Submitted to: Plant Biology Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: July 20, 2006
Publication Date: August 5, 2006
Citation: Hunter, S.C., Cahoon, E.B. 2006. Vitamin e biosynthesis: functional characterization of barley homogentisate geranylgeranyl transferase. American Society of Plant Biologists Annual Meeting. Paper No. P470111. Technical Abstract: Tocopherols and tocotrienols are vitamin E compounds which function as antioxidants in vegetable oils, plants, and nutrition. Tocotrienols are found mainly in monocot seeds, while tocopherols are the principal vitamin E found in leaves and most dicot seeds. Both compounds contain a head group and a lipophilic tail, which is derived from phytyl diphosphate (PDP) in the case of tocopherols or from geranylgeranyl diphosphate (GGDP) in the case of tocotrienols. The committed step of vitamin E biosynthesis is the condensation of homogentisate with either GGDP or PDP. We previously identified an enzyme, "homogentisate geranylgeranyl transferase" (HGGT), which catalyzes the initial step of tocotrienol biosynthesis in monocot seeds. The analogous reaction for tocopherol biosynthesis is catalyzed by homogentisate phytyltransferase (HPT). We have expressed HGGT from barley endosperm (HvHGGT) and HPT from Arabidopsis (AtHPT) in Sf-21 insect cells. Relative activity with GGDP was 4- to 5-fold higher than with PDP in HvHGGT-expressing cells. In contrast, AtHPT-expressing cells had 50- to 80-fold higher activity when PDP (vs. GGDP) was the prenyl donor. To examine its in planta activity, HvHGGT was expressed in a vitamin E null mutant of Arabidopsis. The ratio of tocotrienols to tocopherols detected in these plants varied depending upon the tissue type examined, suggesting that the functional outcome of HGGT activity is also dependent upon the in vivo pool sizes of GGDP and PDP. We are currently conducting domain swaps between HvHGGT and AtHPT to determine the structural basis for their different substrate preferences for GGDP and PDP.