BICH 411 Cholesterol

The mechanism of HMG-CoA reductase involves

- 2 NADPH are used up - form a high energy intermediate

How do statins work?

- HMG-reductase inhibitors that mimic the structure of transition state - form of an inactive lactone that when metabolized in the body with H20 becomes a competitive inhibitor of reductase - form a stable imitation of an unstable transition state

Low density lipoproteins

- bind to tissues to deliver choleseterol - core consists primarily of cholesterol esters

Describe the function of pyrophosphomevalonate decarboxylase

- converts phosphomevalonate to isopentyl pyrophosphate via addition of a 2nd phosphate which causes immediate decarboxylation and dephosphorylation

Bile Salts

- essential for digestion of food - carboxylic acid derviatives of cholesterol - target for drugs which lower reabsorption of 2˚ bile salts which results in the conversion of more cholesterol to bile salts.

Lipoprotein lipases

- glycosol phosphatidylinositol anchored proteins on the extracellular side of intestinal mucosal cells - catalyze hydrolysis of triacylglycerols in chylomicrons and VLDLs to release free FAs to tissues - release glycerol into the bloodstream where it is broken down in the liver - chylomicrons and VLDLs need to have APO-C2

importance of HMG-Coa

- intermediate of both cholesterol and ketone body synthesis - branch point of pathway that either commits to cholesterol synthesis or ketone body formation

Prenyl transferase

- involved into 2 successive condensation reactions - 1st condenses diacylyl PPi with isopentyl PPi to make a 10 carbon geranyl PPi - 2nd condenses geranyl PPI with isopentyl PPi to make a 15 carbon Farnesyl pyrophosphate

HDLs

- made by liver and intestinal mucosal cells - reverse cholesterol transport - pick up excess cholesterol from tissue surface and bring back to liver - excess cholesterol converted to bile salts

Describe the composition of LDLs

- surface cholesterol - apoproteins which provide positive charge to bind electrostatically to LDL receptors - esterfied cholesterol core

Describe the structure of cholesterol

-4 fused hydrocarbon rings attached to a branched hydrocarbon chain -Single hydroxyl group at the C3 serves to give weak hydrophilic character to the molecule -Weakly amphipathic -Very hydrophobic - steroid

VLDLs

-made by the liver and transport triacylglycerols - pick up ApoC2 from HDLs to become active - partially reabsorbed by the liver - partially converted to intermediate density lipoproteins

What are the 5 types of lipoproteins?

1. Chylomicrons 2. Very low-dentsity lipoproteins (VLDLs) 3. Intermediate-density lipoproteins (IDLs) 4. Low-density lipoproteins (LDLs) 5. High-density lipoproteins (HDLs)

List and describe the three ways in which cholesterol synthesis is regulated.

1. Phosphorylatin (hormonal control): pKa inactivates reductase via phosphorylation (glucagon) while phosphatase 2A activates reductase via dephosphorylation (insulin) 2. degradation of HMG-CoA reductase due to short 3 hour half life 3. Gene expression: controlled by cholesterol levels that when high block expression of HMG-CoA reductase

List the steps to make mevalonate

1. Thiolase: condenses two acetyl coA into acetoactyl coA 2. HMG-CoA Synthase: adds another CoA and water to make HMG-Coa 3. HMG Reductase:makes mevalonate/mevalonic acid from HMG-Coa

List the enzymes used in going from mevalonate to squalnene

1. mevalonate kinase 2. phosphomevalonate kinase 3. Pyrophosphatemevalonate decarboxylase 4. Isopentyl pyrophosphate isomerase 5. prenyl transferase x 2

How many carbons does cholesterol have and from where do they originate?

27 carbons all from acetyl coA

How many phosphorylation reactions are involved in the conversion of mevalonate to squalene?

3 (3 ATP are used up)

Familial Hypercholesterolemia

A metabolic disorder that is caused by defective or absent receptors for LDLs on cell surfaces, that is marked by an increase in blood plasma LDLs and by an accumulation of LDLs in the body resulting in an increased risk of heart attack and coronary heart disease, and that is inherited as an autosomal dominant trait. Could be caused by a receptor that is not synthesized, not transported to surface, that fails to bind LDL, fails to cluster, or fails to release LDL.

function of acyl coA cholesterol transferase

Acylates the hydroxyl group on cholesterol, esterifying cholesterol to make it completely hydrophobic and able to be stored inside VLDLs

How does cholesterol affect membrane fluidity?

Increases temperature range for membrane fluidity so that in high temperatures it decreases fluidity and maintains rigidity while in low temperatures it increases fluidity by preventing tight packing of fatty acid

Why is LDL cholesterol bad?

LDLs can become oxidized which reduces their positive charge and makes them unable to bind LDL receptors and deposit cholesterol. Because of this, LDLs accumulate and are taken in by macrophages which become full and form plaques in arteries

Is HMG-CoA reductase active when it is phosphorylated?

No

What is cholesterol a precursor for

bile salts, steroid hormones, vitamin D

squalene synthase

condenses 2 farnesyl pyrophosphates to make squalene

LDL receptor

consists of a binding domain on the N terminus with a net negative charge that allows electrostatic binding to LDL; mutations in receptor result in familial hypercholesteremia

Describe how acetyl coA is transported into the cytosol.

converted to citrate in the matrix and transported out of the cell where it is converted into OAA and acetyl coA which can then be used for biosynthesis

How are excess cholesterols used?

converted to primary bile salts (cholic acid, glycocholic acid, and taurocholic acid) in the liver by cholesterol 7 hydroxylase

Chylomicrons

lowest density, largest size lipoprotein made by intestinal mucosal cells to transport dietary fat must pick of ApoC2 from HDLs in order to be active hydrophobic core has triacylglycerols lipases release triacylglycerols to tissues remnants are taken back in to the liver

Where is cholesterol synthesized?

made in the liver which the primary tissue that makes cholesterol in its cytosol as well as on the surface of the smooth ER; enzymes involved are water soluble or integral membrane proteins with their active sites on the cytosolic side

mechanism of thiolase

ping-pong kinetics, covalent catalysis that forms a thioaceylenzyme intermediamete - claisen condensation

Where is cholesterol synthesis regulated and why?

regulated at HMG-CoA Reductase step which converts HMG-CoA into mevalonic acid; rate limiting step in cholesterol biosynthesis due to formation of a high energy transition step

The density of a lipoprotein is indirectly proportion to ...

the size of the lipoprotein (higher density, smaller size)