Methylene blue is an example of an artificial hydrogen acceptor, or redox indicator.
However, in redox proteins with strong oxidizing centers oxidative decarboxylation of nearby ionized carboxylic groups may be expected to occur to some degree.
Basic concepts in equilibrium geochemistry, isotope geochemistry and redox geochemistry are covered.
We are interested in the essential trace element molybdenum, which is required as a component of the cofactors of various redox enzymes.
To understand what is happening here we need to think about redox reactions.