Journal of Biological Chemistry
Volume 275, Issue 39, 29 September 2000, Pages 30610-30617
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PROTEIN STRUCTURE AND FOLDING
Crystal Structure of Human Squalene Synthase: A KEY ENZYME IN CHOLESTEROL BIOSYNTHESIS *

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Squalene synthase catalyzes the biosynthesis of squalene, a key cholesterol precursor, through a reductive dimerization of two farnesyl diphosphate (FPP) molecules. The reaction is unique when compared with those of other FPP-utilizing enzymes and proceeds in two distinct steps, both of which involve the formation of carbocationic reaction intermediates. Because FPP is located at the final branch point in the isoprenoid biosynthesis pathway, its conversion to squalene through the action of squalene synthase represents the first committed step in the formation of cholesterol, making it an attractive target for therapeutic intervention. We have determined, for the first time, the crystal structures of recombinant human squalene synthase complexed with several different inhibitors. The structure shows that SQS is folded as a single domain, with a large channel in the middle of one face. The active sites of the two half-reactions catalyzed by the enzyme are located in the central channel, which is lined on both sides by conserved aspartate and arginine residues, which are known from mutagenesis experiments to be involved in FPP binding. One end of this channel is exposed to solvent, whereas the other end leads to a completely enclosed pocket surrounded by conserved hydrophobic residues. These observations, along with mutagenesis data identifying residues that affect substrate binding and activity, suggest that two molecules of FPP bind at one end of the channel, where the active center of the first half-reaction is located, and then the stable reaction intermediate moves into the deep pocket, where it is sequestered from solvent and the second half-reaction occurs. Five α helices surrounding the active center are structurally homologous to the active core in the three other isoprenoid biosynthetic enzymes whose crystal structures are known, even though there is no detectable sequence homology.

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Published, JBC Papers in Press, July 13, 2000, DOI 10.1074/jbc.M004132200

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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The atomic coordinates and the structure factors (code 1EZF) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/). .

Present address: Dept. of Biological Chemistry, Johns Hopkins University School of Medicine, 725 Wolfe St., Baltimore, MD 21205.