<p>This family of sequences describe farnesyl-diphosphate farnesyltransferase, also known as squalene synthase, as found in eukaryotes. This family is related to phytoene synthases. Tentatively identified archaeal homologs lack the C-terminal predicted transmembrane region universally conserved among members of this family and therefore are not included in this group. </p><p>These enzymes catalyse the formation of squalene by the reductive dimerisation of two farnesyl diphosphate molecules in a two-step reaction. This reaction occurs at the final branch point of the isoprenoid biosynthesis pathway, and is the first committed step in cholesterol biosynthesis.</p><reaction> 2 farnesyl diphosphate ---&gt; presqualene diphosphate ---&gt; squalene </reaction><p>The human enzyme (<db_xref db="SWISSPROT" dbkey="P37268"/>) is a membrane-bound monomer which folds as a single domain [<cite idref="PUB00017047"/>]. A large channel surrounded by five alpha helices runs through the centre of the protein. One end of this channel is exposed to solvent, while the other is completely enclosed and surrounded by hydrophobic residues. The two half reactions occur at different active sites in this channel. The first half reaction occurs at one end of the channel, then the intermediate is thought to move into the enclosed pocket where the second half reaction occurs.</p> Farnesyl-diphosphate farnesyltransferase