Requires Mg2+ for activity [3]. Involved in the biosynthesis of the vitamin E tocopherols. While the enzyme from the cyanobacterium Synechocystis PCC 6803 has an appreciable activity with geranylgeranyl diphosphate (EC 2.5.1.116, homogentisate geranylgeranyltransferase), the enzyme from the plant Arabidopsis thaliana has only a low activity with that substrate [1,3,4].