Standard apparent reduction potentials for biochemical half reactions as a function of pH and ionic strength

Area of Science:

• Biochemistry
• Chemical Thermodynamics
• Electrochemistry

Background:

• Standard apparent reduction potentials provide a global view of redox reaction driving forces.
• Biochemical reactions are sensitive to environmental factors like pH and ionic strength.
• Understanding these dependencies is key for interpreting biological redox processes.

Purpose of the Study:

• To investigate the impact of pH on standard apparent reduction potentials for biochemical half-reactions.
• To quantify the relationship between pH, pKs, and reduction potentials within a physiological pH range (5-9).
• To analyze these effects across 19 diverse biochemical reactions, including the nitrogenase reaction.

Main Methods:

• Calculations based on known pKs of acid groups in reactants (pH 4-10).
• Analysis of the relationship between pH changes and shifts in standard apparent reduction potentials.
• Evaluation of the proportionality between pH and changes in hydrogen ion binding relative to electron transfer.

Main Results:

• Increasing pH generally decreases standard apparent reduction potentials for affected half-reactions.
• The magnitude of potential change is proportional to the ratio of hydrogen ion binding changes to electrons transferred.
• The nitrogenase reaction exhibits a significant pH dependency, with unfavorable equilibrium constants for nitrogen fixation above pH 8.

Conclusions:

• pH significantly modulates the driving forces of biochemical redox reactions.
• The pKs of reactants are critical for predicting pH-dependent potential shifts.
• These findings have implications for understanding nitrogen fixation and other pH-sensitive biological processes.