Human Squalene synthase,FDFT1 ELI

Catalyzes the condensation of 2 farnesyl pyrophosphate (FPP) moieties to form squalene.

Proceeds in two distinct steps.

In the first half-reaction, two molecules of FPP react to form the stable presqualene diphosphate intermediate (PSQPP), with concomitant release of a proton and a molecule of inorganic diphosphate.

In the second half-reaction, PSQPP undergoes heterolysis, isomerization, and reduction with NADPH or NADH to form squalene.

It is the first committed enzyme of the sterol biosynthesis pathway.

{ECO:0000269|PubMed:10896663, ECO:0000269|PubMed:24531458}.