Cholesterol Synthesis

Hydroxymethylglutaryl-coenzyme A (HMG-CoA) is the precursor for cholesterol synthesis. HMG-CoA is an intermediate on the pathway for synthesis of ketone bodies from acetyl-CoA. The enzymes for ketone body production are located in the mitochondrial matrix. HMG-CoA destined for cholesterol synthesis is made by equivalent, but different, enzymes in the cytosol.

HMG-CoA is formed by condensation of acetyl-CoA & acetoacetyl-CoA, catalyzed by HMG-CoA Synthase. HMG-CoA Reductase then catalyzes production of mevalonate from HMG-CoA.

Mevalonate formation: The carboxyl of HMG that is in ester linkage to the CoA thiol is reduced to an aldehyde, and then to an alcohol, with NADPH serving as reductant in the 2-step reaction. Mevaldehyde is thought to be an active site intermediate, following the first reduction and release of CoA.

The HMG-CoA Reductase reaction, in which mevalonate is formed from HMG-CoA, is rate-limiting for cholesterol synthesis. This enzyme is highly regulated and the target of pharmaceutical intervention (to be discussed later). HMG-CoA Reductase has a cleavable membrane domain that links it to the ER. The isolated catalytic portion of the enzyme forms a tetramer, consisting of 2 dimers, each of which includes 2 active sites. The binding site for HMG-CoA in one monomer is adjacent to the binding site for NADPH in the other. Explore this structure with Chime.

Mevalonate is phosphorylated by 2 sequential Pi transfers from ATP, yielding the pyrophosphate derivative. ATP-dependent decarboxylation, with dehydration, yields isopentenyl pyrophosphate.

Isopentenyl pyrophosphate is the first of several compounds in the pathway that are referred to as isoprenoids, by reference to the compound isoprene.

Isopentenyl Pyrophosphate Isomerase interconverts isopentenyl pyrophosphate & dimethylallyl pyrophosphate. The mechanism involves deprotonation and protonation.

Condensation Reactions Prenyl Transferase catalyzes head-to-tail condensations: Dimethylallyl pyrophosphate & isopentenyl pyrophosphate react to form geranyl pyrophosphate. Condensation with another isopentenyl pyrophosphate yields farnesyl pyrophosphate. Each condensation reaction is thought to involve elimination of PPi to yield a reactive carbocation. Prenyl Transferase (Farnesyl Pyrophosphate Synthase) has been crystallized with the substrate geranyl pyrophosphate in the active site (Chime exercise).

Squalene Synthase: Head-to-head condensation of 2 farnesyl pyrophosphate, with reduction by NADPH, yields squalene.

Squaline epoxidase catalyzes oxidation of squalene to form 2,3-oxidosqualene. This mixed function oxidation requires NADPH as reductant & O2 as oxidant. One O atom is incorporated into substrate (as epoxide) & the other O is reduced to water.

Squalene Oxidocyclase catalyzes a series of cyclization reactions, initiated by donation of a proton to the epoxide. The product is the sterol lanosterol.

Conversion of lanosterol to cholesterol involves 19 reactions, catalyzed by enzymes in ER membranes. Additional modification of cholesterol yields various steroid hormones. Many of these reactions are mixed function oxidations, requiring O2 & NADPH.

In a mixed function oxidation, one O atom of O2 is incorporated into a substrate & the other O atom reduced to H2O. E.g., hydroxylation catalyzed by cyt P450. In a pathway associated with ER membranes, NADPH transfers 2e- to cyt P450 via a Reductase, which has FAD & FMN prosthetic groups. O2 binds to the reduced heme Fe of cyt P450, and hydroxylation is catalyzed.

The heme prosthetic group of cyt P450 has a cysteine S as axial ligand (X or Y). The other axial position, where O2 binds, may be open or have a bound H2O, that is displaced by O2 (Chime exercise). There are many variants of cytochrome P450. Some have broad substrate specificity. Some are in mitochondria. Others are associated with ER membranes. Substrates include steroids & non-polar xenobiotics (drugs & other foreign compounds). Detoxification involves reactions like hydroxylation that increase polarity, so compounds can be excreted by the kidneys.

Farnesyl pyrophosphate, an intermediate on the pathway for cholesterol synthesis, also serves as precursor for synthesis of various isoprenoids: dolichol (role in synthesis of oligosaccharide chains of glycoproteins) coenzyme Q (ubiquinone, role in electron transfer chain) prenylated proteins (geranylgeranyl & farnesyl groups anchor some proteins to membranes).

Regulation of Cholesterol Synthesis HMG-CoA Reductase, the rate-limiting step on the pathway for synthesis of cholesterol, is a major control point. Regulation relating to cellular uptake of cholesterol will be discussed in the next class. Short-term regulation: HMG-CoA Reductase is inhibited by phosphorylation, catalyzed by AMP-Dependent Protein Kinase. This kinase is active when cellular AMP is high, corresponding to when ATP is low. Thus, when cellular ATP is low, energy is not expended in synthesizing cholesterol.

Long-term regulation is by varied transcription and degradation of HMG-CoA Reductase and other enzymes of the pathway for synthesis of cholesterol. The level of of HMG-CoA Reductase is modulated by regulated proteolysis. Degradation of HMG-CoA Reductase is stimulated by oxidized derivatives of cholesterol, mevalonate, & farnesol (dephosphorylated farnesyl pyrophosphate). The membrane domain of HMG-CoA Reductase has a sterol-sensing domain that may have a role in activation of the enzyme’s degradation. Transcription factors called SREBPs (sterol regulatory element binding proteins), particularly SREBP-2, also respond to cell sterol levels.

When sterol levels are low, SREBPs are released by cleavage of precursor proteins in ER membranes. SREBPs translocate into the nucleus where they activate transcription of genes for HMG-CoA Reductase & other cholesterol synthesis enzymes. SCAP (SREBP cleavage-activating protein) has a sterol-sensing domain similar to that of HMG-CoA Reductase. SCAP transports the SREBP precursor to the golgi, where protease S1P cleaves it. A 2nd protease S2P then cleaves in the membrane domain to release the N-terminal SREBP.

Pharmaceutical Intervention Drugs used to inhibit cholesterol synthesis include competitive inhibitors of HMG-CoA Reductase. Examples include various "statin drugs" such as lovastatin (mevacor) and derivatives (e.g., zocor). A portion of each of these compounds is analogous in structure to mevalonate. In addition, it has been suggested that the ring structures of the statin drubs may associated with the NADPH binding site in the enzyme.