17.1: Digestion, Mobilization, and Transport of Fats

Introduction

This chapter will discuss the breakdown of fats to produce ATP. Most of the available chemical energy stored in fats is in the form of highly reduced fatty acids. One source of fatty acid-containing lipids in our diet is triacylglycerols (TAGs) and membrane lipids. Fatty acids, mainly in TAGs, are moved in the circulation by large lipid-carrying vesicles called lipoproteins. Cells can import lipids for storage and energy use.

Another source of fatty acids comes from those synthesized within cells from the small molecule acetyl-CoA. Fatty acids are synthesized by an enzyme complex called fatty acid synthase. This enzyme is most prevalent in adipose (fat) tissue and the liver. It is also highly expressed in the brain, lungs, and mammary glands.

TAGs, stored in lipid droplets, are found in most cells. The primary tissue for TAG storage is adipose (fat) tissue, whose volume consists mainly of lipid droplets. Given the large muscle tissue mass, a considerable amount of TAGs is stored as small lipid droplets in muscle cells. However, skeletal muscle cells don't synthesize fatty acids. They have the genes for fatty acid synthase but do not transcribe them into RNA, so they do not produce the enzyme. They can, however, import them for catabolism. Muscle TAGs can be oxidized for energy, especially during endurance exercise.

TAGs are also stored in the liver in lipid droplets. The liver also assembles lipoproteins, which are then released into the bloodstream. Excess TAGs are stored in the liver in various diseases, including alcoholism and nonalcoholic fatty liver disease (NAFLD), which can progress into nonalcoholic steatohepatitis (NASH), a much worse disease.

In this chapter, we will follow the fate of fatty acids from dietary lipids cleaved from TAGs, loaded into chylomicrons, a lipoprotein assembled in the small intestine, secreted into the circulation, and taken up by the liver. The liver can store incoming fatty acids as TAGs or release them back into circulation as very low-density lipoproteins (VLDL). Circulating VLDL can exchange lipids with other circulating lipoproteins. Lipoproteins deliver fatty acids to cells after interaction with the cell surface of target cells.  The fatty acids enter the cell via either the cleavage of TAGs by a cell membrane-associated enzyme, lipase, followed by fatty acid uptake, or endocytosis of lipoproteins.

Lipoproteins

Before examining the individual steps in lipid processing, let's first consider the various lipoproteins. These large vesicular structures transport fats, which are very insoluble molecules, in the circulation. Unlike normal liposomes or vesicles with a lipid bilayer surrounding an interior aqueous compartment, lipoproteins have only a single monolayer of phospholipids encapsulating a nonaqueous interior filled with TAGs, cholesterol, and cholesterol esters. The protein part of the lipoprotein consists of one or several proteins bound on the outside of the particle. The proteins help solubilize the lipoprotein, limit its size, and prevent lipoprotein aggregation, which could pose a health risk. The structure of a typical lipoprotein is shown in Figure \(\PageIndex{1}\).

Lipoproteins are classified based on density. The lowest-density one, chylomicrons, is the largest and contains the most lipids (mostly triacylglycerols, or TAGs) in its interior compartment. Very low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL) successively decrease in size, have fewer encapsulated lipids, and increase in density. The relative sizes are shown in Figure \(\PageIndex{2}\).

Lipoproteins (except chylomicrons) can be classified as nanoparticles, which typically vary in size from 1 to 100 nm. Larger lipoproteins and chylomicrons form emulsions in the blood, much as milk (also cloudy) is an emulsion of lipid/protein particles. The serum of people with high lipid levels (hyperlipidemia) can appear milky white, especially after eating foods rich in TAGs, when chylomicron levels are very high. Figure \(\PageIndex{3}\) shows the blood of a patient with hyperlipidemia after the addition of EDTA (which binds Ca²⁺ and prevents clotting) that has settled (without centrifugation). The milky-white plasma on top (lower density) likely contains high concentrations of chylomicrons and/or LDL. The lower layer primarily consists of red blood cells.

No X-ray structures of lipoproteins are available. However, the structure of a nascent HDL particle (3k2s) has been determined using small-angle neutron scattering. Figure \(\PageIndex{4}\) shows an interactive iCn3D model of it.

The major protein in HDL, a lipoprotein that protects against cardiovascular disease, is apolipoprotein A-I (apoA-I). Figure \(\PageIndex{4}\) shows that it adopts an antiparallel double superhelix as it wraps around the nascent HDL. The more hydrophobic surfaces of apo A-I are oriented inward, allowing interactions with hydrophobic lipids in the core. It is probably prototypical for nascent lipoproteins. It will give you an idea of how proteins wrap around the particle's surface. Mature lipoproteins are most likely spherical. This nascent HDL in the model contains 200 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholines (POPC) molecules, 20 cholesterol molecules, and a single copy of apolipoprotein A-I (apoA-I).

Figure \(\PageIndex{5}\) below shows a cartoon image of VLDL, assembled from lipids synthesized/taken up by and released from the liver, and chylomicrons, assembled from dietary fats and released from enterocytes in the small intestine (the size of the lipoproteins is not to scale). VLDL has one copy of Apo B-100, while chylomicrons have one copy of Apo B-48.

All lipoproteins, except HDL, are members of the Beta-lipoprotein family as they contain an apo-B protein. The liver synthesizes apo B100, which becomes a permanent part of VLDL (i.e, it is not exchangeable with other lipoproteins) and its metabolic derivatives, so any lipoprotein containing apo B100 arises from the liver. Other proteins on lipoproteins are exchangeable. In contrast, enterocytes in the small intestine produce apo B-48 (48% the size of apo B100), so this protein marks the lipoproteins (chylomicrons and chylomicron remnants) assembled in the small intestine. Apo B100 has over 4500 amino acids and a molecular mass of 555kDa. The gene for intestinal apo B48 is the same as that for apo B100, except that it contains a premature stop codon, resulting in a shorter, truncated apo B-48.

Apoproteins bind to specific receptors on cells, allowing the lipoprotein to be taken up. For example, the LDL receptors bind to the apo B-100 protein on a region removed (by proteolytic processing) from the apo B48 protein of chylomicrons. They also bind ApoE, which is predominantly found on HDL and VLDL, but some is also present in LDL. LDL receptors are also called ApoB/ApoE receptors.

Over 90% of the apoB-containing particles in circulation are LDL. In addition, chylomicrons are present in circulation only after eating. Some apoproteins can act as cofactors and inhibitors for lipoprotein processing.

Lipoproteins and Cardiovascular Risk

High LDL concentrations are associated with increased cardiovascular risk. Chylomicron levels, given their transient and lower concentration levels, do not pose a health risk unless the enzyme required to remove fatty acids from them, lipoprotein lipase, is missing or defective, or if another apoprotein component, apo CII, which mediates the interaction with lipoprotein lipase, is missing. LDL-C (total cholesterol in LDL particles, routinely measured in clinical labs) can be lowered with a healthy diet centered on plant foods. Drugs like statins decrease endogenous cholesterol synthesis, lower LDL levels, and decrease cardiovascular risk.

However, another protein, lipoprotein, also called Lp(a) or LP "little a," is an independent cardiovascular risk factor. Genetics, not diet, regulates its blood concentration. In addition to apo B100, these particles contain apo (a), a protein with a unique repeating structure (up to 40 times) called a kringle, which is also found in some proteins involved in the blood coagulation system. People whose genes encode apo (a) with the fewest number of kringles express lots of that protein, and their Lp(a) particles are smaller. This confers greater cardiovascular risk than expressing proteins with many kringle domains. Figure \(\PageIndex{6}\) (from Amgen) shows models of Lp(a) with different numbers of repeating kringle (kinked) domains.  A recent drug study (September 2026) showed that the drug Pelacarsen significantly lowered Lp(a) levels in high-risk patients but it did not significantly reduce major adverse cardiovascular events, which shows that much is left to learn about cardiovascular risk factors.

From a biochemical perspective, exploring differences in apolipoprotein binding to single-leaflet-encapsulated lipid nanoparticles is of interest compared with interactions of peripheral and integral membrane proteins with intact bilayers (which have two leaflets, as studied in Chapter 12.1). As mentioned above, the more nonpolar surfaces of apo AI in HDL are oriented inward toward the nonpolar lipid core. Presumably, apo B proteins in chylomicrons and LDL also wrap around the entire lipid surface.

HDL's major organizing scaffolding protein is apoA-I (see iCn3D model above). It presumably plays a role similar to apoB in chylomicrons and LDL, but it is exchangeable. (Note: ApoA-I is also found in chylomicrons.) It is also a cofactor for the enzyme lecithin:cholesterol acyltransferase (LCAT), which converts free cholesterol in the single bilayer into cholesterol esters within HDL. In its apo form, it also interacts with the cell-surface transporter ATP-binding cassette A1 (ABCA1), which plays a role in HDL particle assembly. HDL also has apoC and apoE proteins, all of which are exchangeable.

Given their heterogeneity and size, lipoprotein structure is difficult to determine. Apolipoproteins have hydrophobic surfaces that promote self-association and aggregation. Apolipoproteins in the A, C, and E classes have repeating amphiphilic helices that embed to some degree in the lipid particles. In addition, the proteins exhibit significant disorder and can adopt multiple bound conformations.  

Figure \(\PageIndex{7}\) shows an interactive iCn3D model of the AlphaFold predicted structure of human Apo E (P02649) (P06727)

The blue cartoon color indicates high certainty in the AlphaFold-predicted structure, while yellow to orange indicates low certainty. The hydrophobic side chains are shown as sticks, helping illustrate the structure's amphiphilic nature.

Figure \(\PageIndex{8}\) shows an interactive iCn3D model of the AlphaFold predicted structure of human Apo A4 (P02649)

The blue cartoon color indicates high certainty in the AlphFold-predicted structure, while yellow to orange indicates low certainty.  The hydrophobic side chains are shown as sticks.  Again, note the structure's amphiphilic nature.

The exchangeable apolipoproteins share a common genetic structure (four exons and three introns) and amino acid sequences. They have 11-mer amino acid tandem repeats, and some (A-I, A-IV) have 22-mer tandem repeats. These repeats form amphiphilic helices as determined by sequence analysis. The first amino acid in the amphiphilic helix is often positively charged, and a negative one is often found in the middle. Proline, a helix breaker, is often but not always found between the helices.

Figure \(\PageIndex{9}\) shows the primary sequence of apoA-I. An 11-mer repeat is highlighted in yellow. The other highlighted stretches (in different colors) are 22-mer repeats. Note that the repeats are not of identical sequences but rather of sequences that can form amphiphilic helices (i.e., secondary structure repeats).

The bottom part of Figure \(\PageIndex{9}\)shows a helical wheel projection (using Heliquest) of the red-highlighted 22-mer repeat. The arrow shows the hydrophobic moment with the arrowhead pointing to the more nonpolar face. The particular amphiphilic helix shown may or may not facilitate the binding of the bound conformation of the protein.

The relative areas of the hydrophilic and hydrophobic faces in the amphipathic helices influence the lipid-associating properties of the exchangeable apolipoproteins. The arrangement of tandem amphipathic helices is another factor that may influence the lipid-binding capacity of exchangeable apolipoproteins.

Actual amphiphilic helices would bind to the membrane in a parallel fashion, with the nonpolar face anchoring the protein to the lipid surface. Other experimental techniques are used to study how a peptide or protein that can form amphiphilic helices interacts with the lipid surface. These include site-directed mutagenesis studies coupled with spectroscopic (CD, fluorescence) and binding assay methods (using liposomes).

The properties of a membrane-bound amphiphilic helix are affected by the exact size and distribution of the polar/charged and nonpolar side chains. On binding, they sense or induce membrane curvature, interact with specific lipids, and stabilize specific membrane conformations (such as a spherical shape for lipoproteins). Figure \(\PageIndex{10}\) shows how different proteins with amphiphilic membranes interact with membrane surfaces.

A key point to note is the large conformational changes that occur as the protein, or parts of it, transition from the free, more disordered state to the bound state with lipid-associating amphiphilic helices. The figure depicts the following proteins.

1. The peroxisomal membrane protein Pex11 amphiphilic helix distorts the membrane;
2. ARF1 is a small G protein in which only the GTP form localizes and binds through an amphiphilic helix to the membrane;
3. The ALPS motif of the golgin GMAP-210 binds to only highly curved vesicles;
4. The yeast transcriptional repressor Opi1 binds to the endoplasmic reticulum (ER) membrane in part through an amphiphilic helix;
5. The heat shock protein Hsp12 has a long amphiphilic helix, which helps stabilize the membrane;
6. The extremely long amphiphilic helix of perilipin 4 coats lipid droplets and stabilizes them, even without phospholipids.

Dietary uptake and release into the circulation

Now, how are the lipid nanoparticles assembled? We'll start with the dietary lipids: TAGs, glycerophospholipids, and cholesterol esters. The figure below shows key steps which are described in Figure \(\PageIndex{11}\)

Here are some key steps depicted in the figure:

• hydrolysis (lipolysis) of TAGs by pancreatic lipase, cholesterol esters (CE) by cholesterol esterase, and glycerophospholipids (GP) by phospholipase A2 in the lumen of the intestine. These enzymes interact at the interface of the lipid substrates and aqueous surroundings;
• the resulting products, which include free fatty acids (FA), 2-monoacylglycerol (MG), free cholesterol (FC), and lyso-glycerolphospholipids (lyso-GP), aggregate with the help of bile salts to form emulsions (like oil drops in water), which can be taken up by diffusion or possibly endocytosis when present in high amounts. Alternatively, membrane transporters (like FABPs and other proteins) can move them into the cell by facilitated diffusion;
• cytoplasmic transporters like fatty acid binding proteins move the lipolysis product to the ER where free fatty acids are reesterified. The enzymes involved include mono- and diacylglycerol acyltransferases (MGAT, DGAT) and sterol O-acyltransferase 2, also known as acyl-coenzyme A:cholesterol acyltransferase (ACAT-2). Multiple enzymes are involved in the resynthesis of glycerophospholipids);
• Apo B-48 is synthesized by ribosomes bound to the ER and interacts with a heterodimer of microsomal triglyceride transfer protein large subunit two (MTP) and protein disulfide isomerase (PDI). This facilitates apo B-48 folding and lipid loading into pre-chylomicrons with MTP.
• pre-chylomicron vesicles move to the Golgi with the help of Sar1b, a small G-protein (and GTPase) where the particle assembles to the full chylomicron, which is released from the cells as the mature large lipid nanoparticle.

An intriguing feature of lipases is that they operate at the interface between the aqueous and nonaqueous environments (in this case, lipid nanoparticles). Let's briefly consider the hydrolysis mechanism of TAGs by the equine pancreatic lipase. This enzyme employs the same mechanism observed earlier for peptide-bond hydrolysis by serine proteases. A catalytic triad of Asp 176, His 263, and Ser 152 as a nucleophilic catalyst is shown in the partial reaction displayed in Figure \(\PageIndex{12}\).

An acyl-Ser intermediate forms in step B (above) to form the product diacylglycerol after the collapse of an oxyanion intermediate in step A. In the second half of the reaction (not shown in its entirety), water cleaves the acyl-Ser intermediate via hydrolysis, reforming the active enzyme and releasing the free fatty acid, R₃CO₂H. Other lipases also employ the same catalytic triad.

Figure \(\PageIndex{13}\) shows an expanded diagram showing the flow and fate of lipoproteins.

Chylomicrons interact with lipoprotein lipase (LPL), which also uses an Asp-His-Ser catalytic triad, to cleave fatty acid esters, allowing the delivery of free fatty acids to adipose cells. The adipocytes can also undergo de novo fatty acid synthesis. Fatty acids (FA) can also be produced by lipase-mediated lipolysis of stored TGs. These free fatty acids (FA) in the adipocyte have two fates. They can be reesterified to glycerol to form TAGs (TG in the figure) or be exported from the cell, where they bind to a plasma carrier protein and are transported to the liver for storage.  There, various membrane protein importers can take up the fatty acids, as indicated in the figure by the yellow boxes. As in adipose cells, fatty acids can be reesterified to form TAG stores, which can then be packaged into VLDL particles for export. The fatty acid delivered (or synthesized) could also be used for ATP production through the citric acid cycle and oxidative phosphorylation.

VLDL in circulation can undergo lipolysis by lipoprotein lipase to produce fatty acids for uptake in "extrahepatic" tissue (bottom right of the diagram). As fats are removed from VLDL, its density increases as it forms IDL and LDL, which could be considered VLDL "remnants". VLDL is very enriched in TAGs, but after metabolic processing, the resulting LDL is depleted in TAGs and enriched in cholesterol/cholesterol esters. LDL (not shown in the above figure) can be taken up (endocytosed) by the liver and other cells after binding to LDL receptors, which recognize apo-B100 and other apoproteins. This allows the delivery of predominantly cholesterol and cholesterol esters to tissues.

How do adipocytes and hepatocytes determine if free fatty acids should be esterified for storage or released for energy use by other tissues? We'll discuss that in a subsequent section. Still, the short answer is that, in healthy fasting and exercise states, hormones (glucagon and epinephrine) activate lipolysis in the liver and adipose tissue. In contrast, in the fed state, insulin promotes the storage of fatty acids as triacylglycerols.

Adipose cells don't assemble and release lipoproteins. Instead, they release free fatty acids into the circulation, where albumin, the major protein in serum or plasma, carries them. The iCn3D Figure \(\PageIndex{14}\) shows an interactive iCn3D model of the complex of human serum albumin (HSA) binding seven 20:4^Δ5,8,11,14 - arachidonic acids (1gnj ).

Given albumin's multiple fatty acid-binding sites, it is no surprise that it also binds a host of small drugs, including medicinal drugs and toxins such as warfarin (blood thinner), diazepam, ibuprofen, indomethacin, and amantadine.  These appear to bind preferentially to two primary drug-binding sites. This binding may help deliver drugs through circulation, but it may not be effective if they aren't delivered to the target tissue.

We discussed micelles, spherical assemblies of single-chain amphiphiles that act as detergents. Oil from your clothes can enter the nonpolar interior of the detergent micelle, where it is solubilized, effectively forming a nanoparticle with a diameter of 5-15 nm. You should not be surprised to discover that lipoproteins can also carry fat-soluble vitamins, steroid-like endocrine-disrupting substances, and drugs.

Lipoprotein lipase

The enzyme that breaks down TAGs in circulating chylomicrons and VLDL is lipoprotein lipase (LPL). It is a soluble protein secreted by adipocytes and muscle cells, but is made by many cell types. It works at the luminal side of blood vessel endothelial cells. It is recruited to that membrane surface by binding to the glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) and the proteoglycan heparan sulfate at the cell surface.

Notably, GPIHBP1 is synthesized only by endothelial cells. When lipoprotein lipase is secreted from cells, it binds to the extracellular matrix heparan sulfate but dissociates on the cleavage of heparan sulfate by heparinases. GPIHBP1 is highly acidic with an intrinsically disordered N-terminal domain containing a sulfated tyrosine and is highly enriched in glutamates and aspartates, which are often sequential. Here is the single-letter sequence for amino acids 25-50 of the human version of GPIHBP1: EEEEEDEDHGPDDYDEEDEDEVEEEE. This sequence would have electrostatic and binding properties similar to the highly negatively charged heparan sulfate to which it also binds.

LPL also binds Ca²⁺, stabilizing the protein's active dimeric form. ApoC-II activates its enzymatic activity. Like pancreatic lipase, it uses a Ser-132, Asp-156, and His-241 triad to hydrolyze TAGs.

Figure \(\PageIndex{15}\)l below shows an interactive iCn3D model of LPL in complex with GPIHBP1, shown in brown (6E7K). The calcium ion is shown (grey spacefill), as well as the catalytic triad (labeled, sticks, CPK colors). The highly negatively charged stretch of amino acids in GPIHBP1 was not present in the crystal structure.

LDL: Receptor and Uptake

Lipoproteins are taken up by cells via receptor-mediated endocytosis. Given its role in cardiovascular disease, let's focus on the LDL receptor.  Remember that LDL is the primary carrier of cholesterol. The receptor is found in the cell membranes in most tissues. It has many domain repeats, as illustrated in Figure \(\PageIndex{16}\) calculated by SMART.

They include the N-terminal region cysteine-rich LDLa domains, which bind LDL, epidermal growth factor domains, LY (or LDLb) domains, and a transmembrane domain (blue rectangle).

Figure \(\PageIndex{17}\)s shows an interactive iCn3D model of the extracellular domain of the LDL receptor (1n7d).

Four tandem LY (LDLb) domains are in cyan, LDLa domains in magenta, and the EGF domain in dark orange. Glycans are represented in symbolic nomenclature. Zoom into the structure to see the two disulfide bonds in each LDLa domain and the Ca²⁺ ions that stabilize the domains.

LDL binds its receptor at a broad binding interface with multiple LDLa domains. This may explain why it can bind to lipid nanoparticles with apo B100 or apo E. Binding triggers a series of signaling events that lead to receptor internalization via endocytosis through clathrin-coated pits. These eventually fuse with lysosomes, where they are degraded and cholesterol is delivered to the cell. The steps are described in Figure \(\PageIndex{18}\).

The LDL receptor escapes lysosomal degradation and is continually delivered to the plasma membrane, along with newly synthesized receptors. A key protein, proprotein convertase subtilisin/kexin type 9 (PCSK9), a serine protease secreted by the liver, promotes receptor degradation and prevents its recycling to the membrane. It also binds to VLDLR and apolipoprotein E receptors, promoting their degradation. Its action reduces LDL clearance from the blood, increasing cardiovascular risk, so inhibitors of its action might be potent drugs to decrease circulating LDL.

The LDL receptor is just one member of the LDL receptor family. Other family members are illustrated in Figure \(\PageIndex{19}\).

These include the LDL receptor (abbreviated Ldlr in Figure \(\PageIndex{17}\), as well as the VLDL receptor (Vldlr), apolipoprotein E2 receptor (Apoe2), and LDL receptor-related proteins (Lrp)1-4. These also contain an NPxY motif (asterisk in the cytoplasmic domain) and a YWTD/β-propeller domain. Given the similarity in domain structure for the LDL family of receptors, the conformational flexibility of the apolipoproteins (at least free in solution), and similar structures for the exchangeable apolipoproteins, it shouldn't be surprising that the LDL receptor would interact with different classes of lipoproteins, albeit with different affinities.

As mentioned, apoE is abundantly present on HDL, VLDL/chylomicrons and their remnants. It is a ligand that binds to members of the LDL receptor family (remember that LDL generally binds the LDL receptor through apo B100).

Apo E and Alzheimer

Apolipoprotein E has three major variants (alleles) named ε2, ε3, and ε4 (also called ApoE2, 3, and 4). ApoE3 is the most prevalent. ApoE4 is found in only 15% of people, but in more than 50% with Alzheimer's Disease (AD), so it's a risk factor for this disease. AD affects the brain, which also contains up to 30% of the body's cholesterol, so aberrations in cholesterol transport and uptake in the brain are not unexpected in neurodegenerative diseases like AD.

ApoE is secreted by brain microglia (immune) cells and astrocytes (specialized glial cells). It assembles lipids into lipoproteins (HDL-like) and becomes the major vehicle for binding to and importing lipids into neurons, a process initiated by the apoE receptor. The major apoE receptor for the clearance of lipoproteins in the brain is sortilin (SorLA in Figure \(\PageIndex{17}\)).

AD is characterized by the accumulation of a toxic amyloid prion protein called amyloid beta (Aβ). It is derived from selective but abnormal proteolysis of the neural integral membrane protein amyloid precursor protein (APP). Aβ aggregates to form insoluble neurotoxic extracellular Aβ amyloid plaques. Figure \(\PageIndex{20}\) shows the process in normal and diseased cells.

The figure illustrates normal (left) and aberrant APP processing, as well as the family of proteases (secretases) involved. Although LDLR doesn't appear to bind APP or influence its proteolytic processing, it does bind Aβ. LRP1 is much bigger than LDLR, binds many ligands, and can be cleaved with the same enzymes as APP. It is expressed in the liver and brain, especially the brain, and can regulate Aβ removal.

Immune cells in the brain, known as microglia, remove Aβ plaques (extracellular) through phagocytosis. ApoE4 increases the inflammatory response (as measured by cytokine release) of microglia (a good thing if the responses prevent infection or rid Aβ plaques), but also inhibits their ability to phagocytose Aβ plaques and their metabolic activity.

An additional note: Having one ApoE4 allele appears to increase the risk of severe COVID-19 by 5-fold, while being homozygous for E4 increases the risk by 17-fold.

Scavenger Receptors

Patients with homozygous familial hypercholesterolemia (FH) have very high levels of LDL derived from defects in binding and uptake. Patients develop fatty streaks beneath the vascular endothelium, which progress to calcified plaques and fat-filled lesions. Monocytes/macrophages that migrate to sites of vascular injury take up LDL and eventually differentiate into foam cells filled with lipids. Somehow, they have receptors that can bind and internalize LDL when the "normal" LDL receptor can't. Brown and Goldstein found that a specific chemical modification of LDL, acetylation, was necessary for the rapid uptake of "modified" LDL into macrophage receptors. These receptors are now called scavenger receptors (SRs).

Scavenger receptors belong to a large family. They consist of classes A-J proteins that share function but not sequence homology. They are found on macrophages and endothelium. They bind to and help remove "damage" signals, including damage-associated molecular patterns (DAMPs) and chemically modified species generated by reactive oxygen species. Ligands often include polyanions, end-stage glycans, and extracellular matrix proteins. One such example is oxidized-LDL (produced in vivo or by chemical oxidation with malondialdehyde), which binds to the same scavenger receptor, SR-A1 (also called Macrophage scavenger receptor type I), as acetylated-LDL. Figure \(\PageIndex{21}\) shows the domain structure of the SR family.

SR-A1/MSR1 binds acetylated and oxidized LDL and β-amyloid (42), heat shock proteins (43), and PAMPs from some bacteria and viruses.

Conceptualizing all possible combinations of binding interactions among biological molecules is an enormous task. For visual learners and others, it's extremely useful to portray information on structures and interactions visually. An example using STRING, a database of known and predicted protein interactions, for the domain structure and protein:protein interactions of SR-A1/MSR1, is shown in Figure \(\PageIndex{22}\).

Note the interactions with apolipoproteins, apoB, apoE, and apoA1. The right-hand side of the figure also illustrates interactions with the collagen alpha-2(IV) chain (COL4A2), which is present in the extracellular matrix.

HDL metabolism: The Good Cholesterol

High LDL (and Lp(a)) levels pose a cardiovascular risk. In contrast, high HDL and apo A-I levels are cardioprotective. HDL is involved in "reverse" cholesterol transport as it is taken up by the liver and sent to the intestines for elimination from the body. As shown earlier in Figure 2, HDL exists in multiple variants, reflecting its assembly and remodeling by enzymes and lipid transfer proteins. Figure \(\PageIndex{23}\) shows the lifecycle of HDL.

Secreted apo A-1 accretes lipids in the circulation through the transport and delivery of phospholipids and cholesterol from cell membranes by the ATP-binding cassette transporters (ABC) A1 and G1. Another protein, the scavenger receptor BI (SR-BI), a polytopic integral membrane protein, is also involved. It acts as a receptor for various "lipid" ligands, including phospholipids, cholesterol esters, phosphatidylserine (an outer membrane marker for cell apoptosis), and lipoproteins such as HDL.

Other proteins also help assemble and remodel HDL. The lipolytic enzyme lecithin-cholesterol acyltransferase (LCAT) removes a fatty acid from phospholipids and adds it to free cholesterol in the HDL to form cholesterol esters. Two major lipid transfer proteins, phospholipid transfer protein (PLTP) and cholesterol ester transfer protein (CETP), facilitate lipid exchange between HDLs and other lipoproteins. The cholesterol ester transport protein is made and secreted by the liver. It appears to exchange cholesterol esters from HDL for TAGs from VLDL. Other enzymes (lipoprotein lipase and hepatic lipase) also form free fatty acids.

In the final step, HDL can deliver cholesterol esters and cholesterol to the liver by binding to the liver scavenger receptor BI (SR-BI), which is mediated by apo A-I. The liver, adrenal gland, endothelial cells, macrophages, and many other tissues express the protein. The liver does not primarily take HDL up by endocytosis. In contrast, LDL is taken up by LDL receptor-mediated endocytosis. SR-BI facilitates the transfer of cholesterol esters from bound HDL to the liver cell.

Summary

(Summary written by Claude, Sonnet 4.6, Anthropic)

This chapter examines the biochemistry of lipid transport, lipoprotein metabolism, and cellular fatty acid uptake, integrating structural biology, enzymology, and disease pathophysiology to explain how the body moves highly insoluble lipids through an aqueous circulatory system.

Fats are stored primarily in adipose tissue, the liver, and muscle as triacylglycerols (TAGs) in lipid droplets. White adipose tissue (WAT) stores TAGs in a single large lipid droplet and functions as an endocrine organ, secreting adipokines including leptin, adiponectin, TNF-α, and adipsin that regulate systemic metabolism. Brown adipose tissue (BAT), critical for thermogenesis especially in newborns and small animals, contains multiple smaller lipid droplets with a greater surface area for lipolysis and expresses uncoupling protein 1 (UCP1), which dissipates the mitochondrial proton gradient as heat rather than synthesizing ATP — enabling a thermogenic futile cycle. The surface area-to-volume ratio principle explains why BAT thermogenesis is disproportionately important in smaller organisms.

Because fats are essentially insoluble in water, their transport in the circulation requires assembly into lipoproteins — nanoparticles consisting of a single phospholipid monolayer encapsulating a nonaqueous core of TAGs and cholesterol esters, decorated on the outside with one or more apolipoproteins. Lipoproteins are classified by density: chylomicrons (largest, least dense, TAG-rich, assembled in enterocytes with apo B-48), VLDL (assembled in liver with apo B-100), IDL and LDL (VLDL remnants enriched in cholesterol after TAG depletion by LPL), and HDL (smallest, densest, protein-rich). The apo B proteins are permanent constituents that mark lipoprotein origin — apo B-48 (encoded by the same gene as apo B-100 but truncated by a tissue-specific stop codon in enterocytes) identifies chylomicrons, while apo B-100 marks liver-derived lipoproteins. All other apolipoproteins are exchangeable and share a common structural motif: tandem repeating amphipathic helices that embed their hydrophobic faces into the lipid monolayer surface while presenting charged faces to the aqueous environment, with extensive conformational disorder in the free state that collapses into a defined helical structure upon lipid binding.

Dietary lipid processing begins in the intestinal lumen, where pancreatic lipase (using a Ser-His-Asp catalytic triad identical in mechanism to serine proteases), cholesterol esterase, and phospholipase A2 hydrolyze TAGs, cholesterol esters, and glycerophospholipids, respectively. The products — free fatty acids, 2-monoacylglycerol, free cholesterol, and lyso-phospholipids — are emulsified by bile salts and taken up by enterocytes by diffusion and facilitated transport. Within the ER of enterocytes, fatty acids are re-esterified by MGAT, DGAT, and ACAT-2; apo B-48 is co-translationally loaded with lipids by microsomal triglyceride transfer protein (MTP); and pre-chylomicron vesicles are transported to the Golgi via the GTPase Sar1b for final chylomicron assembly before secretion into the lymph and ultimately the bloodstream.

In circulation, chylomicrons and VLDL are processed by lipoprotein lipase (LPL), a secreted enzyme that also uses a Ser-His-Asp catalytic triad and is anchored to the luminal surface of vascular endothelium via GPIHBP1 — a highly acidic intrinsically disordered protein whose negatively charged domain (rich in sequential Glu and Asp residues) mimics heparan sulfate in binding LPL. LPL requires apo C-II as a cofactor and Ca²⁺ for stability. As TAG is removed from VLDL, the particles become progressively denser, converting through IDL to LDL. LDL, the principal cholesterol carrier, is cleared from circulation by the LDL receptor — a multi-domain membrane protein with cysteine-rich LDLa ligand-binding repeats stabilized by Ca²⁺ and disulfide bonds — via clathrin-mediated endocytosis. The protease PCSK9, secreted by the liver, targets LDL receptors for lysosomal degradation and prevents receptor recycling, thereby reducing LDL clearance; PCSK9 inhibitors are potent LDL-lowering drugs. An independent cardiovascular risk factor, Lp(a), contains apo(a) — a protein with up to 40 kringle domain repeats — covalently associated with apo B-100; individuals expressing fewer kringle repeats produce more Lp(a) protein and have higher cardiovascular risk.

HDL mediates reverse cholesterol transport: nascent HDL acquires cholesterol and phospholipids from cell membranes via the ABC transporters ABCA1 and G1, esterifies free cholesterol using LCAT (which transfers fatty acids from phospholipids to cholesterol), exchanges lipids with other lipoproteins via CETP and PLTP, and delivers cholesterol esters to the liver via the scavenger receptor BI (SR-BI) without endocytic uptake of the entire particle. Despite the cardioprotective association of high HDL, drugs that raise HDL have not reliably reduced cardiovascular events — suggesting that HDL function, not just concentration, is the relevant variable.

Modified LDL, particularly oxidized LDL generated by reactive oxygen species in atherosclerotic lesions, evades normal LDL receptor uptake and instead binds to scavenger receptors (especially SR-A1/MSR1) on macrophages. Uncontrolled uptake leads to foam cell formation, the earliest cellular event in atherosclerotic plaque development. Finally, apo E connects lipoprotein metabolism to neurodegeneration: the ApoE4 allele — present in ~50% of Alzheimer's patients but only ~15% of the general population — impairs microglial phagocytosis of amyloid-β plaques, enhances neuroinflammation, and disrupts brain cholesterol transport (mediated by apoE-containing HDL-like particles taken up by neurons via sortilin/LRP receptors), collectively increasing Alzheimer's disease risk. ApoE4 is also associated with a markedly increased risk of severe COVID-19.