Substrate specificities of medium-prenylchain elongating enzymes, hexaprenyl- and heptaprenyl diphosphate synthases

Abstract

In order to investigate substrate specificities of medium-prenylchain elongating enzymes, hexaprenyl diphosphate (HexPP) synthase from Micrococcus luteus B-P 26 and heptaprenyl diphosphate (HepPP) synthase from Bacillus subtilis, we examined the reactivities of 3-alkyl group homologs of allylic or homoallylic substrates for these enzymes. Only one molecule of but-3-enyl diphosphate ( 2b), which lacks a methyl group at the 3-position of isopentenyl diphosphate ( 2a) condensed with farnesyl diphosphate ( 1, FPP) to give E-norgeranylgeranyl diphosphate in the reaction catalyzed by either enzyme. 3-Ethylbut-3-enyl diphosphate ( 2c) reacted as a homoallylic substrate with FPP to give a mixture of (all- E)-3-ethyl-7,11,15-trimethylhexadeca-2,6,10,14-tetraenyl- and (all- E)-3,7-diethyl-11,15,19-trimethyleicosa-2,6,10,14,18-pentaenyl diphosphates by the condensation with one or two molecules of ( 2c), respectively, by the reaction of HexPP synthase. 3-Propylbut-3-enyl diphosphate ( 2d) also reacted with FPP to give a mixture of the corresponding single and double condensation products, respectively, by the action of the same synthase. On the other hand, HepPP synthase reaction of ( 2c) or ( 2d) with FPP gave (all- E)-3-ethyl-7,11,15-trimethylhexadeca-2,6,10,14-tetraenyl diphosphate or (all- E)-3-propyl-7,11,15-trimethylhexadeca-2,6,10,14-tetraenyl diphosphate as a single product, respectively. However, 3-butylbut-3-enyl diphosphate ( 2e), norfarnesyl diphosphate ( 1a) and norgeranylgeranyl diphosphate ( 1b) were never accepted as substrates by either of the enzymes at all.