We don't have synonyms for isoenzyme yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are coenzyme, apoenzyme and proenzyme.
(n) :
(biochemistry) Any of a group of enzymes that catalyze the same reaction but have structures that are (minorly or substantially) different and have physical, biochemical, and immunological properties that are (minorly or substantially) different.
(n) :
(biochemistry) Any of a group of enzymes that catalyze the same reaction but have structures that are (minorly or substantially) different and have physical, biochemical, and immunological properties that are (minorly or substantially) different.