Cholesterol Metabolism Notes (Bulleted)

Cholesterol: Key Roles and Structural Features

• Cholesterol is classified as a cyclopentano-perhydro-phenanthrene ring compound, featuring four fused hydrocarbon rings (three cyclohexane rings, A, B, C, and one cyclopentane D ring) with methyl groups at C10 and C13. A hydrophobic C20–C27 branched isooctyl tail is attached to C17. The 3β-OH (C3 hydroxyl group) imparts slight polarity, making cholesterol amphipathic; this hydroxyl group can be esterified to fatty acids to give cholesterol esters, which are highly hydrophobic and stored in lipid droplets.
• Molecular formula and count: C27 steroid nucleus with a hydrophobic tail; cholesterol is a C-27 compound.
• Functional roles include:
  • Membrane structure and fluidity regulation (approximately $30\%$ of the animal cell membrane cholesterol), interacting with phospholipids to modulate membrane permeability and rigidity, forming lipid rafts important for cellular processes.
  • Precursor for steroid hormones (e.g., pregnenolone, a committed metabolite in steroidogenesis leading to glucocorticoids, mineralocorticoids, and sex hormones).
  • Precursor for vitamin D synthesis (7-dehydrocholesterol converted to cholecalciferol, Vitamin D3, upon UV exposure).
  • Precursor for bile acids (amphipathic detergent molecules essential for fat digestion and absorption in the small intestine).
  • Cholesterol is essential for the formation and function of invaginated caveolae and clathrin-coated pits and participates in both caveola-dependent and clathrin-dependent endocytosis, facilitating cellular uptake of various molecules.
  • Cholesterol can be converted into cholesterol-37 sulfate (cholesterol-SO4) or cholesterol-sulfate, which is crucial for stratum corneum barrier function and desquamation. The cholesterol/cholesterol-sulfate ratio is important for skin differentiation, and deficiency in cholesterol-sulfatase leads to ichthyosis (a condition characterized by a fish-skin appearance due to impaired desquamation).
  • In wound healing, cholesterol and cholesterol-sulfate levels are tightly regulated, influencing keratinocyte migration and proliferation.
  • About $30\%$ of plasma membrane cholesterol is specifically involved in membrane dynamics, and lipid raft formation, and plays a critical role in receptor-mediated signaling.

Cholesterol in Membranes and Endocytosis

• Within the cell membrane, cholesterol organizes membrane lipids into microdomains (lipid rafts), which are critical for intracellular transport, cell signaling (e.g., GPCR signaling, tyrosine kinase receptor activation), and nerve conduction (especially in neuronal membranes where it forms part of the myelin sheath).
• Cholesterol is essential for the structure and function of invaginated caveolae (flask-shaped invaginations rich in caveolin proteins) and clathrin-coated pits (regions of the membrane coated by clathrin protein), and it plays a role in both caveola-dependent and clathrin-dependent endocytosis, processes vital for cellular uptake and recycling of receptors.
• Cholesterol can be found in specialized membrane microdomains that regulate receptor-mediated endocytosis and signaling pathways, providing a platform for protein interactions.

Dietary vs De Novo Cholesterol Synthesis

• Dietary cholesterol contributes to body cholesterol; about $50\%$ of body cholesterol is synthesized de novo from acetyl-CoA in tissues (predominantly liver; less in intestine, reproductive tissues, and adrenal glands).
• Serum cholesterol levels:
  • Desirable: Total cholesterol < $200 ext{ mg/dL}$; LDL < $100 ext{ mg/dL}$; HDL > $60 ext{ mg/dL}$ (for protective effect); Triglycerides < $150 ext{ mg/dL}$.

Hypercholesterolemia and Cardiovascular Risk

• Hypercholesterolemia, an elevated level of cholesterol in the blood, is a major risk factor for cardiovascular disease (CAD) and related symptoms, most notably atherosclerosis.
• Atherosclerosis involves the progressive narrowing and hardening of arteries due to the buildup of lipid-rich plaques. The process begins with endothelial dysfunction, allowing LDL particles to enter the arterial wall, become oxidized (oxLDL), and be taken up by macrophages, transforming them into lipid-laden foam cells. Advanced plaques have a necrotic lipid core with inflammatory cells and a fibrous cap (consisting of smooth muscle cells and collagen) that stiffens the artery wall and narrows the arterial lumen, impeding blood flow and potentially leading to thrombotic events upon rupture.
• Clinical signs associated with hypercholesterolemia include arcus senilis and xanthelasma.
  • Arcus senilis: A grey or white opaque ring around the corneal limbus, representing cholesterol deposits in the corneal stroma. While common in elderly individuals as an age-related finding, its presence in younger individuals (<$50$ years) can indicate significant hyperlipidemia and warrant further investigation.
  • Xanthelasma (yellow cholesterol-rich deposits, typically on the eyelids) and Xanthomas (localized subcutaneous lipid deposits, often on tendons, such as xanthoma tendinosum) can indicate severe hypercholesterolemia, particularly familial hypercholesterolemia, a genetic disorder.

Major Cholesterol Pathway Overview: From Acetyl-CoA to Cholesterol

• Overall framework: The mevalonate pathway (also known as the isoprenoid pathway) is a central metabolic pathway responsible for the biosynthesis of isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), which are universal isoprenoid building blocks. These isoprenoids are then assembled into squalene, which subsequently undergoes cyclization to lanosterol, and finally, a series of further modifications lead to cholesterol.
• De novo synthesis steps are traditionally divided as:
  1) Mevalonate (MVA) formation from acetyl-CoA.
  2) IPP/DMAPP generation (isoprenoid units) through sequential phosphorylation and decarboxylation of mevalonate.
  3) Squalene synthesis (linear isoprenoid assembly) via condensation of farnesyl pyrophosphate units.
  4) Cyclization to lanosterol and subsequent multi-step transformations (demethylation, double bond reduction/shift) to cholesterol.
• Total de novo synthesis comprises about $37$ steps:
  • $18$ steps from acetyl-CoA to lanosterol.
  • $19$ steps from lanosterol to cholesterol.
• The process is energetically costly, consuming $18 ext{ ATP}$ molecules in total for the pathway (specifically from mevalonate toward IPP synthesis) and requiring NADPH as a reducing agent for several reductase steps, most notably the HMG-CoA reductase-catalyzed step.
• The rate-limiting and highly regulated step in cholesterol biosynthesis is the cytosolic conversion of HMG-CoA to mevalonate catalyzed by HMG-CoA reductase (HMGCR).

Key Enzymes and Intermediates (From Acetyl-CoA to Cholesterol)

• Cytosolic steps leading to HMG-CoA:
  • Two molecules of acetyl-CoA condense to form acetoacetyl-CoA catalyzed by thiolase (a reversible reaction).
  • Acetoacetyl-CoA then condenses with another acetyl-CoA molecule to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA), catalyzed by HMG-CoA synthase (specifically the cytosolic isoform for cholesterol synthesis, distinct from the mitochondrial isoform involved in ketogenesis).
• HMG-CoA reductase step:
  • The enzyme HMG-CoA reductase (HMGCR) catalyzes the reduction of HMG-CoA to mevalonate:
    $ext{HMG{-}CoA} + 2 ext{ NADPH} + 2 ext{ H}^+ \rightarrow ext{Mevalonate} + ext{CoA} + 2 ext{ NADP}^+$
  • This reaction is the committed and rate-limiting step in cholesterol biosynthesis and is the primary target of statin drugs.
  • The HMGCR gene is located on chromosome $5q13.3$ and contains $22 ext{ exons}$, encoding two isoforms: HMGR isoform 1 (888 amino acids) and HMGR isoform 2 (835 amino acids).
• Mevalonate activation and conversion to IPP (isoprenoid units):
  • Mevalonate is sequentially phosphorylated:
  • Mevalonate → Mevalonate-5-phosphate (MVA-5-P) via mevalonate kinase (ATP-dependent).
  • Mevalonate-5-P → Mevalonate-5-pyrophosphate (MVA-PP) via phosphomevalonate kinase (ATP-dependent).
  • MVA-PP undergoes ATP-dependent decarboxylation to form isopentenyl pyrophosphate (IPP), a 5-carbon isoprenoid unit, also known as "activated isoprene."
  • IPP is the fundamental isoprenyl donor for prenyl transferases and for subsequent steps toward squalene synthesis.
  • IPP is isomerized to its more reactive structural isomer, dimethylallyl pyrophosphate (DMAPP), by isopentenyl pyrophosphate isomerase (IPPI). Both IPP and DMAPP are 5-carbon units.
• Condensation and head-to-tail assembly:
  • IPP and DMAPP condense to form geranyl pyrophosphate (GPP, a 10-carbon molecule).
  • GPP then condenses with another IPP molecule to form farnesyl pyrophosphate (FPP, a 15-carbon molecule).
  • Two molecules of FPP condense head-to-tail to form squalene (a 30-carbon linear isoprenoid) via squalene synthase (SQS):
    $2 ext{ FPP} \rightarrow ext{squalene} + 2 ext{ Pyrophosphate} + ext{NADPH} \rightarrow ext{NADP}^+$
  • Squalene is then converted to 2,3-oxidosqualene by squalene monooxygenase (also known as squalene epoxidase), requiring $ext{O}_2$ and NADPH.
  • The cyclization of 2,3-oxidosqualene to lanosterol (a 30-carbon tetracyclic triterpenoid) is catalyzed by lanosterol synthase (oxidosqualene cyclase).
  • Lanosterol is then converted to cholesterol through a complex series of 19 steps involving multiple enzymatic reactions, including demethylation (removal of methyl groups at C4 and C14), reduction of double bonds, and migration of the C8-C9 double bond to C5-C6 to form the characteristic $ext{B}^5$ double bond of cholesterol.
• Storage and transport intermediates:
  • While sitosterol and other plant sterols exist and are present in the diet, they are absorbed to a much lesser extent than cholesterol and are not major contributors to the mammalian cholesterol pool. They appear in diagrams as competing sterols or are subject to efflux mechanisms (like ABCG5/G8 transporters) that prevent their accumulation within human cells.
• Practical summary: The mevalonate pathway provides the universal isoprenoid backbone. The key regulatory and drug-target step is HMG-CoA reductase. Downstream steps funnel toward squalene and lanosterol, eventually leading to cholesterol. Isoprenoids are also precursors for other vital molecules like CoQ10, dolichols, and farnesyl/geranylgeranyl groups for protein prenylation.

Regulation of HMG-CoA Reductase (HMGCR)

• Metabolite regulation (feedback):
  • Mevalonate, the direct product of HMGCR, and cholesterol, the end product of the pathway, exert feedback inhibition on HMGCR activity. This occurs through both direct allosteric inhibition of the enzyme and by influencing its degradation and gene expression.
• Hormonal control:
  • Insulin: An anabolic hormone that stimulates cholesterol synthesis. Insulin promotes the dephosphorylation and thus activation of HMGCR by increasing the activity of cellular phosphatases (e.g., protein phosphatase 1).
  • Glucagon: A catabolic signal that suppresses cholesterol synthesis. Glucagon activates cAMP-dependent protein kinase (PKA), leading to the phosphorylation of HMGCR, which renders it inactive.
• Energy-sensing regulation (AMP-activated protein kinase, AMPK):
  • Increased AMP levels (indicating low cellular energy) activate AMP-activated kinase (AMPK).
  • AMPK phosphorylates HMGCR (forming HMGCR-P), rendering it inactive. This mechanism conserves ATP when energy is scarce by shutting down an energetically costly pathway like cholesterol synthesis.
  • Conversely, when energy (ATP) is abundant, insulin signaling promotes the dephosphorylation and activation of HMGCR, favoring cholesterol synthesis.
• Overall: HMGCR activity is tightly governed by a sophisticated interplay of metabolite levels (feedback inhibition), hormonal signals (insulin/glucagon influencing phosphorylation state), and cellular energy status via AMPK phosphorylation.

Gene-Level Regulation of HMGCR (Transcriptional Control)

• The synthesis of HMGCR and the LDL receptor (LDLR) is primarily regulated at the transcriptional level by sterol regulatory element-binding proteins (SREBPs), which are transcription factors.
• When cholesterol is low:
  • The SREBP protein, complexed with SCAP (SREBP cleavage-activating protein) and INSIG-1 (insulin-induced gene 1 protein), resides in the endoplasmic reticulum (ER) membrane.
  • At low cholesterol levels, INSIG-1 dissociates from the SREBP-SCAP complex in the ER, which allows the SREBP-SCAP complex to be transported from the ER to the Golgi apparatus.
  • In the Golgi, SREBP undergoes a two-step proteolytic cleavage by site-1 protease (S1P) and site-2 protease (S2P).
  • The processed, active N-terminal domain of SREBP (a transcription factor) then translocates to the nucleus.
  • In the nucleus, it binds to specific DNA sequences called Sterol Response Elements (SRE) located in the promoter regions of target genes, including HMGCR and LDL receptor (LDLR) genes. This binding boosts their transcription, thereby upregulating de novo cholesterol synthesis and the uptake of dietary/circulating cholesterol via the LDL receptor.
• In the presence of high cholesterol:
  • High cholesterol levels cause INSIG-1 to bind firmly to the SREBP-SCAP complex in the ER.
  • This binding retains the SREBP-SCAP complex in the endoplasmic reticulum, preventing its transport to the Golgi for processing.
  • Consequently, SREBP cannot be cleaved, and its active nuclear form is not generated, leading to reduced transcription of HMGCR and LDLR genes and a decrease in cholesterol synthesis and uptake.

Statins, Side Effects, and Therapeutic Context

• Statins: A class of drugs that are potent competitive inhibitors of HMG-CoA reductase (e.g., atorvastatin, lovastatin, pitavastatin, pravastatin, rosuvastatin, simvastatin).
  • Mechanism: Statins resemble HMG-CoA and competitively bind to the active site of HMGCR, blocking the conversion of HMG-CoA to mevalonate. This reduction in mevalonate production significantly decreases downstream cholesterol synthesis in the liver. The resulting intracellular cholesterol depletion upregulates LDL receptor expression, leading to increased uptake of circulating LDL cholesterol from the blood, thereby lowering serum LDL levels.
  • Side effects: Common side effects include myalgia (muscle pain) and myopathy (muscle weakness), and in rare cases, severe rhabdomyolysis. They can also impact mitochondrial function due to reduced synthesis of coenzyme Q10 (CoQ10), which shares a biosynthetic pathway with cholesterol (both are isoprenoid-derived). CoQ10 is crucial for the mitochondrial electron transport chain.
  • CoQ10 supplementation is sometimes recommended to mitigate statin-associated myopathy by supporting mitochondrial electron transport and oxidative phosphorylation.
• Cholestyramine: A bile acid sequestrant (resin) that binds to bile acids in the intestine.
  • Mechanism: By binding bile acids, cholestyramine prevents their reabsorption in the ileum, reducing their enterohepatic recirculation. This increases the fecal excretion of bile acids, forcing the liver to convert more cholesterol into new bile acids to replenish the bile acid pool. This diversion of cholesterol utilization leads to decreased intracellular cholesterol levels in hepatocytes, upregulating LDL receptor expression and subsequently lowering serum LDL cholesterol.
• Bile acid dynamics: Primary bile acids (cholic acid and chenodeoxycholic acid) are synthesized in the liver from cholesterol (rate-limited by cholesterol 7$\alpha$-hydroxylase). They are then conjugated to glycine or taurine to form more soluble bile salts (e.g., glycocholate, taurocholate), which are stored in the gallbladder. Upon a meal, cholecystokinin (CCK) stimulates gallbladder contraction, releasing bile salts into the small intestine. Most bile salts are reabsorbed in the terminal ileum via specific transporters and returned to the liver via the portal vein (enterohepatic circulation). A small fraction escapes reabsorption and is excreted, necessitating their resynthesis from cholesterol, which is a major pathway for cholesterol elimination.
• Drug and dietary implications:
  • Inhibiting HMGCR significantly lowers downstream isoprenoid synthesis (e.g., FPP, GPP), which can affect membrane protein anchoring (prenylation), dolichol synthesis (essential for N-linked glycosylation of proteins), and CoQ10 production (impacting mitochondrial function).
  • Alterations to bile acid synthesis and enterohepatic circulation influence overall cholesterol homeostasis and can impact gallbladder health, potentially increasing the risk of cholelithiasis (gallstones).
• Cholelithiasis (gallstones): Result from an imbalance in bile composition. Decreased levels of phosphatidylcholine (lecithin) and/or bile acids/salts relative to cholesterol can lead to supersaturation of bile with cholesterol, promoting cholesterol precipitation and the formation of cholesterol gallstones in the gallbladder.
  • Cholestyramine and dietary fiber (which can also bind bile acids) can both enhance bile acid excretion, thereby diverting more cholesterol toward bile acid synthesis and lowering serum cholesterol levels.
• Enterohepatic circulation specifics:
  • Approximately $95\%$ of secreted bile acids are reabsorbed from the intestinal lumen (mainly the ileum) and returned to the liver via the portal circulation.
  • The remaining ~$5\%$ escape reabsorption and enter the colon, where they are acted upon by colonic bacteria. These bacteria deconjugate and dehydroxylate primary bile acids to form secondary bile acids (e.g., deoxycholic acid from cholic acid, and lithocholic acid from chenodeoxycholic acid), which are then mostly excreted in feces.
  • The daily bile salt pool in humans is approximately $1.5 - 3.5 ext{ g}$, which is secreted into the intestine multiple times a day (total daily secretion can be $15 - 30 ext{ g}$). Approximately $0.5 ext{ g}$ of bile acids are lost in feces daily, which the liver must resynthesize.

Bile Acids and Bile Salts: Synthesis, Function, and Recycling

• Bile acids are amphipathic molecules with both hydrophilic (hydroxyl groups) and hydrophobic (methyl groups and steroid nucleus) faces, enabling them to act as biological detergents. This property is crucial for the emulsification of dietary fats (triglycerides) into smaller fat globules, increasing the surface area for lipase action, and for the formation of mixed micelles, which facilitate the absorption of digested fats and fat-soluble vitamins.
• Hormonal trigger for bile release:
  • Cholecystokinin (CCK), a hormone released by the duodenal mucosa in response to fatty acids and amino acids, signals the gallbladder to contract, releasing stored bile (containing bile salts) into the small intestine via the common bile duct.
• Liver synthesis and conjugation:
  • Primary bile acids (cholic acid and chenodeoxycholic acid) are synthesized in the liver from cholesterol, with cholesterol 7$\alpha$-hydroxylase being the rate-limiting enzyme.
  • These primary bile acids are then conjugated in the liver to either glycine (e.g., glycocholate) or taurine (e.g., taurocholate) to form more polar and effective bile salts. These bile salts are then stored and concentrated in the gallbladder.
  • Primary bile acids are regenerated after reabsorption (re-esterified to bile salts) and secreted back into bile to maintain the bile salt pool.
• Secondary bile acids:
  • Are produced by intestinal bacteria, primarily in the colon, through a series of modifications including deconjugation (removal of glycine or taurine) and dehydroxylation (removal of hydroxyl groups).
  • Examples include deoxycholic acid (from cholic acid) and lithocholic acid (from chenodeoxycholic acid).
• Health implications:
  • An adequate bile acid pool is essential for proper fat digestion and absorption. Imbalances in bile acid composition or changes in intestinal bacterial dysbiosis can influence the levels of secondary bile acids, which some studies suggest may influence colon cancer risk (e.g., deoxycholate has been implicated).

Pathophysiology: Atherosclerosis and Related Signs

• Atherosclerosis is a chronic inflammatory disease characterized by the progressive narrowing of arterial lumens by lipid-rich plaques. The process is initiated by endothelial damage (e.g., due to hypertension, smoking, high LDL), which leads to increased permeability and the deposition of oxidized LDL particles in the subendothelial space. This triggers an inflammatory response, with the recruitment of monocytes that differentiate into macrophages. Macrophages engulf oxLDL, transforming into lipid-laden foam cells, which are a hallmark of early atherosclerotic lesions (fatty streaks). Over time, these plaques grow, accumulating more lipids, inflammatory cells, smooth muscle cells, and calcification, culminating in a mature plaque with a fibrous cap. This plaque narrows the arterial lumen, restricts blood flow, and can rupture, leading to acute thrombotic events like myocardial infarction or stroke.
• Arcus senilis and xanthelasma are important clinical indicators of dyslipidemia and increased cardiovascular disease risk, signaling underlying cholesterol accumulation.
• Xanthomas (e.g., xanthelasma, xanthoma tendinosum, eruptive xanthomas) may indicate various forms of hyperlipidemia, including familial hypercholesterolemia, and warrant comprehensive lipid panel testing and potentially genetic evaluation.

7-Dehydrocholesterol Reductase (DHCR7) and Smith-Lemli-Opitz Syndrome (SLOS)

• DHCR7 deficiency, caused by mutations in the DHCR7 gene, results in Smith-Lemli-Opitz Syndrome (SLOS).
• SLOS is a devastating developmental disorder characterized biochemically by elevated levels of 7-dehydrocholesterol (7-DHC), the immediate precursor to cholesterol, and significantly reduced cholesterol levels (often only $15\% - 27\%$ of normal). This deficiency leads to a wide spectrum of developmental malformations, including craniofacial anomalies, limb abnormalities, intellectual disability, growth retardation, and behavioral problems (e.g., autism spectrum characteristics).
• Clinical implication: Cholesterol synthesis defects, such as in SLOS, have profound developmental consequences due to the critical role of cholesterol in cell membrane structure, myelin formation, and its function as a precursor for steroid hormones and vitamin D, all of which are essential for normal embryogenesis and neurological development.

Alternative/Investigational Therapies and Other Notes

• Alternative inhibitors of cholesterol biosynthesis:
  • Squalene synthase inhibitors (e.g., TAK-475, Zaragozic acids): These compounds target SQS, blocking the condensation of two FPP molecules to squalene, thus preventing further cholesterol synthesis.
  • Lapaquistat: Another squalene synthase inhibitor.
• These approaches aim to reduce the flux through the cholesterol biosynthetic pathway at different points than statins, potentially impacting not only cholesterol levels but also the availability of other isoprenoids like FPP and GPP, which are essential for membrane protein anchoring (prenylation), dolichol synthesis for N-linked glycosylation, and mitochondrial function (via CoQ10 synthesis).
• Heme A (a prosthetic group for cytochrome c oxidase, Complex IV of the mitochondrial electron transport chain) synthesis is distinct from the cholesterol pathway, but CoQ10 and other isoprenoids derived from the mevalonate pathway directly influence mitochondrial function and energy production. Therefore, disruptions in this pathway can have broader metabolic consequences.
• Vitamin D synthesis (from 7-dehydrocholesterol) and pregnenolone formation (the first step in steroid hormone synthesis) are directly dependent on cholesterol metabolism, highlighting the broad physiological importance of maintaining cholesterol homeostasis.

Summary: Core Concepts and Takeaways

• All carbon atoms in cholesterol derive from acetyl-CoA via the mevalonate pathway and subsequent isoprenoid assembly steps. This pathway produces various vital isoprenoid intermediates.
• The HMG-CoA reductase step is the keystone regulatory point and the primary target of statin therapy. Its activity is rigorously regulated at metabolic (feedback), hormonal (insulin/glucagon), and transcriptional levels.
• SREBP-mediated transcriptional control integrates cellular cholesterol status with the capacity for both de novo cholesterol synthesis (via HMGCR gene) and cholesterol uptake (via LDLR gene regulation) to maintain flux.
• Bile acid synthesis from cholesterol in the liver and their subsequent enterohepatic circulation are crucial mechanisms for cholesterol elimination and maintaining overall cholesterol homeostasis. Disturbances in this process can lead to gastrointestinal issues like gallstones and altered lipid metabolism.
• Clinical signs such as arcus senilis, xanthelasma, and xanthomas are visible indicators of dyslipidemia and increased cardiovascular risk. Genetic disorders like Smith-Lemli-Opitz Syndrome (SLOS) underscore the profound developmental importance of cholesterol in cell structure and signaling.
• CoQ10 supplementation may therapeutically mitigate statin-associated myopathy by supporting compromised mitochondrial function. Cholestyramine acts by reducing enterohepatic recirculation of bile acids, thereby diverting more cholesterol toward bile acid production and lowering serum cholesterol.
• The detailed synthesis (e.g., $37$ steps total, $18$ steps to lanosterol, $19$ steps to cholesterol) illustrates the significant complexity and substantial energy cost (approximately $18 ext{ ATP}$ per cholesterol molecule) of de novo cholesterol production.
• Alternative inhibitors targeting downstream steps (like Squalene Synthase inhibitors) offer potential new therapeutic routes to modulate cholesterol synthesis and the production of its downstream isoprenoids, opening up new avenues for lipid management.

Quick Reference: Key Equations and Numbers

• HMG-CoA reductase reaction (rate-limiting):
  $ext{HMG{-}CoA} + 2 ext{NADPH} + 2 ext{H}^+ \rightarrow ext{Mevalonate} + ext{CoA} + 2 ext{NADP}^+$
• Mevalonate pathway energy cost: $18 ext{ ATP}$ total for IPP production from mevalonate.
• Squalene synthesis (head-to-tail condensation of two FPP molecules):
  $2 ext{ FPP} \rightarrow ext{squalene}$
• Primary bile acids: cholic acid and chenodeoxycholic acid (synthesized in liver, rate-limited by cholesterol 7$\alpha$-hydroxylase); conjugated to glycine or taurine to form bile salts.
• Enterohepatic circulation: Approximately $95\%$ of bile acids are reabsorbed; about $5\%$ are excreted, mainly as secondary bile acids (deoxycholic acid, lithocholic acid) formed by colonic bacteria.
• Gallstone risk factors: Decreased phosphatidylcholine (lecithin) and/or bile acids/salts relative to cholesterol can cause cholesterol supersaturation and precipitation, leading to gallstones; cholestyramine binds bile acids to promote cholesterol excretion.
• Clinical signs of hypercholesterolemia: Arcus senilis (cholesterol deposits in cornea), xanthelasma (yellow eyelid deposits); familial hypercholesterolemia may prominently present with xanthomas (subcutaneous/tendinous lipid deposits).
• SLOS: Characterized by elevated 7-dehydrocholesterol and significantly reduced cholesterol (often $15\% - 27\%$ of normal) due to DHCR7 deficiency, leading to severe developmental defects.
• Statins: HMGCR inhibitors; potential side effects include myalgia and myopathy, partly due to reduced CoQ10 synthesis; CoQ10 supplementation may help mitigate these muscle-related side effects.
• Important regulatory genes for cholesterol homeostasis: SREBP, SCAP, INSIG-1; SREBP processing involves sequential cleavage by S1P and S2P in the Golgi; active SREBP binds SREs in the promoter regions of HMGCR and LDLR genes to regulate transcription.
• Enzyme and gene details: HMGCR gene on chromosome $5q13.3$; encodes two HMGR isoforms (888 aa and 835 aa).
• Heme A is a prosthetic group in cytochrome c oxidase (Complex IV) of the mitochondrial electron transport chain.