<p>Proteins in this entry are members of a superfamily of phytoene and squalene synthases which catalyse the head-to-head condensation of polyisoprene pyrophosphates. They are often encoded in the same genetic locus as squalene-hopene cyclase genes, and are never associated with genes for the metabolism of phytoene. In the organisms <taxon tax_id="542">Zymomonas mobilis</taxon> and <taxon tax_id="375">Bradyrhizobium japonicum</taxon> they have been characterised as squalene synthases (farnesyl-pyrophosphate ligases) [<cite idref="PUB00042985"/>]. HpnC is often encoded in tandem with the gene for HpnD, which appears to have resulted from an ancient gene duplication event. Presumably these proteins form a heteromeric complex, but this has not yet been experimentally demonstrated.</p> Squalene synthase HpnC