Cholesterol, Lipoprotein, and Steroid Metabolism: Biochemistry Study Notes

Cholesterol: Overview and Biological Importance

General Properties and Functions

Cholesterol is a characteristic steroid alcohol found in animal tissues. It is a vital component of all cell membranes, modulating membrane fluidity, and serves as a precursor for bile acids, steroid hormones, and vitamin D in specialized tissues. Cholesterol plays a central role in the regulation of the body's metabolic homeostasis.

• Major Sources: The hepatic cholesterol pool is maintained by dietary cholesterol and de novo synthesis.

• Transport: Cholesterol is exported from the liver as unmodified cholesterol in the bile, converted to bile salts, or as a component of plasma lipoproteins.

• Homeostasis: The balance between cholesterol influx and efflux is not precise, leading to gradual accumulation in tissues, especially in blood vessel walls.

• Clinical Relevance: Excess cholesterol deposition in arteries leads to plaque formation, narrowing of blood vessels (atherosclerosis), and increased risk of cardiovascular disease.

Cholesterol Structure

Chemical Features

Cholesterol is a very hydrophobic molecule, consisting of a steroid nucleus and a hydrocarbon tail.

• Steroid Nucleus: Four fused hydrocarbon rings (A, B, C, D).

• Side Chain: An eight-carbon, branched hydrocarbon chain attached to carbon 17.

• Functional Groups: Hydroxyl group at carbon 3; double bond between carbon 5 and carbon 6.

Formula: $C_{27}H_{46}O$

Classification: Sterols are steroids with a hydroxyl group at carbon 3 and a side chain at carbon 17.

Dietary Sterols and Absorption

Plant Sterols (Phytosterols) vs. Animal Sterols

Plant sterols, such as β-sitosterol, are structurally similar to cholesterol but are poorly absorbed by humans. They are transported back into the intestinal lumen, reducing dietary cholesterol absorption.

• Mechanism: Plant sterols compete with cholesterol for absorption, lowering plasma cholesterol levels.

• Dietary Strategy: Consumption of plant sterol esters is used to manage plasma cholesterol.

Example: Ezetimibe is a drug that inhibits cholesterol uptake by blocking the Niemann-Pick C1-like 1 protein.

Cholesteryl Esters

Formation and Transport

Most plasma cholesterol is in an esterified form (cholesteryl esters), which are even more hydrophobic than free cholesterol.

• Enzyme: Acyl-CoA:cholesterol acyltransferase (ACAT) catalyzes the formation of cholesteryl esters.

• Location: Cholesteryl esters are not found in membranes and are present only in low levels in cells; they must be transported in lipoprotein particles or solubilized by phospholipids and bile salts in bile.

Reaction: $\text{Cholesterol} + \text{Fatty Acyl-CoA} \xrightarrow{\text{ACAT}} \text{Cholesteryl Ester} + \text{CoA}$

Cholesterol Synthesis

Overview and Key Steps

Cholesterol is synthesized by virtually all human tissues, with the liver, adrenal cortex, and reproductive tissues making significant contributions. The process is energetically expensive and tightly regulated.

• Precursors: Acetyl-CoA provides carbon atoms; NADPH provides reducing equivalents.

• Key Enzyme: HMG-CoA reductase catalyzes the rate-limiting step, converting HMG-CoA to mevalonate.

• Regulation: Cholesterol synthesis is regulated by feedback inhibition, gene expression, enzyme degradation, phosphorylation, and hormonal control.

Key Reaction: $\text{HMG-CoA} + 2\,\text{NADPH} + 2\,\text{H}^+ \rightarrow \text{Mevalonate} + 2\,\text{NADP}^+ + \text{CoA}$

Regulation of Cholesterol Synthesis

Transcriptional and Enzymatic Control

Cholesterol biosynthesis is primarily regulated at the level of HMG-CoA reductase.

• SREBP-2: Sterol regulatory element-binding protein-2 increases HMG-CoA reductase expression when cholesterol is low.

• SCAP and INSIG: These proteins retain SREBP-2 in the ER when cholesterol is high, downregulating synthesis.

• Phosphorylation: AMP-activated protein kinase (AMPK) inactivates HMG-CoA reductase by phosphorylation; dephosphorylation activates it.

• Statins: Drugs such as atorvastatin and simvastatin are competitive inhibitors of HMG-CoA reductase, used to lower cholesterol.

Cholesterol Degradation and Bile Acid Synthesis

Bile Acids and Bile Salts

Cholesterol is not metabolized to CO2 and water; instead, its steroid nucleus is eliminated by conversion to bile acids, which are excreted in bile.

• Bile Composition: Bile contains bile salts, phosphatidylcholine (PC), and other organic compounds.

• Bile Acid Structure: 24 carbons, hydroxyl groups, and a carboxyl-terminated side chain.

• Primary Bile Acids: Cholic acid and chenodeoxycholic acid, synthesized in the liver.

• Conjugation: Bile acids are conjugated to glycine or taurine, forming bile salts (e.g., glycocholic acid, taurocholic acid).

• Function: Bile salts are effective detergents, aiding in fat emulsification and absorption.

Key Reaction: $\text{Cholesterol} \xrightarrow{7\alpha\text{-hydroxylase}} \text{Cholic Acid}$

Lipoproteins: Structure and Function

Classification and Composition

Lipoproteins are macromolecular complexes of lipids and proteins (apolipoproteins) that transport lipids in plasma.

• Major Classes: Chylomicrons, very-low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL).

• Structure: Neutral lipid core (TAG, cholesteryl esters) surrounded by a shell of amphipathic apolipoproteins, phospholipids, and cholesterol.

• Density: Chylomicrons are largest and least dense; HDL is smallest and most dense.

Lipoprotein Table

Lipoprotein	Main Lipid	Major Apolipoprotein	Function
Chylomicron	TAG	Apo B-48	Transport dietary TAG to tissues
VLDL	TAG	Apo B-100	Transport endogenous TAG to tissues
IDL	TAG, Cholesteryl esters	Apo B-100, Apo E	Intermediate in VLDL to LDL conversion
LDL	Cholesteryl esters	Apo B-100	Deliver cholesterol to peripheral tissues
HDL	Cholesteryl esters	Apo A-I	Reverse cholesterol transport to liver

Lipoprotein Metabolism

Chylomicron Metabolism

Chylomicrons are assembled in intestinal mucosal cells from dietary lipids and released into the lymphatic system.

• Key Apolipoproteins: Apo B-48 (structural), Apo C-II (activates lipoprotein lipase), Apo E (remnant uptake).

• Enzyme: Lipoprotein lipase (LPL) hydrolyzes TAG, releasing fatty acids for storage or energy.

• Remnant Clearance: Chylomicron remnants are taken up by the liver via Apo E-mediated endocytosis.

VLDL, IDL, and LDL Metabolism

• VLDL: Synthesized in the liver, transports endogenous TAG; receives Apo C-II and Apo E from HDL.

• IDL: Formed from VLDL after TAG removal; intermediate in LDL formation.

• LDL: Delivers cholesterol to peripheral tissues; taken up by LDL receptors recognizing Apo B-100.

• Clinical Note: Defects in LDL receptor or Apo B-100 cause familial hypercholesterolemia.

HDL Metabolism and Reverse Cholesterol Transport

• HDL: Synthesized in the liver and intestine; serves as a reservoir for Apo C and E.

• Reverse Transport: HDL removes cholesterol from peripheral tissues, esterifies it via LCAT, and delivers it to the liver.

• Key Proteins: Lecithin:cholesterol acyltransferase (LCAT), scavenger receptor class B type 1 (SR-B1).

Clinical Disorders Related to Lipoprotein Metabolism

Hyperlipoproteinemias and Atherosclerosis

• Type I: Deficiency of LPL or Apo C-II leads to chylomicronemia.

• Type IIa: LDL receptor defects cause familial hypercholesterolemia.

• Type III: Apo E defects cause dysbetalipoproteinemia.

• Atherosclerosis: Accumulation of LDL in arterial walls leads to plaque formation and cardiovascular disease.

Steroid Hormone Biosynthesis

Classes and Sites of Synthesis

Cholesterol is the precursor for all steroid hormones, including glucocorticoids, mineralocorticoids, and sex hormones (androgens, estrogens, progestins).

• Sites: Adrenal cortex (cortisol, aldosterone, androgens), ovaries and placenta (estrogens, progestins), testes (testosterone).

• Rate-Limiting Step: Conversion of cholesterol to pregnenolone by cholesterol side-chain cleavage enzyme (cytochrome P450scc).

• Regulation: Controlled by hypothalamic and pituitary hormones (e.g., ACTH, GnRH, LH, FSH).

Key Reaction: $\text{Cholesterol} \xrightarrow{\text{P450scc}} \text{Pregnenolone}$

Mechanism of Steroid Hormone Action

• Transport: Steroid hormones are hydrophobic and transported in blood bound to carrier proteins.

• Cellular Action: Hormones diffuse across the plasma membrane, bind to cytosolic or nuclear receptors, and regulate gene transcription by binding to hormone response elements (HREs).

Example: Cortisol regulates metabolism and immune response; aldosterone controls sodium and water balance.

Summary Table: Cholesterol and Lipoprotein Disorders

Disorder	Defect	Clinical Features
Type I Hyperlipoproteinemia	LPL or Apo C-II deficiency	Elevated chylomicrons, pancreatitis
Type IIa Hyperlipoproteinemia	LDL receptor defect	Elevated LDL, premature atherosclerosis
Type III Hyperlipoproteinemia	Apo E defect	Elevated IDL, chylomicron remnants
Cholelithiasis	Excess cholesterol in bile	Gallstones, biliary colic

Key Equations and Reactions

• $\text{HMG-CoA} + 2\,\text{NADPH} + 2\,\text{H}^+ \rightarrow \text{Mevalonate} + 2\,\text{NADP}^+ + \text{CoA}$

• $\text{Cholesterol} + \text{Fatty Acyl-CoA} \xrightarrow{\text{ACAT}} \text{Cholesteryl Ester} + \text{CoA}$

• $\text{Cholesterol} \xrightarrow{7\alpha\text{-hydroxylase}} \text{Cholic Acid}$

• $\text{Cholesterol} \xrightarrow{\text{P450scc}} \text{Pregnenolone}$

Additional info: These notes expand on the original slides and text by providing definitions, mechanisms, and clinical context for cholesterol, lipoprotein, and steroid metabolism, suitable for biochemistry exam preparation.