Lipid Metabolism

What are the three stages of cholesterol synthesis and where do they occur?

Stage 1: Synthesis of isopentenyl pyrophosphate from mevalonate (occurs in the cytoplasm).

Stage 2: Condensation of six molecules of isopentenyl pyrophosphate to form squalene (occurs in the ER).

Stage 3: Cyclization of squalene into cholesterol (occurs in the ER).

What is the committed step of cholesterol synthesis?

The formation of mevalonate from HMG-CoA, catalyzed by the enzyme HMG-CoA reductase.Regulation & Clinical Application

What are the four mechanisms that regulate HMG-CoA reductase?

Transcription: Control of the synthesis of mRNA for the enzyme.

Translation: Regulation of the rate of translation by dietary cholesterol and mevalonate metabolites.

Proteolytic Degradation: Increased cholesterol levels trigger the breakdown of the enzyme.

Phosphorylation: Inactivation by AMP-dependent kinase when cellular energy is low.

How do statins work at a biochemical level?

Statins act as competitive inhibitors of HMG-CoA reductase, preventing the synthesis of new cholesterol.

What role does PCSK9 play in cholesterol regulation?

PCSK9 is a protein that binds to LDL receptors and prevents them from recycling, leading to their degradation. Blocking PCSK9 allows receptors to recycle, lowering blood LDL levels.Lipoproteins & Transport

Compare the primary roles of LDL and HDL.

LDL (Low-Density Lipoprotein): The major carrier of cholesterol in the blood, delivering it to peripheral tissues via receptor-mediated endocytosis.

HDL (High-Density Lipoprotein): Carries cholesterol from peripheral tissues (like macrophages) back to the liver for excretion or use as bile salts; this is known as reverse cholesterol transport.

What are "foam cells" and how do they contribute to cardiovascular disease?

Foam cells are macrophages that have ingested excess oxidized LDL. They become trapped in blood vessels, contributing to the development of atherosclerosis and cardiovascular disease.

What molecule serves as the precursor for storage lipids (triacylglycerols) and membrane lipids?

Phosphatidate (formed from glycerol 3-phosphate and two fatty acids).

What important biochemicals are synthesized from cholesterol?

Steroid hormones.Vitamin D.Bile salts (which act as detergents for lipid diges

How is phosphatidate formed?

the addition of two fatty acids to glycerol 3-phosphate

What are the three stages of cholesterol being synthesized from acetyl Coenzyme A

1. isopentenyl pyrophosphate is synthesized from mevalonate (cytoplasm)

2. six molecules of isopentyl pyrophosphate condense to form squalene (endoplasmic reticulum)

3. squalene cyclizes and is converted into cholesterol (ER)

What is the committed step in cholesterol synthesis and why?

formation of mevalonate by HMG CoA reductase that

What is mevalonate converted into?

isopentennyl pyrophosphate (an active isoprene)

How is Cholesterol Synthesis Regulated

• liver is major site of cholesterol biosynthesis

• controlling the amount and activity of HMG-CoA reductase

How do lipids move around

in chylomicrons (a specific type of lipoprotein)

The regulatory enzyme of cholesterol synthesis is least active when

• Phosphorylated

• Dephosphorylated

phosphorylated

How are cholesterol and triacylglycerols transported into the blood

in the form of lipoprotein particles

What do low-density lipoproteins deliver

cholesterol to peripheral tissues which then enter the cell by receptor mediated endocytosis

Steps of how LDL enters cell

1. LDL binds to the LDL receptor on the cell surface

2. the receptor LDL complex is internalized, a process called endocytosis

3. the LDL is hydrolyzed in lysosomes, whereas the LDL receptor is returned to the cell surface ot find another LDL particle

What genetic disease results from the absence of the functional LDL receptor

familial hypercholesterolemia

Based on the content of the document and the screenshot you shared, here is a second set of flashcards focused on Phosphatidate synthesis and the broader Lipid Metabolism map.

Phosphatidate & Membrane Lipids

What are the two main precursors for Phosphatidate synthesis?

: Glycerol 3-phosphate and Fatty Acyl CoA.

In which cellular organelle is Phosphatidate primarily synthesized?

: The Endoplasmic Reticulum (ER).

Phosphatidate is a common precursor for which two major lipid classes?

: 1. Triacylglycerols (Storage lipids)
2. Phospholipids (Membrane lipids)

Clinical & Hormonal Connections

What is the relationship between cholesterol and Vitamin D?

: Cholesterol serves as the precursor molecule for the synthesis of Vitamin D (as well as steroid hormones and bile salts).

What clinical condition results from excess cholesterol precipitating in the gallbladder?

: Cholelithiasis (gallstones), which can lead to inflammation of the gallbladder.

How do bile salts assist in lipid digestion?

: They act as detergents that emulsify dietary lipids, making them more accessible to breakdown by lipases.

Pathway Integration (The Metabolism Map)

According to the metabolism map, which molecule acts as the "hub" connecting carbohydrate metabolism (glycolysis) to lipid synthesis?

: Acetyl CoA.

What is the significance of the "Reverse Cholesterol Transport" pathway?

: It is the process where HDL removes excess cholesterol from peripheral tissues (like macrophages in blood vessels) and returns it to the liver, reducing the risk of atherosclerosis.

Enzyme Regulation Redux

Why does low cellular energy (high AMP levels) inhibit cholesterol synthesis?

: Because AMP-activated protein kinase (AMPK) phosphorylates and inactivates HMG-CoA reductase to conserve energy.

True or False: Dietary cholesterol can influence the translation rate of HMG-CoA reductase mRNA.

: True. High levels of dietary cholesterol (and mevalonate-derived intermediates) inhibit the translation of the e

How does HDL protect against atherosclerosis

it removes cholesterol from macrophages and returns it to the liver for use as bile salts or excretion

What are the two ways to control high cholesterol

1. bile slat readsorption from the intestine is prevented by ingesting a resin that binds to the bile salts

2. new cholesterol synthesis is prevented by inhibition of HMG-CoA reductase by a class of drugs called statins

What are the three primary types of biochemicals synthesized from cholesterol?

1. Steroid hormones
2. Vitamin D
3. Bile salts

Card 2: Bile Salt Function

What is the primary physiological role of bile salts in digestion?

They act as detergents that emulsify dietary lipids, making them more accessible for digestion by lipases.

Card 3: Physiology of Bile

Where are bile salts synthesized and where are they stored?

* Synthesized: Liver

Stored: Gallbladder (until secreted into the small intestine)

Card 4: Clinical Correlation (Cholelithiasis)

What occurs when there is excess cholesterol in the bile?

The cholesterol may precipitate to form gallbladder stones, leading to inflammation of the gallbladder—a condition known as cholelithiasis.

Card 5: Chemical Structure

Look at the molecule Glycocholate shown in the image. What major precursor molecule provides its core four-ring structure?

Cholesterol. (Note the addition of hydroxyl groups and the glycine conjugation that makes it a bile