Cholesterol and Lipoproteins
Cholesterol and Lipoproteins: Synthesis, Function, and Atherosclerosis
Cholesterol Synthesis and Regulation
Functions of Cholesterol
Maintains membrane fluidity.
Precursor for all steroid hormones (e.g., estradiol, testosterone).
Used to form bile salts.
Backbone for Vitamin D.
Precursor Molecule: Acetyl CoA
Cholesterol synthesis is an anabolic process.
Acetyl CoA is a versatile molecule derived from carbohydrate oxidation and fat catabolism.
Pathways of Acetyl CoA:
Oxidized in the citric acid cycle.
Converted to malonyl CoA for lipogenesis.
Forms beta-hydroxybutyrate (ketone bodies).
Helps make cholesterol (in the fed state, dictated by insulin and glucose).
Key Enzyme: HMG CoA Reductase
The rate-limiting enzyme for cholesterol synthesis.
Found in almost every cell in the body, allowing most cells to synthesize their own cholesterol.
Major sites of synthesis:
Liver: Synthesizes a significant amount for bile salts, steroid hormone precursors, and Vitamin D precursors.
Gonads (testes, ovaries): Synthesize cholesterol for sex hormones (testosterone, estrogen).
Regulation of HMG CoA Reductase Activity
Insulin (Fed State): Induces phosphatase enzymes, which dephosphorylate HMG CoA reductase, activating it. This helps dispose of excess Acetyl CoA by shunting it towards cholesterol formation when glucose is abundant.
Energy Deprived State: Increases phosphorylation of the enzyme, rendering it inactive. This prevents wasteful cholesterol synthesis and allows Acetyl CoA to be burned for energy.
Allosteric Feedback Inhibition: Cholesterol itself can allosterically inhibit HMG CoA reductase. When cells have met their cholesterol needs (e.g., incorporated it into membranes), they
Cholesterol Synthesis and Regulation
Functions of Cholesterol
Maintains membrane fluidity by intercalating between phospholipid molecules, preventing them from packing too closely or moving too freely, thus regulating membrane permeability and structural integrity.
Precursor for all steroid hormones (e.g., estradiol, testosterone, cortisol, aldosterone).
Used to form bile salts, essential for the emulsification and absorption of dietary fats and fat-soluble vitamins in the small intestine.
Backbone for Vitamin D synthesis, which is further activated in the skin upon exposure to ultraviolet B (UVB) radiation.
Precursor Molecule: Acetyl CoA
Cholesterol synthesis is an anabolic process, requiring significant energy.
Acetyl CoA is a versatile two-carbon molecule centrally involved in metabolism, derived from:
Complete oxidation of carbohydrates (via glycolysis and pyruvate dehydrogenase complex).
Beta-oxidation of fatty acids (fat catabolism).
Catabolism of certain amino acids.
Pathways of Acetyl CoA:
Oxidized in the citric acid cycle to generate ATP.
Converted to malonyl CoA for de novo lipogenesis (fatty acid synthesis).
Forms beta-hydroxybutyrate and acetoacetate (ketone bodies) during prolonged fasting or starvation.
Shunted towards cholesterol synthesis primarily in the fed state, dictated by high insulin levels and abundant glucose, signaling an energy surplus.
Key Enzyme: HMG CoA Reductase
The rate-limiting and committed step enzyme for cholesterol synthesis, controlling the conversion of HMG CoA to mevalonate.
Found in the endoplasmic reticulum of almost every nucleated cell in the body, allowing most cells to synthesize their own cholesterol for membrane maintenance.
Major sites of synthesis:
Liver: Synthesizes the largest amount of cholesterol, providing precursors for bile salts, steroid hormones, and Vitamin D. It also packages cholesterol into VLDL for distribution to peripheral tissues.
Gonads (testes, ovaries): Synthesize cholesterol locally for sex hormone production (e.g., testosterone, estrogen).
Adrenal cortex: Synthesizes cholesterol for the production of corticosteroid hormones (e.g., cortisol, aldosterone).
Regulation of HMG CoA Reductase Activity
Insulin (Fed State): High insulin levels activate phosphatase enzymes, which dephosphorylate HMG CoA reductase, thereby activating it. This promotes cholesterol synthesis when energy and carbon sources (glucose, Acetyl CoA) are abundant, signaling an anabolic state.
Glucagon/Energy Deprived State: In states of low energy (e.g., fasting), hormones like glucagon and activation of AMP-activated protein kinase (AMPK) lead to phosphorylation of the enzyme, rendering it inactive. This conserves ATP by preventing wasteful cholesterol synthesis and prioritizes Acetyl CoA for energy production.
Allosteric Feedback Inhibition: Cholesterol itself, and other sterols, can allosterically inhibit the activity of HMG CoA reductase. When intracellular cholesterol levels are high, cholesterol binds to a site on the enzyme, causing a conformational change that reduces its catalytic activity. This provides a rapid, short-term regulatory mechanism.
Transcriptional Regulation (SREBP pathway): This is the primary long-term regulation mechanism. Sterol Regulatory Element-Binding Proteins (SREBPs) are transcription factors that, when cholesterol levels are low, translocate from the endoplasmic reticulum to the Golgi apparatus, where they are cleaved. The active N-terminal domain then moves to the nucleus and binds to Sterol Regulatory Elements (SREs) in the promoter regions of target genes, including HMG CoA reductase, leading to increased gene expression and enzyme synthesis. Conversely, high cholesterol levels keep SREBPs sequestered in the ER, preventing their activation and thus reducing HMG CoA reductase synthesis.
Proteasomal Degradation: High intracellular cholesterol levels also lead to increased ubiquitination and proteasomal degradation of HMG CoA reductase, further reducing its quantity.