GERANYLGERANYL DIPHOSPHATE SYNTHASE CATALYZING THE SINGLE CONDENSATION BETWEEN ISOPENTENYL DIPHOSPHATE AND FARNESYL DIPHOSPHATE

Geranylgeranyl diphosphate synthase was purified 191-fold from bovine brain by Mono Q column chromatography followed by preparative isoelect ric focusing electrophoresis and Superose 12 gel filtration. The synth ase had a pI value at 6.0, and it was made free of farnesyl diphosphat e synthase, the pI of which was 5.1. The partially purified enzyme cat alyzed the formation of geranylgeranyl diphosphate from isopentenyl di phosphate and farnesyl diphosphate with the K(m) values for isopenteny l diphosphate and farnesyl diphosphate being 14 and 0.8 muM, respectiv ely. Dimethylallyl diphosphate and geranyl diphosphate were poor subst rates with velocities of only 0.003 and 0.03, respectively, relative t o that of farnesyl diphosphate. These results indicate that geranylger anyl diphosphate synthase catalyzes a single condensation between isop entenyl diphosphate and farnesyl diphosphate and that farnesyl diphosp hate is the common intermediate at the branch point for the synthesis of geranylgeranylated proteins as well as cholesterol, ubiquinone, dol ichol, and farnesylated proteins. The enzyme required Mg2+ or Mn2+ for maximum activity. Octylglucoside showed a stimulatory effect on the e nzyme activity.