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ABSTRACT3-hydroxy-3-methylglutaryl-CoA (HMGCoA) reductase is the rate-limiting enzyme and the first committed step in the biosynthesis of cholesterol in mammals. We have determined the crystal structures of two nonproductive ternary complexes of HMG-CoA reductase, HMG-CoA͞ NAD ؉ and mevalonate͞NADH, at 2.8 Å resolution. In the structure of the Pseudomonas mevalonii apoenzyme, the last 50 residues of the C terminus (the f lap domain), including the catalytic residue His381, were not visible. The structures of the ternary complexes reported here reveal a substrate-induced closing of the f lap domain that completes the active site and aligns the catalytic histidine proximal to the thioester of HMG-CoA. The structures also present evidence that Lys267 is critically involved in catalysis and provide insights into the catalytic mechanism.The reaction catalyzed by 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA), the conversion of (S)-HMG-CoA to (R)-mevalonate, represents a major point of control for isoprenoid biogenesis (for a review, see ref. 1). Because in mammals this reaction is the first committed step in cholesterol biosynthesis, HMG-CoA reductase is a primary target enzyme for chemotherapy of hypercholesterolemias (2). The crystal structure of the HMG-CoA reductase from Pseudomonas mevalonii previously was solved at 3.0 Å resolution (3). The structure revealed a tightly bound dimer that brings together conserved residues implicated in binding and catalysis at the subunitinterface active site. Each monomer is composed of two major domains. The large domain (residues 1-108 and 220-375) binds HMG-CoA and consists of a central 24-residue ␣-helix surrounded by three roughly triangular walls. The small NAD(H) binding domain (residues 110-215) has a nonclassical dinucleotide-binding fold. This domain consists of a fourstrand antiparallel ␤-sheet with two crossover helices that lie on one side of the sheet. Connecting the third strand and the second helix in the small domain, there is a highly conserved sequence, the DAMG loop (residues 180-186), which is analogous to the G-rich loop in the classic dinucleotidebinding domain.Knowledge of the spatial location of catalytic residues is crucial for understanding the mechanism of this enzyme. However, the last 50 residues of the C terminus (377-428), which include the catalytic His381 (4, 5), were not visible in the electron density maps of the HMG-CoA reductase apoenzyme and were presumably disordered. It was proposed that these C-terminal residues form a flap domain that closes over the active site when substrates are bound (3). We have now determined the crystal structures of two nonproductive ternary complexes, HMG-CoA͞NAD ϩ and mevalonate͞NADH, at 2.8 Å resolution. The structures demonstrate that the flexible flap domain closes on binding of the substrates and positions the catalytic residue His381 close to the scissile bond of HMG-CoA, completing the active site in its catalytic conformation. MATERIALS AND METHODSCrystallization. P. mevalonii HMG-CoA reductase was crystalliz...

SummaryThe enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase catalyzes the conversion of HMG-CoA to mevalonate, a four-electron oxidoreduction that is the rate-limiting step in the synthesis of cholesterol and other isoprenoids. The enzyme is found in eukaryotes and prokaryotes; and phylogenetic analysis has revealed two classes of HMG-CoA reductase, the Class I enzymes of eukaryotes and some archaea and the Class II enzymes of eubacteria and certain other archaea. Three-dimensional structures of the catalytic domain of HMG-CoA reductases from humans and from the bacterium Pseudomonas mevalonii, in conjunction with sitedirected mutagenesis studies, have revealed details of the mechanism of catalysis. The reaction catalyzed by human HMG-CoA reductase is a target for anti-hypercholesterolemic drugs (statins), which are intended to lower cholesterol levels in serum. Eukaryotic forms of the enzyme are anchored to the endoplasmic reticulum, whereas the prokaryotic enzymes are soluble. Probably because of its critical role in cellular cholesterol homeostasis, mammalian HMG-CoA reductase is extensively regulated at the transcriptional, translational, and post-translational levels.

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