Figure 1. The reaction catalyzed by the precorrin reductase CobK in the context of the eight methyltransferase reactions leading from uroporphyrinogen III to hydrogenobyrinic acid which is subsequently converted to cobalamin (vitamin B12).

Precorrin-6A is transformed into precorrin-6B the substrate for the methyltransferase CobL in a reaction dependent on the coenzyme NADPH.