The Increasingly Complex Mechanism of HMG-CoA Reductase

Abstract

CONSPECTUS The seven decades of research on the mechanism of HMG-CoA Reductase (HMGR) provided a detailed reaction pathway for what is one of the most biomedically important and mechanistically most complex enzymes. HMGR is the target of statins that are prescribed to improve the quality of life of millions of people worldwide and, more recently, has been identified as a target for the development of antimicrobial agents. The impact of advances in diverse research areas, such as molecular biology and computational chemistry, are reflected in the maturation of mechanistic proposals for HMGR. Thus, the development of state-the-art methods in enzyme mechanism research can be traced by following the development of the HMGR mechanism. Similarly, the questions raised about these mechanism proposals reflect the limitations of these methods. The mechanism of HMGR, a four-electron oxidoreductase, has been uncovered to be unique and far more complex than originally thought. The reaction contains multiple chemical steps, coupled to large- scale domain motions of the homodimeric enzyme. The first proposals for the HMGR mechanism were based on kinetic and labeling experiments, drawing analogies to the mechanism of known dehydrogenases. Advances in molecular biology and bioinformatics enabled site-directed mutagenesis experiments and protein sequence analysis which identified catalytically important glutamate, aspartate, and histidine residues that in turn generated new and more complicated mechanistic proposals. With the development of protein crystallography, HMGR crystal structures were solved to reveal the spatial organization of the active site with an unexpected lysine residue lying at its center. The multitude of crystal structures led to more and more complex mechanistic proposals but the inherent limitations of the protein crystallography left a number of questions unresolved. For example, the proposed mechanisms change based on the protonation state of the active site glutamate residue, which cannot be clearly determined from the crystal structures. As computational analysis of large biomolecules become more feasible, application of methods such as hybrid quantum mechanics/molecular mechanics (QM/MM) calculations to the HMGR mechanism have led to the most detailed mechanistic proposal yet. As these methodologies continue to improve, their power to study enzyme mechanism in conjunction with protein crystallography is enormous. Nevertheless, even the most current mechanistic proposal is yet incomplete due to limitations of the current computational methodologies. Thus, HMGR serves as a model for how combination of increasingly sophisticated experimental and computational methods can elucidate very complex enzyme mechanisms.

Paeoniflorin Protects against Nonalcoholic Fatty Liver Disease Induced by a High-Fat Diet in Mice

“…3-Hydroxy-3-methyl coenzyme A reductase (HMG-CoA reductase) is the rate-limiting enzyme for cholesterol synthesis. 24) Inhibiting its activity could reduce the LDL-c synthesis. But what is different is our study has found serum LDL levels decreased and LDLR gene expressions was also significantly reduced after paeoniflorin administration, which is inconsistent with the usual findings.…”

Evidence of a sequestered imine intermediate during reduction of nitrile to amine by the nitrile reductase QueF from Escherichia coli

“…While in terms of reaction catalyzed, QueF nitrile reductases share commonalities with other four-electron reductases, such as 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (29)(30)(31) and the non-ribosomal peptide synthetase module myxalamid reductase (32,33), that use NAD(P)H to convert thioester-activated forms of carboxylic acids into primary alcohols via an aldehyde intermediate, they are individuated strongly from these other enzymes in having to deal with a highly decomposition-prone intermediate. The central role of intermediate sequestration in the QueF mechanism seems to be reflected in the fact that structural devices used to keep a firm grip on the imine intermediate are likewise involved directly in the catalytic events of the enzymatic reaction.…”

Phytochemistry and biosynthesis of δ-lactone withanolides

“…HMGR (EC 1.1.1.34) catalyzes an irreversible conversion of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) into MVA, which is the key ratecontrolling and first committed step of isopentenyl diphosphate (IPP) biosynthesis via the MVA pathway (Chappell 1995 (Kato-Emori et al 2001;Haines et al 2013). HMGR is an enzyme used in the initial stages of isoprenoid biosynthesisgenes encoding HMGRs have been isolated and characterized from many species since 1996 (Korth et al 1997;Maldonado-Mendoza et al 1997;KatoEmori et al 2001;Liao et al 2004;Shen et al 2006;Wang et al 2007;Lin et al 2009;Cao et al 2010;Dai et al 2011;Akhtar et al 2013).…”