The 3-hydroxy-3-methylglutaryl coenzyme-A (HMG-CoA) reductases

Abstract

The 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 site-directed 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.

Figure 1

Schematic representation of the human hmgr gene and the human HMGR_H and P. mevalonii HMGR_P proteins. (a) The exon-intron structure of the human hmgr gene, which extends from position 74717172 to position 74741998 of the human genome; positions refer to the Ensembl Transcript ID for the human hmgr gene (ENST00000287936 [22]). The numbers indicate the start and end of each exon and intron and refer to the position in the human genome sequence, omitting the first three digits (747); exons are indicated as filled boxes. Exon 1 is an untranslated region (UTR), as are the last 1,758 nucleotides of exon 20. The exons encoding the membrane-anchor domain, a flexible linker region, and the catalytic domain are indicated below the gene structure. (b) Human HMGR protein (HMGR_H) is comprised of three domains: the membrane anchor domain, a linker domain, and a catalytic domain; within the catalytic domain subdomains have been defined. The N domain connects the L domain to the linker domain; the L domain contains an HMG-CoA binding region; and the S domain functions to bind NADP(H). The cis-loop (indicated by an asterisk), a region present only in HMGR_H but not HMGR_P, connects the HMG-CoA-binding region with the NADPH-binding region. (c) The HMGR_P protein does not contain the membrane-anchor domain or the linker domain but has a catalytic domain containing a large domain with an HMG-CoA binding region, and a small, NAD(H)-binding domain. The active site of HMG-CoA reductase is present at the homodimer interface between one monomer that binds the nicotinamide dinucleotide and a second monomer that binds HMG-CoA. The numbers underneath the diagrams in (b,c) denote amino acids (in the single-letter amino-acid code) that are implicated in catalysis; S872 of HMGR_H is reversibly phosphorylated. At the extreme carboxyl terminus of each enzyme is a flap domain (approximately 50 amino acids in HMGR_P and 25-30 amino acids in HMGR_H) that closes over the active site during catalysis; the flap domain is indicated by shading in (b,c).

Figure 2

A phylogenetic tree of HMGRs. The tree includes 98 selected organisms that have hmgr genes; for plants, which have multiple isoforms, only isoform 1 of each species is included in the tree. Roman numerals indicate the division of the family into two classes [2,3]. Phylogenetic analysis was performed using aligned amino-acid sequences of HMGR catalytic domains; membrane anchor domains were excluded from analysis. Amino-acid sequence alignments were generated using ClustalW [23] and the phylogenetic tree constructed with TreeTop [24] using the cluster algorithm with PHYLIP tree-type output. Full species names and GenBank accession numbers of the sequences used are provided in Table 1.

Figure 3

Structures of lovastatin, a statin drug that competitively inhibits HMGR, and of HMG-CoA. It can be seen that the portion of the drug shown here at the top resembles the HMG portion of HMG-CoA.

Figure 4

Proposed reaction mechanism for HMGR_P [7,18]. The side groups of the key catalytic residues, Lys267, Asp283, Glu83, and His381, are shown, and the substrate and products are shown with R representing the HMG portion. The reaction follows three stages (see text for details). A basically similar mechanism has been proposed for HMGR_H [4].