Farnesyl-diphosphate farnesyltransferase
class of enzymes
Squalene synthase (SQS) or farnesyl-diphosphate:farnesyl-diphosphate farnesyl transferase is an enzyme localized to the membrane of the endoplasmic reticulum. SQS participates in the isoprenoid biosynthetic pathway, catalyzing a two-step reaction in which two identical molecules of farnesyl pyrophosphate (FPP) are converted into squalene, with the consumption of NADPH. Catalysis by SQS is the first committed step in sterol synthesis, since the squalene produced is converted exclusively into various sterols, such as cholesterol, via a complex, multi-step pathway. SQS belongs to squalene/phytoene synthase family of proteins.
Diversity
Squalene synthase has been characterized in animals, plants, and yeast. In terms of structure and mechanics, squalene synthase closely resembles phytoene synthase (PHS), another prenyltransferase. PHS serves a similar role to SQS in plants and bacteria, catalyzing the synthesis of phytoene, a precursor of carotenoid compounds.
Structure
Squalene synthase (SQS) is localized exclusively to the membrane of the endoplasmic reticulum (ER). SQS is anchored to the membrane by a short C-terminal membrane-spanning domain. The N-terminal catalytic domain of the enzyme protrudes into the cytosol, where the soluble substrates are bound. Mammalian forms of SQS are approximately 47kDa and consist of ~416 amino acids.
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