The 3-hydroxy-3-methylglutaryl coenzyme-A (HMG-CoA) reductases
© BioMed Central Ltd 2004
Published: 1 November 2004
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.
Gene organization and evolutionary history
Details of the sequences used for the phylogenetic tree in Figure 2
Mus musculus (mouse)
Mesocricetus auratus (hamster)
Rattus norvegicus (rat)
Homo sapiens (human)
Gallus gallus (chicken)
Xenopus laevis (frog)
Drosophila melanogaster (fruit fly)
Homarus americanus (lobster)
Blatella germanica (cockroach)
Dendroctonus jeffreyi (Jeffrey pine beetle)
Ips pini (bark beetle)
Ips paraconfusus (bark beetle)
Raphanus sativus (radish)
Arabidopsis thaliana (thale-cress)
Oryza sativa (rice)
Lycopersicon esculentum (tomato)
Nicotinia tabacum (tobacco)
Cucumis melo (muskmelon)
Hevea brasiliensis (rubber tree)
Pisum sativum (pea)
Solanum tuberosum (potato)
Tagetes erecta (African marigold)
Catharanthus roseus (Madagascar periwinkle)
Artemisia annua (wormwood)
Gossypium hirsutum (cotton)
Taxus × media (yew)
Andrographis paniculata (Indian herb)
Malus × domestica (apple)
Capsicum annuum (pepper)
Saccharomyces cerevisiae (baker's yeast)
Schizosaccharomyces pombe (fission yeast)
Trypanosoma cruzi (trypanosome)
Leishmania major (trypanosome)
Strongylocentrotus purpuratus (sea urchin)
Dicentrarchus labrax (European sea bass)
Characteristic structural features
The HMGRs of different organisms are multimers of a species-specific number of identical monomers. High-resolution crystal structures have been solved for the Class I human enzyme (HMGRH) [4, 5] and for the Class II enzyme of Pseudomonas mevalonii (HMGRP) [6, 7], including protein forms bound to either the HMG-CoA substrate or the coenzyme (NADH or NADPH) or both, or bound to statin drugs, which are potent competitive inhibitors of HMGR activity and thus lower cholesterol levels in the blood [8, 9]. As reviewed in detail by Istvan , structural comparisons reveal both similarities and significant differences between the two classes of enzyme. The human HMGR has three major domains (catalytic, linker and anchor), whereas the P. mevalonii HMGR has only the catalytic domain (Figure 1).
Both HMGRH and HMGRP have a dimeric active site with residues contributed by each monomer, and a non-Rossman-type coenzyme-binding site (a three-dimensional structural fold that contains a nucleotide-binding motif and is found in many enzymes that use the dinucleotides NADH and NADPH for catalysis). The core regions containing the catalytic domains of the two enzymes have similar folds. Despite differences in amino-acid sequence and overall architecture, functionally similar residues participate in the binding of coenzyme A by the two enzymes, and the position and orientation of four key catalytic residues (glutamate, lysine, aspartate and histidine) is conserved in both classes of HMGR.
Unlike the central cores, the amino- and carboxy-terminal regions of the catalytic domains show little similarity between the human and P. mevalonii HMGR structures. The active site of HMG-CoA reductase is at the interface of the homodimer between one monomer that binds the nicotinamide dinucleotide and a second monomer that binds the HMG-CoA. In human HMGR, the catalytic lysine is found on the monomer that binds the HMG-CoA and comes from the so-called cis-loop (a section that connects the HMG-CoA-binding region with the NADPH-binding region). In contrast, the P. mevalonii HMGR lacks the cis-loop and the catalytic lysine is contributed by the monomer that binds the nicotinamide dinucleotide. HMGRP crystallizes as a trimer of dimers (which are composed of identical subunits), but HMGRH crystallizes as a tetramer (of identical units). HMGRP uses NADH as a coenzyme, whereas HMGRH uses NADPH, but mutation to alanine of the aspartyl residue of HMGRP that normally blocks binding of NADPH can allow NADPH to serve - albeit poorly - as the coenzyme for HMGRP. A 180° difference in the orientation of the nicotinamide ring of the coenzyme suggests that that the stereospecificity of the HMGRH hydrogen transfer is opposite to that of HMGRP.
Localization and function
HMGRs of eukaryotes are localized to the endoplasmic reticulum (ER), and are directed there by a short portion of the amino-terminal domain (prokaryotic HMGRs are soluble and cytoplasmic). In humans, the reaction catalyzed by HMGR is the rate-limiting step in the synthesis of cholesterol, which maintains membrane fluidity and serves as a precursor for steroid hormones. In plants, a cytosolic HMG-CoA reductase participates in the synthesis of sterols, which are involved in plant development, certain sesquiterpenes, which are important in plant defense mechanisms against herbivores, and ubiquinone, which is critical for cellular protein turnover. In plastids, however, these compounds are synthesized via a pathway that does not involve mevalonate or HMGR . Various plant HMGR isozymes function in fruit ripening and in the response to environmental challenges such as attack by pathogens. In yeast, either of the two ER-anchored HMGR isozymes can provide the mevalonate needed for growth.
The reaction catalyzed by HMGR is:
(S)-HMG-CoA + 2 NADPH + 2 H+ → (R)-mevalonate + 2 NADP+ + CoA-SH.
with the (S)-HMG-CoA and (R)-mevalonate designations referring to the stereochemistry of the substrate and product (enzymatic reactions are stereospecific and the (R)-HMG-CoA isomer is not a substrate for HMGR). This three-stage reaction involves two reductive stages and the formation of enzyme-bound mevaldyl-CoA and mevaldehyde as probable intermediates:
Stage 1: HMG-CoA + NADPH + H+ → [Mevaldyl-CoA] + NADP+
Stage 2: [Mevaldyl-CoA] → [Mevaldehyde] + CoA-SH
Stage 3: [Mevaldehyde] + NADPH + H+ → Mevalonate + NADP+
A highly regulated enzyme, HMGRH is subject to transcriptional, translational, and post-translational control  that can result in changes of over 200-fold in intracellular levels of the enzyme. The transcription factor sterol regulatory element-binding protein 2 (SREBP-2) participates in regulating levels of HMGRH mRNA in response to the level of sterols ; the regulatory process is as follows. At the ER membrane or the nuclear envelope, SREBP-2 binds to SREBP cleavage activating protein (SCAP) to form a SCAP-SREBP complex that functions as a sterol sensor. The proteins Insig-1 and Insig-2 bind to SCAP when cellular cholesterol levels are high and prevent movement of the SCAP-SREBP complex from the ER to the Golgi. In cells depleted of cholesterol, Insig-1 and Insig-2 allow activation of the SCAP-SREBP complex and its translocation to the Golgi, where SREBP is cleaved at two sites. Cleavage releases the amino-terminal basic helix-loop-helix (bHLH) domain, which enters the nucleus, where it functions as a transcription factor that recognizes non-palindromic decanucleotide sequences of DNA called sterol-regulatory elements (SREs). Binding of the bHLH domain of SREBP to an SRE promotes transcription of the hmgr gene.
Degradation of HMGRH involves its transmembrane regions : removal of two or more transmembrane regions abolishes the acceleration of HMGRH degradation that occurs under certain conditions [12, 15]: degradation is induced by a non-sterol, mevalonate-derived metabolite alone or by a sterol plus a mevalonate-derived non-sterol metabolite, possibly farnesyl pyrophosphate or farnesol. Four conserved phenylalanines in the sixth membrane span of the transmembrane region are essential for degradation of HMGRH . Insig-1 also functions in the degradation of HMGRH : when cholesterol levels are high, SCAP and HMGRH compete for binding to Insig-1. If SCAP binds Insig-1, the SCAP-Insig-1 complex is retained in the Golgi, whereas if HMGRH binds Insig-1, HMGRH is ubiquinated on lysine 248 and is rapidly degraded through a ubiquitin-proteasome mechanism .
The catalytic activity of the HMGRs of higher eukaryotes is attenuated by phosphorylation of a single serine, which in the case of HMGRH is at position 872 . The location of this serine - six residues from the catalytic histidine, a spacing conserved in all higher eukaryote HMGRs - suggests that the phosphoserine may interfere with the ability of this histidine to protonate the inhibitory CoAS- thioanion that is released in stage 2 of the reaction mechanism. Alternatively, it may interfere with closure of the flap domain, a carboxy-terminal region that is thought to close over the active site to facilitate catalysis, a step thought to be essential for formation of the active site . Subsequent dephosphorylation restores full catalytic activity. HMGR kinase (also called AMP kinase) phosphorylates HMGR; the primary phosphatase in vivo is thought to be protein phosphatase 2A (PP2A), but both phosphatases 2A and 2B can catalyze dephosphorylation of vertebrate HMGR in vitro . HMGRH activity therefore responds to hormonal control through AMP levels and PP2A activity. Phosphorylation of serine 577 of A. thaliana HMGR isozyme 1 by a plant HMGR kinase that does not require 5'-AMP attenuates activity, and restoration of HMGR activity follows from dephosphorylation . As many plant genes encode a putative target serine surrounded by an apparent AMP kinase recognition motif, it is probable that most plant HMGRs are regulated by phosphorylation. Yeast HMGR activity is, however, unaffected by AMP kinase. The phosphorylation state of HMGR does not affect the rate at which the protein is degraded.
Several basic unresolved questions concern how phosphorylation controls the catalytic activity of HMGRs; solution of the structures of phosphorylated HMGRs should reveal more of the precise mechanism. The protein kinases, phosphatases, and signal-transduction pathways that participate in short-term regulation of HMGR activity are yet to be elucidated. Finally, the physiological roles served by the multiple ways in which HMGR is regulated require clarification. On the medical side, continuing intense competition between drug companies for a share of the lucrative worldwide market for hypercholesterolemic agents should result in new statin drugs with modified pharmacodynamic and metabolic properties that not only lower plasma cholesterol levels more effectively but more importantly minimize undesirable side effects.
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- Bochar DA, Stauffacher CV, Rodwell VW: Sequence comparisons reveal two classes of 3-hydroxy-3-methylglutaryl coenzyme A reductase. Mol Genet Metab 1999, 66:122–127. This article reported the classification of HMG-CoA reductases into Class I and Class II enzymes on the basis of sequence comparison. The authors utilized phylogenetic analysis to analyze a plethora of genomic sequences of various organisms.View ArticlePubMed
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- Istvan ES, Palnitkar M, Buchanan SK, Deisenhofer J: Crystal structure of the catalytic portion of human HMG-CoA reductase: insights into regulation of activity and catalysis. EMBO J 2000, 19:819–830. This article and  reported the crystal structure of the human HMG-CoA reductase catalytic domain, providing numerous insights into catalysis by a Class I HMG-CoA reductase.View ArticlePubMed
- Istvan ES, Deisenhofer J: The structure of the catalytic portion of human HMG-CoA reductase. Biochim Biophys Acta 2000, 1529:9–18. See PubMed
- Lawrence CM, Rodwell VW, Stauffacher CV: The crystal structure of Pseudomonas mevalonii HMG-CoA reductase at 3.0 Å resolution. Science 1995, 268:1758–1762. This article reports the first HMG-CoA reductase structure that was solved.View ArticlePubMed
- Tabernero LD, Bochar DA, Rodwell VW, Stauffacher CV: Substrate-induced closure of the flap domain in the ternary complex structures provides new insights into the mechanism of catalysis by 3-hydroxy-3-methylglutaryl-CoA reductase. Proc Natl Acad Sci USA 1999, 96:7167–7171. The original structure of P. mevalonii HMG-CoA reductase  lacked a portion of the enzyme known to be critical for catalysis. This article provided insight into the catalytic mechanism by solving the structure of the original missing region.View ArticlePubMed
- Istvan ES, Deisenhofer J: Structural mechanism for statin inhibition of HMG-CoA reductase. Science 2001, 292:1160–1164. This article reports a structural explanation for inhibition of human HMG-CoA reductase by statins, which are widely prescribed drugs for hypercholesterolemia.View ArticlePubMed
- Tabernero L, Rodwell VW, Stauffacher CV: Crystal structure of a statin bound to a class II hydroxymethylglutaryl-CoA reductase. J Biol Chem 2003, 278:19933–19938. The authors detail the interaction of P. mevalonii HMG-CoA reductase, a Class II enzyme, with statins.View ArticlePubMed
- Istvan ES: Bacterial and mammalian HMG-CoA reductases: related enzymes with distinct architectures. Curr Opin Struct Biol 2001, 11:746–751. A review that provides insight into the relationships between Class I and Class II HMG-CoA reductases, both in terms of structure and evolution.View ArticlePubMed
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- Jingami H, Brown MS, Goldstein JL, Anderson RJ, Luskey KL: Partial deletion of membrane-bound domain of 3-hydroxy-3-methylglutaryl coenzyme A reductase eliminates sterol-enhanced degradation and prevents formation of crystalloid endoplasmic reticulum. J Cell Biol 1987, 104:1693–1704. The original report of the sterol-mediated regulation of HMG-CoA reductase degradation and localization of the region responsible for mediating this degradation.View ArticlePubMed
- Xu L, Simoni RD: The inhibition of degradation of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase by sterol regulatory element binding protein cleavage-activating protein requires four phenylalanine residues in span 6 of HMG-CoA reductase transmembrane domain. Arch Biochem Biophys 2003, 414:232–243. A study of the structure-function relationships between HMG-CoA reductase degradation and the sterol cleavage activating protein (SCAP).View ArticlePubMed
- Sever N, Yang T, Brown MS, Goldstein JL, DeBose-Boyd RA: Accelerated degradation of HMG-CoA reductase mediated by binding of insig-1 to its sterol-sensing domain. Mol Cell 2003, 11:25–33. The authors identified the role of the protein insig-1 in regulation of HMG-CoA reductase by degradation.View ArticlePubMed
- Sever N, Song BL, Yabe D, Goldstein JL, Brown MS, DeBose-Boyd RA: Insig-dependent ubiquitination and degradation of mammalian 3-hydroxy-3-methylglutaryl-CoA reductase stimulated by sterols and geranylgeraniol. J Biol Chem 2003, 278:52479–52490. This study described the relationship between ubiquitination, degradation, and the protein insig-1 in HMG-CoA reductase degradation.View ArticlePubMed
- Sato R, Goldstein JL, Brown MS: Replacement of serine-871 of hamster 3-hydroxy-3-methylglutaryl-CoA reductase prevents phosphorylation by AMP-activated kinase and blocks inhibition of sterol synthesis induced by ATP depletion. Proc Natl Acad Sci USA 1993, 90:9261–9265. In this study, the authors identified the specific amino acid of mammalian HMG-CoA reductase that is phosphorylated and mediates regulation of HMG-CoA reductase by reversible phosphorylation.View ArticlePubMed
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