Chapter 21: Lipid Biosynthesis: Fatty Acids, Membrane Phospholipids, Cholesterol, and Steroids

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Welcome to The Deep Dive, the show that takes complex information, distills it, and gives you that shortcut to being truly well -informed.

Today we're embarking on a deep dive into a world that's often misunderstood, maybe sometimes even villainized, but absolutely essential to life, the world of fats and lipids.

Beyond just energy storage, these molecules play these incredible multifaceted roles in our bodies.

Indeed, it's a really fascinating area of biochemistry, much more intricate and elegant than many people realize.

Our goal today is to unravel the molecular mechanisms and metabolic pathways behind lipid biosynthesis.

We'll be drawing insights from Nelson and Cox's Leninger Principles of Biochemistry.

Think of this as your sort of self -contained pedagogically structured guide, understanding how life builds its essential fatty molecules.

Perfect for, say, an upper division undergraduate, a seasoned professional, or just anyone intensely curious looking for that aha moment.

Okay, let's unpack this.

When we think of fats, we often jump straight to energy storage, right?

Right, and that's true.

Lipids are the principal form of stored energy in most higher organisms.

They're also major building blocks of

But what's truly surprising, I think, is their sheer variety of other crucial cellular roles.

Absolutely.

What's fascinating here is just how diverse these roles are.

Lipids aren't just passive components, they are active players.

They serve as pigments, like retinal in our eyes or carotene in plants, essential co -factors, like vitamin K for blood clotting, detergents, you know, such as the bile salts that help us digest food, and even potent hormones like vitamin D derivatives and sex hormones.

Plus, they're vital intra - and extracellular messengers signaling molecules telling cells what to do, even anchors for membrane proteins.

It's, well, an incredible versatility essential for life.

So we're talking about building these amazing molecules.

You mentioned the nabolism earlier, which is the process of building things up.

How does building fats differ from breaking them down?

It can't just be the reverse, can it?

That's a really crucial insight.

And nabolism or biosynthesis, it's never simply the reverse of catabolism or breakdown.

Biosynthetic pathways are endergonic, meaning they require an input of energy, typically from ATP, and they are reductive, meaning they use electron carriers like NADPH to add electrons.

For fatty acids, this division is particularly striking.

Breakdown happens in the mitochondria, but biosynthesis occurs in the cytosol.

This spatial separation is a critical point.

It helps the cell regulate and control these distinct processes.

That's a brilliant bit of cellular organization, and I hear there's a unique intermediate involved in building fats that's just not present when you're breaking them down.

Yes, biosynthesis crucially involves a three -carbon intermediate called melonal CoA.

This molecule is completely absent in fatty acid breakdown.

It serves as a clear chemical signature of synthesis.

So how does the cell create this crucial starting point, melonal CoA?

Melonal CoA is formed from acetyl CoA and bicarbonate.

It's an irreversible three -step process catalyzed by acetyl CoA carboxylase.

This enzyme in animal cells is a single large multifunctional polypeptide, and it contains a biotin prosthetic group.

Biotin is like a molecular hand, you could say.

It picks up CO2 and then transfers it to acetyl CoA to yield melonal CoA.

But wait, why go to all that trouble adding CO2 just to remove it later in the process?

It seems bit, I don't know, redundant.

What's the clever trick here?

That raises an important question, and it's a beautiful example of biochemical elegance, the decarboxylation, the removal of that CO2 from the melonal group in a later step.

Well, it makes that reaction much more thermodynamically favorable.

It provides crucial energy boost.

It essentially pulls the synthesis forward, like adding a small energetic kick to ensure the whole process proceeds efficiently and in the right direction.

That makes so much more sense.

So once we have melonal CoA, how do we actually build those long fatty acid chains?

That's done by a repeating four -step sequence, catalyzed by an enzyme system called fatty acid synthase.

Each cycle extends the fatty acyl chain by two carbons.

We primarily see two types of these systems, FASI and FAS2.

FASI is the one found in mammals and fungi, right?

Right.

And it sounds like a highly efficient molecular assembly line.

Exactly.

The mammalian FASI system is a single massive multifunctional peptide.

It has seven active sites all working together.

It's incredibly efficient and typically produces a single specific product, palmitate, a 16 -carbon fatty acid.

In contrast, FAS2 found in plants, most bacteria, and even vertebrate mitochondria is a dissociated system.

Each step is catalyzed by a separate enzyme.

This allows it to yield a much wider variety of fatty acids.

And central to this whole process is the acyl carrier protein, or ACP.

It sounds like a molecular shuttle, almost like a flexible arm, keeping everything moving along the assembly line.

Precisely.

ACP, with its long flexible arm ending in a four -out phosphopentethene prosthetic group, which is quite similar to coenzyme A, actually acts as a tether.

It holds onto the growing fatty acyl chain and physically swings intermediates between the various active sites of the fatty acid synthase complex.

This ensures that no intermediates are released.

It makes the process highly channeled and incredibly efficient.

Fascinating.

So let's walk through the four steps of chain elongation itself.

What happens first?

The first step is condensation.

An acetyl group, initially held by the enzyme, condenses with the malonyl group attached to ACP.

Crucially, as they join, the CO2 from the malonyl group is released.

This is that thermodynamic advantage we talked about.

Makes the step highly energetic and irreversible.

So that's where the CO2 comes off.

What's next?

Next is reduction.

The newly formed carbonyl group on the growing chain is reduced by NADPH, forming a hydroxyl group.

It's important to note the specific D stereochemistry here.

It's opposite to the L intermediate found in fatty acid breakdown.

Okay, D versus L, and after that we get a double bond.

Yes.

The third step is dehydration.

Water is removed, creating a double bond in the fatty acyl chain.

And finally, a second reduction.

The fourth step is another reduction.

The double bond is reduced by NADPH, yielding a fully saturated acyl group.

It's now two carbons longer and ready for the next cycle of elongation.

So seven cycles of these four reactions ultimately produce the common 16 -carbon fatty acid, palmitate.

What's the energy cost for all this?

It seems like a lot.

Synthesizing just one palmitate molecule is quite energy intensive, yes.

It requires significant investments of both ATP and NADPH.

It's a major energetic commitment for the cell, which really highlights the importance of fat storage.

That leads to the next question.

Where does all this happen in the cell?

You mentioned cytosol earlier for animal cells.

Right.

In animal cells, fatty acid synthesis primarily occurs in the cytosol.

This effectively keeps it separate from the breakdown pathways in the mitochondria.

In plants, the synthesis happens in the chloroplast stroma.

That makes perfect sense in plants, where chloroplasts get NADPH from light reactions.

But in animal cells, where do we get all that NADPH in the cytosol?

You need a lot to power these reductions.

Good question.

In animals, cytosolic NADPH is largely generated by two key pathways,

the pentose phosphate pathway and the malic enzyme.

This ensures a high NADPH and NADP plus ratio in the cytosol, creating the strongly reductive environment needed for building these molecules.

And what about the starting material, acetyl CoA?

Most of it is made in the mitochondria, but the inner mitochondrial membrane is impermeable to it.

How does it get to the cytosol where synthesis happens?

Ah, that's where a clever mechanism comes in, often called the group shuttle.

Acetyl CoA in the mitochondria combines with oxaloacetate to form citrate.

Citrate then moves out to the cytosol via a specific transporter.

In the cytosol, an ATP -dependent enzyme called citrate -liase regenerates acetyl CoA and oxaloacetate.

Now, oxaloacetate is then often converted to malate and then to pyruvate by malic enzyme, which conveniently generates more of that crucial NADPH right there in the cytofol.

Wow, so the cell is incredibly strategic, making sure it has both the building blocks and the power.

Given this complexity, how is fatty acid biosynthesis regulated?

What's the main control point?

This raises an important question about metabolic control.

The enzyme acetyl CoA carboxylase, the one that forms malonyl CoA, is the REIT limiting step.

It's a major regulatory hub.

When the cell has more metabolic fuel than needed, excess gets converted to fatty acids, and this enzyme is tightly controlled to manage that process.

So it's like the gatekeeper.

What are the key signals that regulate it?

Precisely.

It's regulated by both allosteric control and hormonal mechanisms.

Allosteric control means molecules binding to the enzyme but not at the active site.

For instance, palmitoyl CoA, the final product.

The fatty acid itself.

Right.

It acts as a feedback inhibitor, signals that enough product has been made.

Conversely, citrate acts as an allosteric activator.

High citrate levels indicate an abundance of metabolic fuel.

It also inhibits glycolysis, essentially diverting carbon flow towards storage.

And hormones play a big role, too, I imagine, linking this to the body's overall energy state.

Absolutely.

Phosphorylation, triggered by hormones like glucagon and epinephrine or by low energy signals like high AMP levels,

inactivates the enzyme.

This slows down fatty acid synthesis when the body needs energy.

On the other hand, dephosphorylation, promoted by insulin, activates it.

That signal's fuel is plentiful and can be stored.

This enzyme also dramatically changes shape, forming long, active filaments when dephosphorylated and dissociating into inactive monomers when phosphorylated.

It's quite dynamic.

And it seems like the cell also has mechanisms to prevent futile cycles, making sure it doesn't break down fats at the same time as trying to build them.

Right.

That would be wasteful.

That's another critical aspect of its elegance.

Melonal CoA, that very first intermediate in fatty acid synthesis.

It also inhibits carnitine assault transferase II.

This enzyme is essential for transporting fatty acids into the mitochondria for breakdown.

So the mere presence of Melonal CoA effectively shuts down fatty acid oxidation, prevents a futile cycle, conserves energy.

There's also longer term regulation at the gene expression level.

Certain polyunsaturated fatty acids can suppress the expression of lipogenic enzymes.

Okay.

We've talked about making palmitate, the 16 carbon fatty acid, but our bodies need longer chains and unsaturated fats.

How are those made?

Palmitate serves as the primary precursor.

It can be elongated to stearate and 18 carbon fatty acid and even longer ones.

This happens via fatty acid elongation systems located in the smooth ER and mitochondria.

While the enzyme systems differ slightly and coenzyme A is the carrier, the mechanism is pretty similar.

Two carbons added from Melonal CoA followed by a sequence of reductions and dehydration.

And what about introducing double bonds?

That's called desaturation.

Correct.

Palmitate and stearate can be desaturated to form mono unsaturated fatty acids like palmitoliate and oleate.

Both get a cis double bond at a specific position.

This process is catalyzed by fatty acyl CoA desaturase, which is a type of mixed function oxidase.

This means it uses molecular oxygen and NADPH to directly insert a double bond into the fatty acid chain.

Now here's a critical point for human health.

Mammals can't make all fatty acids.

We hear a yes, this is where it gets really interesting for us.

Mammals, including humans, cannot synthesize linoleate and omega -6 fatty acid or ilinoleate and omega -3 fatty acid.

These must be obtained directly from our diet, primarily from plants.

They are absolutely crucial.

They serve as precursors for a huge range of other polyunsaturated fatty acids and important signaling molecules, including the icosanoids.

Icosanoids.

These are the powerful short range messengers like local hormones that such potent effects in our bodies, inflammation, pain.

Exactly.

Icosanoids are derived from 20 and 22 carbon polyunsaturated fatty acids, especially arachidonate.

In response to various stimuli, an enzyme called phospholipase A2 releases arachidonate from our cell membrane phospholipids.

Then other enzymes in the smooth ER convert this arachidonate into various icosanoids, orchestrating a whole host of cellular responses.

Let's talk about the cyclic pathway that leads to prostaglandins and thromboxanes, because this is where a lot of common medications come into play, like aspirin.

That pathway begins with cyclooxygenase, or COX, also known as prostaglandin H2 synthase.

This is a fascinating bifunctional enzyme.

It has both cyclooxygenase and peroxidase activities.

It first converts arachidonate to PG2, then to PGH2, which then serves as the precursor for other prostaglandins and thromboxanes.

And this is where the story of common pain relievers comes in.

There are different COX isoforms, right?

COX1 and COX2?

Yes, mammals have two main isozymes, COX1 and COX2.

COX1 generally regulates routine physiological functions, like protecting the stomach lining, you know, housekeeping roles.

COX2, on the other hand, is primarily responsible for mediating inflammation, pain, and fever.

It's often induced in response to injury or infection.

And this is why nonsteroidal anti -inflammatory drugs, NSAIDs, like aspirin and ibuprofen, are so effective for pain relief, they block COX.

But there's also a cautionary tale here with some of the more specific COX2 inhibitors, isn't there?

Indeed.

Aspirin, a truly remarkable molecule, irreversibly inactivates both COX isozymes.

It does this by permanently modifying a specific serine residue in their active sites.

That inhibits prostaglandin and thromboxane synthesis.

Other NSAIDs, like ibuprofen, work similarly, though reversibly.

Initially, specific CRX2 inhibitors were developed to avoid QOX1's gastric side effects.

Sounded like a good idea.

However, some, like roficoxib, remember Vioxx, were later withdrawn due to an increased risk of heart attack and stroke.

It really highlights the complex interconnected nature of these signaling pathways.

It's not always simple.

So it's not always as straightforward as just blocking one pathway.

What about thromboxane specifically?

What's their role?

Thromboxanes are formed from PGH2, primarily in blood platelets.

They induce blood vessel constriction and promote platelet aggregation, which is crucial for blood clotting.

This is precisely why low doses of aspirin, taken regularly, can reduce the risk of heart attacks and strokes.

By reducing thromboxane production, it prevents unwanted clot formation.

And then there's the linear pathway for leukotrienes, different enzymes involved.

Exactly.

Leukotrienes are formed by lipoxogenases, a different family of enzymes.

Unlike the cyclic pathway, this linear pathway is not inhibited by aspirin or other NSAIDs, meaning they affect different aspects of the inflammatory response.

We often hear about inflammation as a negative thing, but it's actually a necessary process for healing.

And I've heard that different eicosanoids play roles in both initiating and resolving it.

Is that right?

That's a crucial distinction.

Omega -6 eicosanoids are indeed critical for initiating the acute inflammatory response, recruiting immune cells, getting the defense started.

But inflammation also needs to be actively resolved, a process called catabasis.

You can't just leave it going.

This is actively promoted by certain leukotrienes and prostaglandins, and importantly, by an exciting newer class of eicosanoids called specialized proresolving mediators,

or SPMs.

SPMs.

That sounds incredibly promising for new therapies.

What are they?

It truly is.

These include molecules like lipoxins, resolvins, protectins, and maresins.

They are derived from essential fatty acids, both omega -3 and omega -6, and they actively promote the resolution of inflammation.

This means clearing debris, microbes, dead cells, restoring blood vessel integrity, promoting tissue regeneration.

They actively turn off the inflammation.

They also reduce pain and fever.

They're being actively explored as potential new pharmaceutical targets to combat chronic inflammatory diseases.

Interestingly, even plants use similar fatty acid derivatives like jasmineate for signaling roles in defense and development.

It's a common theme.

So fatty acids are built, elongated, desaturated, and give rise to these powerful signaling molecules.

But what are their main fates once they're synthesized?

Storage or membranes?

Exactly.

Fatty acids synthesized or ingested are primarily destined for one of two fates.

Long -term energy storage as triacylglycerols or membrane construction as phospholipids.

And triacylglycerols really are the ultimate energy vault, aren't they?

It's amazing how much energy we can pack into them compared to, say, glycogen.

They are incredibly efficient.

Humans can store only maybe a few hundred grams of glycogen, enough for about 12 hours of energy needs, maybe a day at most.

But a 70 kilogram human stores around 15 kilograms of triacylglycerol.

That can support basal energy needs for up to 12 wints.

They pack the highest energy content of all stored nutrients.

So whenever we consume excess carbohydrate, it's efficiently converted to triacylglycerols and tucked away in adipose tissue.

How do triacylglycerols and phospholipids get started?

Do they share initial building blocks?

They do.

Both share fatty acyl -CoA and L -glycerol 3 -phosphate as precursors.

Most glycerol 3 -phosphate comes from dihydroxyacetone phosphate, which is an intermediate from glycolysis.

In the liver and kidneys, some also comes directly from glycerol via an enzyme called glycerolchitis.

So how do we get from these precursors to triacylglycerols?

What's the pathway?

It's essentially a three -step process.

First, two fatty acyl -CoA is attached to glycerol 3 -phosphate, forming phosphitic acid that's a key intermediate.

Okay.

Then the phosphitic acid is hydrolyzed by an enzyme called phosphatidic acid phosphatase, also known as lipin, to form diacylglycerol.

Right.

Finally, a third fatty acyl -CoA is added to that diacylglycerol to yield the final triacylglycerol.

And how is all this fat storage regulated?

Hormones, again, I'm guessing, coordinating with the body's energy needs.

Insulin, maybe?

Absolutely.

Insulin strongly promotes the conversion of excess carbohydrate to triacylglycerols.

It signals fuel abundance.

In stark contrast, in diabetes, a lack of insulin or insulin resistance leads to impaired fatty acid synthesis.

The body then relies on increased fat oxidation, sometimes leading to ketone body formation.

Conversely, hormones like glucagon and epinephrine, signaling low energy, stimulate triacylglycerol breakdown that's lipolysis and fatty acid release.

It's interesting that even during starvation, there's constant recycling of fatty acids, something like 75 % being reesterified rather than fully consumed.

This triacylglycerol cycle sounds like a bit of a metabolic mystery.

What's it for?

It really is an enigmatic loop.

About three -quarters of fatty acids released from adipose tissue are reesterified, even during starvation,

constantly cycling between adipose tissue and the liver.

Its precise function isn't fully understood, but it might represent a rapidly mobilizable energy reserve,

ready to be deployed for emergencies without the full commitment of breakdown, maybe fine -tuning fatty acid levels.

And this leads to another puzzle.

During starvation,

when glycolysis is suppressed, how do you get the glycerol 3 -phosphate needed for this reesterification?

Where does it come from, if not glucose?

Ah, that's where a fascinating pathway called glyceroneogenesis comes in.

It's basically a shortened version of gluconeogenesis.

It converts pyruvate, not glucose, into dihydroxyacetone phosphate and then to glycerol 3 -phosphate.

This pathway plays multiple roles, controlling fatty acid release from adipose, providing substrate for dermogenesis and brown fat, and supporting liver reesterification during fasting.

And hormones like cortisol, a glucocorticoid, affect this, don't they?

Yes.

Glucocorticoids reciprocally regulate a key enzyme in this pathway called PEP carboxykinase.

They increase its expression in the liver, stimulating gluconeogenesis and glyceroneogenesis there, but they suppress it in adipose tissue.

This coordinated regulation resulted in increased flux through the triacylglycerol cycle, maintaining a delicate balance.

And this pathway even has therapeutic implications for type 2 diabetes.

Indeed.

Thiazoladenionase, a class of drugs used to treat type 2 diabetes, actually stimulate glycerineogenesis in adipose tissue.

They do this by activating a nuclear tripector called PEPAERO, which increases PEP carboxykinase activity.

This leads to more triacylglycerol resynthesis in fat cells, reduced circulating free fatty acids, and ultimately improved insulin sensitivity.

However, some have been linked to increased cardiac risk, again, underscoring the complexities of fiddling with metabolism.

So fatty acids can be

Let's shift to how membranes, specifically phospholipids, are built.

What are the general rules for assembly?

Phospholipid assembly follows a few basic patterns.

First, you synthesize the backbone, either glycerol or sphingosine.

Second, attach the fatty acids.

Third, add the hydrophilic head group via a phosphodaster linkage.

And finally, there's ongoing remodeling.

This largely occurs on the smooth ER and inner mitochondrial membrane surfaces.

And there are two main strategies for attaching the head groups, part of what's called the Kennedy pathway, using CDP.

Correct.

Both strategies involve activating either the diacylglycerol or the head group with citidine diphosphate, CDP, which acts as an energy carrier.

One strategy involves CDP diacylglycerol.

This is used for certain phospholipids in eukaryotes and pretty much all phospholipids in bacteria.

The second strategy, common in mammals, uses CDP head groups to salvage free ethylamine and choline to synthesize phosphatidylethanolamine and phosphatidylcholine, kind of a recycling pathway.

So, different phospholipids have interconnected synthesis pathways depending on the organism or even what building blocks are available at the time.

Yes.

For example, in mammals, phosphatidylzisurine is derived from phosphatidylethanolin or phosphatidylcholine via simple head group exchange reactions in the ER.

That's different from direct synthesis from CDP diacylglycerol, which happens in yeast.

This constant interconversion makes membranes highly dynamic structures.

And speaking of dynamic, membranes aren't static, are they?

There's constant remodeling, famously known as the land cycle, swapping fatty acids.

That's a crucial point.

The fatty acyl groups, especially the polyunsaturated ones at a specific position, usually C2, are continually replaced.

An enzyme like phospholipase A2 removes a fatty acid, creating a lysophospholipid.

Then another enzyme, like lysophosphatidylcholine acyltransferases or replaces it with a different fatty acyl -CoA.

And LPCA3, for example, seems to be involved in regulating overall fat synthesis and even appetite, linking these seemingly simple remodeling enzymes to broader health issues like atherosclerosis, obesity and cancer.

Beyond the common phospholipids, there are also more specialized lipids like plasmalogens and sphingolipids.

Briefly, what are they?

Plasmalogens are unique because they contain an ether linkage instead of the typical ester linkage at C1 of the glycerol backbone.

Their synthesis primarily occurs in peroxisomes.

Sphingolipids are found widely in cell membranes and are important for signaling.

They're synthesized in four stages, starting from palmitoyl -CoA and serine, forming a core molecule called ceramide, and then diverse head groups are attached.

So how do all these water -insoluble lipids get to their final destinations within the cell?

They can't just diffuse through the watery cytoplasm, right?

No, they can't.

They're transported within the cell primarily via vesicles that bud from the Golgi and fuse with target membranes.

Or they can be carried by specific lipid -binding proteins in the cytosol that act like molecular chaperones, shielding the hydrophobic parts.

All right, now for the star of the show,

maybe the celebrity molecule of the lipid world, often notorious for its link to heart disease, but also absolutely crucial,

cholesterol.

It's true.

Cholesterol gets a challenging reputation, but it's an absolutely essential molecule for life.

All our cells can synthesize it from a simple precursor, acetate.

It's a critical component of cellular membranes, persisting rigidity and fluidity control, and it's the precursor for all steroid hormones and bile acids.

We can't live without it.

And building this complex 27 -carbon molecule is an intricate journey.

It happens in four main stages, starting from those simple acetate units.

Stage one.

Stage one involves building mevalinate from acetate.

Several acetyl -CoA molecules condense in a series of reactions, eventually forming all -hydroxymethylgluteryl -CoA or HMG -CoA.

And HMG -CoA reductase is the committed step, the major point of regulation for cholesterol synthesis, right?

This is where statin drugs work.

Exactly.

This enzyme reduces HMG -CoA to mevalinate, consuming two NADPH molecules.

Its activity is very tightly controlled, making it a critical choke point in the entire cholesterol synthesis pathway.

Then we move to stage two, activating the isoprene units.

What are those?

Yes.

Stage two converts mevalinate into two activated isoprene units.

These are five -carbon building blocks.

Dase, 3 -isopentanilpyrophosphate, and dimethylallylpyrophosphate.

This process consumes three ATPs per mevalinate, highlighting the significant energy investment even at this early stage.

And then these five -carbon isoprene units polymerize to form the 30 -carbon squan in stage three.

Right.

In stage three, these five -carbon activated isoprene units undergo successive head -to -tail condensations, forming 10 -carbon, then 15 -carbon chains.

The fascinating final step is when two 15 -carbon units, farnesyl pyrophosphates, join head -to -head to form the 30 -carbon linear squalling.

This entire synthesis up to squalling is energetically very expensive, takes a lot of ATP.

In stage four is where that linear squalling gets its characteristic four rings, forming the recognizable steroid structure.

That seems like magic.

It's pretty amazing biochemistry.

An enzyme called squalene monoxygenase adds an oxygen, creating an epoxide.

Then a remarkable concerted cyclization reaction spontaneously converts this linear squalene epoxide into the four -ring spheroid nucleus, forming lanosterol in animals.

Lanosterol is then converted to cholesterol through about 20 complex reactions involving subtle molecular rearrangements.

Plants and fungi use similar pathways, but produce other types of sterols like ergosterol.

Once cholesterol is made, mostly in the liver, what happens to it?

What are its diverse fates?

Most cholesterol is exported from the liver in one of three crucial forms.

First, as bile acids, principal components of bile.

They act as natural detergents to emulsify fats for digestion and are a primary way the body removes excess cholesterol.

Dietary fiber actually helps this process.

Second, some is directly secreted as biliary cholesterol.

Third, it's transported as cholesterol are more hydrophobic, formed by an enzyme called acetate for storage in lipid droplets or transport in lipoproteins.

And cholesterol is also the foundational precursor for a whole suite of important hormones.

Absolutely.

It's the precursor for all steroid hormones.

Mineralocorticoids, regulating salt balance, glucocorticoids like cortisol -managing stress, the sex hormones like estrogen and testosterone, and even vitamin D hormone.

Since lipids are insoluble in water, how do they get transported around the body in our blood?

Lipoproteins.

Exactly.

They are carried as plasma lipoproteins.

These are micromolecular complexes of specific carrier proteins called apolipoproteins, along with phospholipids, cholesterol, cholesterol esters, and triacylglycerols.

Think of them as spherical particles with a hydrophobic core full of lipids and a hydrophilic surface of proteins and phospholipid head groups, making them soluble in based on density, right?

Chylomicrons first.

Yes.

Chylomicrons are the largest and least dense.

They transport dietary fats from the intestine to tissues.

Their apolipoproteins, like apacetoo, activate lipoprotein lipase on capillary walls.

This enzyme releases fatty acids to adipose and muscle tissue.

The chylomicron remnants then go to the liver.

That's the exogenous pathway.

Then there's VLDL for fats made in the liver.

Right.

Very low density lipoproteins, VLDL, transport liver -synthesized triacylglycerols and cholesterol to peripheral tissues.

This is the endogenous pathway.

They are essentially the liver's way of exporting excess fat and cholesterol.

And VLDL eventually leads to LDL, the infamous bad cholesterol.

Exactly.

As VLDL circulate and lose triacylglycerols, they become denser and eventually transform into LDL.

Low density lipoproteins, LDL, are rich in cholesterol and cholesterol esters, and their main job is to cholesterol to extrahepatic tissues.

Cells take up LDL via specific LDL receptors through a process called receptor -mediated endocytosis.

The receptor recognizes ApoB100 on the LDL particle.

And I remember hearing about familial hypercholesterolemia in connection with these LDL receptors.

What does a genetic defect mean?

Why is it so bad?

This highlights the critical role of these receptors.

Individuals with familial

hypercholesterolemia, FH, have defective LDL receptors.

This leads to very high blood levels of LDL because cholesterol cannot be efficiently cleared from the bloodstream by cells.

This dramatically increases the risk of severe early onset atherosclerosis.

It's a clear example of how a single molecular defect can have profound health consequences.

So if LDL is bad, HDL must be good.

How does high density lipoprotein HDL function?

What makes it protective?

HDL is indeed the good cholesterol and is key in reverse cholesterol transport.

It originates in the liver and small protein -rich particles.

As it circulates, it actively picks up excess cholesterol from peripheral cells, including those macrophages that accumulate cholesterol in plaques within arteries.

This is crucial.

Enzymes on HDL like LCAT then esterify this cholesterol, making it even more hydrophobic so it stays inside the HDL core.

This cholesterol is then returned to the liver via receptors like SRBI for excretion, often as bile salts.

It actively removes cholesterol from circulation and from plaques.

So HDL really helps clear out the excess, almost like a cleanup crew actively fighting plaque formation.

Absolutely.

HDL's reverse cholesterol transport pathway plays a critical role in preventing and potentially even reversing plaque formation.

It protects against atherosclerosis.

Genetic conditions with very low HDL levels like Tangier disease involving the ABCA1 transporter underscore its immense protective role.

Given how vital cholesterol is and how problematic too much can be, its metabolism must be incredibly tightly controlled.

What's the master switch again?

HMG -CoA reductase?

It is, at multiple levels.

HMG -CoA reductase, that committed step enzyme, is central.

Short -term regulation involves phosphorylation.

A kinase called AMPK, triggered by low ATP, glucagon, and epinephrine, inactivates it, signals low energy.

Conversely, insulin promotes dephosphorylation and activation when energy is abundant.

And there's elegant long -term regulation at the gene expression level too.

The SREBP system sounds quite sophisticated.

How does that work?

The SREBP system is a marvel of feedback control.

Sterile regulatory element binding proteins, SREBPs, are normally held in the ER membrane by other proteins like SKP and INSIG.

When cellular cholesterol is low, SREBP is released by proteolytic cleavage, travels to the nucleus, and activates genes for HMG -CoA reductase and LDL receptors.

This essentially boosts cholesterol synthesis and uptake to restore levels.

When cholesterol levels are high, this release is blocked, SREBP stays in the ER, and the existing HMG -CoA reductase enzyme is even targeted for degradation.

So it's a brilliant feedback group.

Are there other major regulatory players that coordinate this whole system of receptors?

Yes.

The liver X receptor, LXR, which is activated by molecules called oxisterols, signaling high cholesterol, works with another receptor, RXR.

This complex activates genes for fatty acid synthesis enzymes, cholesterol transport proteins like ABCA1 for HDL, and even glucose transporters.

It's part of a broader network signaling abundance.

Conversely, the farnesoid X receptor, FXR, responds to bile acids, often with reciprocal effects to LXR, helping maintain bile acid homeostasis.

These systems are complex and interconnected, working to keep everything in balance.

All these tightly controlled pathways.

But still, things can go wrong.

Disregulation of cholesterol metabolism is a huge problem leading to cardiovascular disease.

How does that happen at a molecular level?

Atherosclerosis.

It's a major health challenge globally.

When cholesterol, particularly LDL, is in excess, it can lead to plaque formation in blood vessels

atherosclerosis.

Oxidized LDL tends to accumulate in the artery wall.

This attracts immune cells, monocytes, which differentiate into macrophages.

These macrophages engulf the oxidized LDL, becoming bloated foam cells because they're engorged with lipids.

As these foam cells accumulate and die, they contribute to plaque growth, inflammation, narrowing, and potentially blocking arteries, leading to heart attacks or strokes.

And this is where the statin story comes in.

A real medical marvel.

How exactly do they work again?

It truly is a game changer.

Statins, initially discovered from fungi, are powerful competitive inhibitors of HMG CoA reductase.

They effectively mimic mevalinate, the enzyme's natural substrate, and bind very tightly to the active site.

This dramatically lowered serum cholesterol by inhibiting its synthesis in the liver.

The liver cell then compensates by putting more LDL receptors on its surface, pulling more LDL out of the blood.

It's a double win.

And they do more than just lower cholesterol.

It seems they have broader benefits, pleiotropic.

That's right.

What's fascinating here is that beyond direct cholesterol lowering, statins have these broader positive effects.

They can improve blood vessel function, enhance the stability of atherosclerotic plaques, making them less likely to rupture, reduce platelet aggregation, and lessen vascular inflammation.

These are critical additional benefits in reducing cardiovascular risk.

They do have potential side effects, like muscle pain for some individuals, which needs monitoring.

So HDL really is the good guy, actively fighting plaque formation by removing cholesterol.

Absolutely.

That reverse cholesterol transport pathway is key.

HDL actively removes cholesterol from peripheral tissues, including those foam cells and nascent plaques, and returns it to the liver for excretion.

This active removal is crucial for countering plaque formation and protecting against atherosclerosis.

Before we get into it, let's talk about cholesterol.

It's astounding versatility.

Those five carbon -activated isoprene units aren't just for cholesterol.

They are precursors for over 20 ,000 diverse isoprenoid compounds found across all forms of life.

These include essential vitamins like A, E, and K, vibrant plant pigments like carotene and chlorophyll's tail, natural rubber, aromatic essential oils, insect hormones, and even crucial electron carriers in our own bodies like ubiquinone or coenzyme Q.

And these isoprenoid groups can even be attached to proteins, right, to anchor them to membranes.

Pre -annulation.

Yes, that process is called pre -annulation.

Isoprenoid groups like the 15 -carbon farnesyl or 20 -carbon geranol -geranol group are covalently attached to specific proteins, often near their C -terminus.

This acts as a hydrophobic molecular anchor, tethering them to the inner surface of cellular membranes, which is absolutely vital for their proper localization and function, especially in signaling pathways like those involving raised proteins.

It's another crucial role for these derivatives of the cholesterol pathway.

Wow.

We've truly taken a deep dive today, uncovering the incredible complexity and elegance of lipid biosynthesis from the fundamental fatty acids to the multifaceted cholesterol and its family of derivatives.

We've seen how every step, every enzyme, every molecule plays a vital role.

Energy storage, membrane integrity, cellular signaling, overall health,

it's all connected.

It's clear that the interconnectedness of these precise regulatory mechanisms and their surprising real -world applications, you know, from understanding disease to developing life -saving drugs like satins, is profoundly impactful.

It really illustrates how molecular details translate directly into physiological outcomes.

And it makes you wonder, as we continue to uncover the secrets of our biochemistry, what new essential molecules or maybe specialized pro -resolving mediators might we discover next?

And how might a deeper understanding of these intricate pathways continue to revolutionize our approach to health and medicine in the future?

It feels like there's still so much more.

It's an exciting prospect, absolutely.

It reminds us how much there is still to learn and how interconnected all these biological processes truly are.

Never a dull moment in biochemistry.

Thank you for joining us on this deep dive into lipid biosynthesis.

We really appreciate you being part of the Last Minute Lecture family.