PP8 redox run pt.2

Microbes utilize various methods for survival in different environments through redox reactions.
- Key components: different electron donors and electron acceptors.
- Main focus: the energy yield from specific reductants and oxidants.

Microbes prefer powerful reductants and effective oxidants to maximize energy harvest from reactions.
Objective: Quantification of energy obtained from specific redox reactions.

Definition: E°' is the experimentally determined likelihood of a molecule accepting electrons.
Characteristics: Higher E°' indicates a stronger tendency to accept electrons.
Example: Molecular oxygen is typically the best oxidant in non-photosynthetic conditions.

When oxygen is unavailable, nitrate is used but is less effective in energy yield.
Protons are poor electron acceptors with negative E°'.

E°' signifies conditions at 1 M concentrations, pH 7, and 25°C.
Comparison with E°: E° refers to conditions where all reactants are at 1 M—less applicable to life forms.
Importance of E°': Reflects organisms operating under natural conditions (not typically 1 M).

Utilization of a redox tower to visualize E°' values for various half-reactions.
Structure: Oxidized species - Reduced species; E°' value; number of electrons.
- Example: Nitrate/Nitrite reaction: +0.42 V, requires 2 electrons.
- Oxygen to water: +0.82 V, requires 2 electrons.

Positive E°' values signify good oxidants (e.g., oxygen), negative values denote good reductants (e.g., glucose).
Energy yield correlates with distance on the redox tower: greater distance equals more energy obtainable.

Method: Assess the difference in E°' between the oxidant and reductant to find ΔE°'.
Example with Hydrogen and Oxygen: Combining half-reactions to derive a balanced overall reaction.
- Need to balance charges and atoms to produce: H_2 + O_2
  ightarrow 2H_2O

Key Equation: ext{ΔG°'} = -nF ext{ΔE°'}
- Where:
- n = number of electrons transferred,.
- F = Faraday's constant (charge per mole of electrons).
Immediate effect of doubling electron transfer: energy released (ΔG°') increases proportionately.

Energy captured and utilized via two primary forms:
- ATP production by cleavage of high-energy bonds.
- Proton motive force (PMF) across membranes for ATP synthase activity.

Structured in four major complexes, differing among species based on terminal electron acceptor.
- Primary involvement of NADH and succinate (from TCA cycle) as electron donors.
- Key steps involve transferring electrons via intermediate carriers (e.g., quinones).

NADH - primary electron donor for complex one.
- Reversibly reduced from NAD+, enabling electron donation to the chain.
Flavins - can be reduced by hydride or via one-electron transfers allowing it to connect two-electron and one-electron processes.
Cytochromes - contain heme groups that facilitate one-electron transfer.
Iron-Sulfur Clusters - allow single-electron transfers, depending on redox environment (tunable potentials).
Quinones - act as mediators within the membrane, transferring electrons through one-electron and two-electron processes.

Illustrates how protons are translocated across membranes due to electron transfer dynamics.
Electron Bifurcation: One electron is sent uphill (energetically unfavorable) while another goes downhill, facilitating energy capture for proton translocation.
- Result: More protons outside the membrane facilitates ATP synthase.

Fermentation allows recycling of NAD+ by using internal organic molecules as electron acceptors when external ones are unavailable.
- Common example: Glucose converting to lactate in yogurt bacteria.
- Outcome: Less energy yield compared to respiration due to less complete oxidation.

The process recycles NAD+ but results in less ATP production compared to when terminal oxidants are available.
- Distinct fermentation products arise depending on specific organisms and conditions.

Metabolic pathways are tailored to environmental conditions utilizing various electron donors and acceptors to maximize energy efficiency for microbial survival.