Complete saturation of the geranylgeranyl groups of biosynthetic interme-diates of archaeal membrane lipids is an important reaction that confers chemical stability on the lipids of archaea, which generally inhabit extreme conditions. An enzyme encoded by theAF0464gene of a hyperthermophi-lic archaeon, Archaeoglobus fulgidus, which is a distant homologue of plant geranylgeranyl reductases and an A.