5.3.3.2 metabolism IPP isomerase activity is a rate-limiting step in terpenoid synthesis 5.3.3.2 metabolism the enzyme plays a critical role in terpenoid metabolic pathways 5.3.3.2 metabolism the plastidial enzyme plays an important function in carotenoid biosynthesis 5.3.3.2 physiological function involved in terpenoid metabolism 5.3.3.2 physiological function isopentenyl diphosphate and dimethylallyl diphosphate are essential precursors for terpenoid biosynthesis