Biochemistry: Cholesterol

Where is cholesterol found?

Membrane bilayers (a structural component of cell membranes)

What is cholesterol a precursor to?

Steroid hormones and bile salts

How is cholesterol in blood packaged?

Inside a lipoprotein, as free cholesterol and cholesteryl ester

Dietary cholesterol goes to the _____ in what form?

Liver; cholesteryl esters in chylomicron remnants

Endogenous cholesterol synthesis is done primarily by what organ?

Liver

T/F: The ring structure of cholesterol can be broken down in humans for energy.

False

What are the two lipoproteins in fasting blood of clinical relevance?

LDL and HDL

What are chylomicrons formed from?

Dietary cholesterol and fat

What is the major apoprotein of chylomicrons?

ApoB-48

What is recognized by chylomicron remnant receptors in the liver?

ApoB-48

Chylomicron remnants are taken up by the _____.

Liver

The liver puts out a lipoprotein called ____.

VLDL

What lipoprotein does the liver most directly produce to deliver cholesterol and fatty acids to extrahepatic tissues?

VLDL

Lipid transport out of the liver

Liver -> VLDL -> (metabolized in circulation) -> LDL

What is the final product in the metabolism of VLDL?

LDL

What enzyme participates in converting VLDL to LDL?

Hepatic lipase (HTGL)

What activates LPL (lipoprotein lipase)?

ApoC-II

LDL is the major transporter of _________.

Cholesterol (as cholesteryl esters)

Where does LDL carry cholesterol?

Peripheral tissues, including the adrenals and sex glands (converted to steroid hormones), or back to the liver (may be converted to bile acids)

What is the major apoprotein of LDL?

ApoB-100

The ApoB-100 protein on LDL is recognized by what?

The LDL receptor (found on a wide variety of cells)

What is the key to the delivery of LDL-cholesterol to peripheral tissues?

Binding of ApoB-100 to LDL receptors

LDL delivers cholesteryl esters to cells and is taken up by the ___ receptor.

LDL

The LDL receptor is recycled to the cell surface every __ minutes.

About 10 minutes

T/F: A little of the LDL receptor gets degraded each recycling cycle.

True

Cholesterol _______ LDL receptor synthesis.

Reduces (negative feedback)

Low cholesterol leads to ____ LDL receptor.

More

Cholesterol-deficient cells increase the LDL receptor, which does what?

Removes more LDL from the blood

A patient has high LDL. Their cells can't bind LDL from control serum, anti-LDL-receptor antibodies bind their cells, and control cells degrade the patient's LDL normally. Most likely cause?

A mutant LDL receptor that cannot bind LDL

What does HDL do?

Removes excess cholesterol from cells and carries it to the liver

Why is HDL considered cardioprotective?

It removes cholesterol from cells and carries it to the liver

HDL is released as an "______ _____" by the liver and intestine.

Empty shell

HDL transfers which proteins?

ApoC-II and ApoE, to other lipoproteins such as VLDL

HDL removes excess cholesterol from cell surfaces via ____.

LCAT

What does LCAT stand for?

Lecithin-cholesterol acyltransferase

What does LCAT do?

Adds a fatty acid to cholesterol at a cell surface, forming a cholesteryl ester that is transported onto HDL

What activates LCAT?

ApoA-1

Where does mature HDL get taken up?

Liver and adrenals

When serum cholesterol is too abundant, what happens to circulating LDL?

It increases

What is increased LDL a root cause of?

Atherosclerosis

Why is elevated LDL a risk factor for heart disease?

It causes atherosclerosis

What happens to excess LDL?

It is oxidized

Why is oxidized LDL important?

It is a key step in atherosclerotic plaque formation

How can LDL levels be controlled?

Decreasing synthesis (statins, i.e., HMG-CoA reductase inhibitors); decreasing dietary cholesterol; promoting excretion of excess cholesterol (e.g., dietary fiber)

Why might dental work differ for a patient with heart disease?

Risk of oral bacteria spreading to a compromised heart

Do we need cholesterol in the diet?

No; we can make plenty from carbohydrate-derived food sources

Why do older patients often have high LDL despite diet and lifestyle changes?

Endogenous synthesis (and genetics) plays a major role, not just diet

Cholesterol is synthesized in the _________ from what?

Cytoplasm; acetyl-CoA

Cholesterol synthesis uses cytoplasmic _________.

Acetyl-CoA

Where is acetyl-CoA generated?

The mitochondria

Why is the citrate shuttle needed?

Acetyl-CoA has no transporter to cross the mitochondrial membrane, so it is moved to the cytosol as citrate

Where does cytoplasmic acetyl-CoA come from?

Citrate exported from the mitochondria, split by citrate lyase

Citrate shuttle pathway

Glucose -> 2 pyruvate -> (pyruvate dehydrogenase) -> mitochondrial acetyl-CoA + oxaloacetate -> citrate -> exported to cytosol -> citrate lyase regenerates acetyl-CoA

Citrate lyase reaction

Citrate + ATP + CoA + H2O -> acetyl-CoA + ADP + Pi + oxaloacetate

How is NADPH generated for cholesterol synthesis?

Oxaloacetate + NADH -> malate (malate dehydrogenase); malate + NADP+ -> pyruvate + CO2 + NADPH (malic enzyme)

Cytoplasmic acetyl-CoA is converted to…?

HMG-CoA (3-hydroxy-3-methylglutaryl-CoA)

Full cholesterol synthesis pathway

Acetyl-CoA -> HMG-CoA -> (HMG-CoA reductase) -> mevalonate -> IPP (C5) -> geranyl PP (C10) -> farnesyl PP (C15) -> squalene (C30) -> lanosterol -> cholesterol

What are the key intermediates in cholesterol synthesis?

Mevalonate, IPP, geranyl PP, farnesyl PP, squalene, lanosterol

What is the major regulated step in cholesterol synthesis?

HMG-CoA reductase

What is the first committed step in cholesterol biosynthesis?

HMG-CoA reductase

Cholesterol synthesis is regulated at…

HMG-CoA reductase

What is the product of HMG-CoA reductase?

Mevalonate

Which is false about HMG-CoA reductase? (practice question)

Its product is acetyl-CoA (false; its product is mevalonate)

What does HMG-CoA reductase oxidize?

NADPH to NADP+

What inhibits HMG-CoA reductase?

Free cholesterol, phosphorylation, and statin drugs

What drug inhibits HMG-CoA reductase?

Statins

Why are statins effective cholesterol-lowering drugs?

They inhibit HMG-CoA reductase, the major regulated step in cholesterol synthesis

Which statement about cholesterol regulation is false? (practice question)

Statin drugs inhibit cytoplasmic HMG-CoA synthase (false; statins inhibit HMG-CoA reductase)

Cholesterol acts as a feedback regulator to ________ the synthesis of HMG-CoA reductase.

Decrease (reducing the cell's ability to make new cholesterol)

What are the two ways cholesterol suppresses HMG-CoA reductase?

Inhibits its activity (feedback inhibition) AND decreases synthesis of the enzyme (amount)

HMG-CoA reductase is controlled by the rate of _________ and ___________.

Synthesis and degradation (short half-life)

What is the half-life of HMG-CoA reductase?

About 3 hours

HMG-CoA reductase is under extensive __________ regulation.

Allosteric

What is mevalonate converted to?

A 5-carbon building unit, isopentenyl pyrophosphate (IPP)

Mevalonate is decarboxylated using what, to form what?

3 ATP; the 5-carbon 3-isopentenyl pyrophosphate (IPP)

What is squalene?

The 30-carbon linear precursor to cholesterol

What is the first reaction in the synthesis of squalene?

IPP condenses with its isomer 3,3-dimethylallyl pyrophosphate (DMAPP) to form the 10-carbon geranyl pyrophosphate

Geranyl pyrophosphate condenses with another IPP to form what?

The 15-carbon farnesyl pyrophosphate

What is the final reaction of squalene synthesis?

Condensation of 2 farnesyl pyrophosphates, using NADPH

How many carbons are in squalene?

30

Carbon count sequence in squalene synthesis

C5 -> C10 -> C15 -> C30

T/F: Squalene can be depicted in a pseudo-cyclic fashion, which enables the cyclization reactions that lead to sterols.

True

Squalene is oxidized and undergoes ___________ to form __________.

Cyclization; lanosterol

Lanosterol (C30) undergoes demethylation and rearrangement to form the 27-carbon sterol ___________.

Cholesterol

A defect in which enzyme is least likely to inhibit cholesterol biosynthesis?

7-alpha-hydroxylase (used to make bile salts)

What are the 2 fates of HMG-CoA?

1) In the cytoplasm: HMG-CoA reductase -> 5-carbon units -> cholesterol

2) In liver mitochondria (separate pool): ketone bodies

_______ can appear in someone's breath.

Acetone

Ketone bodies are synthesized via what?

HMG-CoA (a separate mitochondrial pool in the liver)

When does the liver convert fatty acids to ketone bodies?

When fatty acids are released from adipose: fasting, diabetes, or a diet much higher in fat than sugar

Why does the TCA cycle slow in the liver during ketogenesis?

Oxaloacetate is diverted to make glucose (gluconeogenesis)

In the liver, the TCA cycle slows and acetyl-CoA accumulates in the ___________.

Mitochondria

Excess acetyl-CoA is converted to ___________, and then into what?

Acetoacetyl-CoA; then the ketone bodies acetoacetate + beta-hydroxybutyrate

Where does acetone come from?

Acetoacetate; it gives the breath a fruity smell

A patient has fruity breath and was pre-diabetic 4 years ago. What is appearing in the breath?

A ketone body (acetone)

T/F: Ketone bodies are soluble in water.

True

T/F: Ketone bodies can be carried in the blood to other tissues.

True

T/F: Ketone bodies can be converted back to acetyl-CoA to provide energy.

True

Where can ketone bodies NOT be converted back to acetyl-CoA?

The liver (it makes them but cannot use them)

Why are ketone bodies especially important for the brain?

Fatty acids cannot cross the blood-brain barrier, and blood glucose is low during fasting

Why is cholesterol synthesis relevant to dental practice?

Cholesterol-lowering medications (statins) may affect tooth pulp density