(S)-Geranylgeranylglyceryl phosphate synthase. Purification and characterization of the first pathway-specific enzyme in archaebacterial membrane lipid biosynthesis

The first pathway-specific step in the biosynthesis of the core membrane diether lipids in archaebacteria is the alkylation of the primary hydroxyl group in (S)-glyceryl phosphate by geranylgeranyl diphosphate. The reaction is catalyzed by (S)-3-O-geranylgeranylglyceryl phosphate ((S)-GGGP) synthase. The cytosolic enzyme was purified to homogeneity from the moderately thermophilic archaebacterium Methanobacterium thermoautotrophicum by a combination of ammonium sulfate precipitation, four chromatographic steps (DE52, Q-Sepharose, phenyl-Superose, and Protein Pak), and native polyacrylamide gel electrophoresis. SDS-polyacrylamide gel electrophoresis of gel-purified GGGP synthase gave a single band at 29 kDa. The enzyme requires Mg2+ for optimal activity, although prenyltransfer is also seen in buffers containing Mn2+ or Zn2+. A well defined pH optimum occurs between 6.0 and 7.5. Maximal activity is seen at 50-65 degrees C. The Michaelis constants for GGGP synthase are Vmax = 4.1 +/- 0.5 mumol min-1 mg-1, KMGGPP = 4.1 +/- 1.1 microM, and KMGP = 41 +/- 5 microM.