There are three ways to modulate the redox potential of a metalloenzyme: Changing ligands, changing geometry, and changing solvent. When I introduce this topic in Bioinorganic, I try to give my students concrete examples of each. I love this one because it applies what they learned in Gen Chem about the Nernst Equation to a biological problem. Granted, I don't use a metalloenzyme as my example, but I do pull the biological chemistry into it at the end, by referrring to the cytochrome oxidase/O2 couple.
In a Gen Chem class this could be used as an in class exercise or a (challenging) test question.
- A student should be able to balance a redox half-reaction in either acidic or basic conditions.
- A student should know the conditions defined as standard conditions in aqueous electrochemical cells.
- A student should understand the connection between the Nernst equation and potentials for reactions at other than standard conditions.
- A student should be able to calculate the cell potential from two half-reaction potentials.
- A student should be able to explain the connection between redox potential and strength as an oxidizing (or reducing) agent.
- A student should be able to calculate the free energy derived (or cost) from a given redox couple.
- A student should be able to relate these calculations to the affect of changing solvent (either pH, or concentrations of substrate or even availability of water in a microenvironment around the active site of a metalloenzyme) on the redox potential (and therefore the free energy of a biological redox couple).