD+ accepts electrons (and a proton) to become NADH, which can then donate electrons in other metabolic pathways.

NAD+: Oxidized form, accepts electrons.
NADH: Reduced form, donates electrons.
Example: In the conversion of lactate to pyruvate, NAD+ is reduced to NADH.

Lactate to pyruvate oxidation-reduction reaction

ATP Generation and Phosphorylation

ATP: The Energy Currency

ATP (adenosine triphosphate) is the primary molecule for energy storage and transfer in cells. It is generated by the phosphorylation of ADP (adenosine diphosphate) with the input of energy.

ATP Formation: ADP + Pi + Energy → ATP
Phosphorylation: Addition of a phosphate group to ADP.

Equation:

ADP to ATP phosphorylation diagram

Types of Phosphorylation

Substrate-level phosphorylation: ATP is generated when a high-energy phosphate group is directly transferred from a substrate to ADP.
Oxidative phosphorylation: Electrons are transferred through an electron transport chain, releasing energy to generate ATP.
Photophosphorylation: Occurs in photosynthetic cells; light energy is used to generate ATP.

Electron transport chain in oxidative phosphorylation Photophosphorylation in thylakoid membrane

Carbohydrate Catabolism: Respiration and Fermentation

Overview of Metabolic Pathways

The breakdown of carbohydrates to release energy occurs in three principal stages: glycolysis, the Krebs cycle, and the electron transport chain. These pathways are central to both respiration and fermentation.

Glycolysis: Oxidation of glucose to pyruvic acid, producing ATP and NADH.
Krebs Cycle: Further oxidation of pyruvic acid, generating more NADH, FADH2, and ATP.
Electron Transport Chain: Electrons from NADH and FADH2 are transferred through a series of carriers, resulting in ATP production.

Overview of respiration and fermentation pathways

Glycolysis

Glycolysis is the first stage of carbohydrate catabolism, converting glucose into pyruvic acid. It consists of two main phases: the preparatory stage and the energy-conserving stage.

Preparatory Stage: 2 ATP are used to split glucose into two 3-carbon molecules.
Energy-Conserving Stage: The 3-carbon molecules are oxidized, producing 4 ATP and 2 NADH, resulting in a net gain of 2 ATP per glucose.

Glycolysis overview: glucose to pyruvic acid, ATP and NADH Preparatory stage of glycolysis Energy-conserving stage of glycolysis Glyceraldehyde 3-phosphate (GP) structure Summary diagram of glycolysis stages

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid cycle (TCA), is a series of reactions that further oxidize pyruvic acid, producing NADH, FADH2, ATP, and CO2.

Acetyl-CoA Formation: Pyruvic acid is converted to acetyl-CoA, releasing NADH and CO2.
Cycle Reactions: Acetyl-CoA enters the cycle, generating NADH, FADH2, ATP, and CO2 through a series of enzyme-catalyzed steps.

Acetyl-CoA formation from pyruvic acid Krebs cycle diagram

Electron Transport Chain (ETC)

The electron transport chain is a series of membrane-bound carriers that transfer electrons from NADH and FADH2 to a final electron acceptor, generating ATP through oxidative phosphorylation.

Electron Flow: Electrons move from high-energy carriers to lower-energy carriers, releasing energy.
ATP Synthesis: Energy released is used to pump protons across the membrane, creating a gradient that drives ATP synthesis.
Final Electron Acceptor: In aerobic respiration, O2 is the final acceptor; in anaerobic respiration, other molecules are used.

Electron transport chain overview Electron transport chain in prokaryotes and eukaryotes

ATP Yield from Respiration

The total ATP yield from aerobic respiration of one glucose molecule in prokaryotes is typically 38 ATP. Each NADH can produce 3 ATP, and each FADH2 can produce 2 ATP via the electron transport chain.

Glycolysis: 2 ATP (substrate-level), 6 ATP (oxidative phosphorylation).
Krebs Cycle: 2 ATP (substrate-level), 22 ATP (oxidative phosphorylation).
Total: 38 ATP per glucose.

Source	ATP Yield (Method)
Glycolysis	2 ATP (substrate-level), 6 ATP (oxidative phosphorylation)
Preparatory Step	6 ATP (oxidative phosphorylation)
Krebs Cycle	2 ATP (substrate-level), 18 ATP (oxidative phosphorylation), 4 ATP (oxidative phosphorylation)
Total	38 ATP

ATP yield from respiration diagram Table of ATP yield during prokaryotic aerobic respiration

Aerobic vs Anaerobic Respiration

Aerobic respiration uses molecular oxygen (O2) as the final electron acceptor in the electron transport chain, yielding more energy. Anaerobic respiration uses other molecules as the final electron acceptor and yields less energy, as only part of the Krebs cycle operates under anaerobic conditions.

Aerobic Respiration: Final electron acceptor is O2.
Anaerobic Respiration: Final electron acceptor is not O2 (e.g., nitrate, sulfate).
Energy Yield: Anaerobic respiration yields less ATP than aerobic respiration.

Example: Facultative anaerobes can switch between aerobic and anaerobic respiration depending on oxygen availability.