Microbial Metabolism: Redox Reactions, Energy Conservation, and Central Metabolic Pathways

Microbial Metabolism

Introduction to Microbial Metabolism

Microbial metabolism encompasses the chemical processes that occur within microorganisms to maintain life. These processes include the acquisition and use of energy, the synthesis of cellular components, and the regulation of metabolic pathways. Understanding microbial metabolism is fundamental to microbiology, as it explains how microbes grow, interact with their environment, and contribute to global biogeochemical cycles.

• Metabolism refers to all biochemical reactions occurring in a cell.

• Microbes utilize diverse metabolic strategies to obtain energy and carbon.

• Central metabolic pathways are highly conserved among different organisms.

Redox Reactions and Energy Conservation

Standard Redox Potential (E0')

Redox (reduction-oxidation) reactions are central to microbial energy conservation. The standard redox potential (E0') measures a molecule's tendency to donate or accept electrons under standard conditions.

• Electron Donating: A more negative E0' indicates a better electron donor.

• Electron Accepting: A more positive E0' indicates a better electron acceptor.

• Pairs of electron donor and acceptor are called conjugate redox pairs.

• In a redox reaction, the electron donor is oxidized and the electron acceptor is reduced.

• Example half-reactions:

  • H2 → 2e- + 2H+ (donor)

  • O2 + 2e- → 2O2- (acceptor)

Electron Movement and Reduction Potential

The movement of electrons from donors to acceptors is associated with changes in free energy, which can be harnessed for cellular work.

• The difference in E0' between donor and acceptor determines the change in Gibbs free energy (ΔG°').

• Equation for free energy change: $\Delta G^{\circ'} = -nF[\Delta E_0']$ Where:

  • $\Delta G^{\circ'}$ = Gibbs free energy change

  • n = number of electrons transferred

  • F = Faraday’s constant (96.5 kJ/V·mol)

  • $\Delta E_0'$ = difference in standard redox potential

• Electrons flow spontaneously from lower to higher E0' (from better donors to better acceptors).

• Example: NADH (E0' = -0.32 V) donates electrons to O2 (E0' = +0.815 V), yielding a large ΔE0' and significant free energy release.

Electron Carriers

Electron carriers such as NAD+ and FAD are essential for transferring electrons during metabolic reactions.

• NAD+/NADH and FAD/FADH2 act as coenzymes in redox reactions.

• They shuttle electrons between metabolic pathways and the electron transport chain.

• NADPH/NADP+ is primarily used in anabolic (biosynthetic) reactions.

• Electron carriers are recycled and not consumed in the process.

ATP and Energy Coupling

ATP Hydrolysis and Free Energy

ATP (adenosine triphosphate) is the universal energy currency in cells. Its hydrolysis releases free energy that can drive endergonic (energy-requiring) reactions.

• ATP hydrolysis reaction: $\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P_i} + \mathrm{H}^+$ $\Delta G^{\circ'} = -7.3$ kcal/mol

• Exergonic breakdown of ATP is coupled to endergonic reactions to make them energetically favorable.

• Example: Glucose phosphorylation $\mathrm{Glucose} + \mathrm{P_i} \rightarrow \mathrm{Glucose-6-phosphate}$ $\Delta G^{\circ'} = +3.3$ kcal/mol Coupled with ATP hydrolysis, the net ΔG°' is negative, allowing the reaction to proceed.

Types of Microbial Metabolism

Classification by Energy Source

Microorganisms are classified based on their energy and electron sources.

• Chemotrophs: Obtain energy from chemical compounds.

• Chemoorganotrophs: Use organic compounds (e.g., glucose, acetate) as energy and electron sources.

• Chemolithotrophs: Use inorganic compounds (e.g., H2S, Fe2+, NH3) as energy and electron sources.

• Examples:

  • Escherichia coli (chemoorganotroph)

  • Thiobacillus thioparus (chemolithotroph)

Central Metabolic Pathways

Overview of Central Metabolism

Central metabolism includes the core pathways that process carbohydrates, proteins, and lipids to generate energy and biosynthetic precursors.

• Main pathways: Glycolysis, Citric Acid Cycle (TCA/Krebs), and Pentose Phosphate Pathway.

• Pathways are highly regulated and interconnected.

• Each step is catalyzed by a specific enzyme or ribozyme.

Glycolysis (Embden–Meyerhof–Parnas Pathway)

Glycolysis is a universal pathway for glucose catabolism, converting glucose to pyruvate and generating ATP and NADH.

• Occurs in the cytoplasm of both prokaryotes and eukaryotes.

• Consists of two stages:

  • Preparatory (Investment) Phase: Glucose is phosphorylated and split into two 3-carbon molecules, consuming 2 ATP.

  • Payoff (Energy-yielding) Phase: 3-carbon molecules are oxidized to pyruvate, producing 4 ATP and 2 NADH.

• Net yield per glucose: 2 ATP (substrate-level phosphorylation), 2 NADH, 2 pyruvate.

Fate of Pyruvate and NADH

The end products of glycolysis (pyruvate and NADH) can be further metabolized via fermentation or respiration, depending on the organism and environmental conditions.

• Fermentation: Incomplete oxidation of pyruvate; electrons from NADH are transferred to organic molecules derived from pyruvate (e.g., lactate, ethanol).

• Respiration: Complete oxidation of pyruvate to CO2 via the citric acid cycle; electrons from NADH are transferred to inorganic terminal electron acceptors (e.g., O2, NO3-, SO42-).

• Aerobic respiration: O2 is the terminal electron acceptor.

• Anaerobic respiration: Other inorganic molecules serve as terminal electron acceptors.

Citric Acid Cycle (TCA/Krebs Cycle)

The citric acid cycle completes the oxidation of pyruvate to CO2, generating NADH, FADH2, and GTP/ATP.

• Pyruvate is decarboxylated to acetyl-CoA, which enters the cycle.

• Each turn of the cycle (per pyruvate) yields:

  • 3 CO2

  • 4 NADH

  • 1 FADH2

  • 1 GTP (or ATP)

• Per glucose (2 pyruvate): 6 CO2, 8 NADH, 2 FADH2, 2 GTP/ATP.

• Much greater ATP yield than fermentation (up to 38 ATP vs. 2 ATP per glucose).

Electron Transport Chain and ATP Yield

NADH and FADH2 produced in glycolysis and the citric acid cycle are oxidized in the electron transport chain, driving ATP synthesis via oxidative phosphorylation.

• Electrons are transferred through a series of membrane-bound carriers to the terminal electron acceptor.

• Proton motive force generated across the membrane is used by ATP synthase to produce ATP.

• Overall ATP yield from aerobic respiration can reach up to 38 ATP per glucose molecule.

Summary Table: ATP and Reducing Equivalent Yields

Pathway	ATP (net)	NADH	FADH2	CO2
Glycolysis (per glucose)	2	2	0	0
Citric Acid Cycle (per glucose)	2	8	2	6
Total (Aerobic Respiration)	~38	10	2	6

Additional info: The above table summarizes the main energy and reducing equivalent yields from central metabolic pathways in bacteria. Actual ATP yield may vary depending on organism and conditions.