Lipids and their metabolism

After ingested fats (lipids) are cleaved by enzymes, lipids are absorbed in the small intestine and transported via the lymphatic system into the bloodstream. During this transport process, lipids are bound to special hydrophilic apolipoproteins. These lipoproteins control fat metabolism and have different proportions of bound fat as well as different functions. Elevated low-density lipoprotein (LDL) and triglycerides are associated with an increased risk of atherosclerosis; however, an increase in high-density lipoprotein (HDL) has a positive effect on the vessels. Treatment of elevated lipid levels usually involves the administration of lipid‑lowering agents (e.g., statins). Lifestyle changes also play an important role.

Lipids

• Definition: organic molecules that are soluble in nonpolar solvents but do not dissolve in polar solvents (e.g., water)
• Include:
  • Fatty acids, e.g., branched-chain fatty acids: saturated fatty acids that have ≥ 1 methyl group that branch off of the main carbon chain
  • Triglycerides
  • Structural lipids: phospholipids, sphingolipids, and glycolipids
  • Steroids/sterols: cholesterol, bile acids, and steroid hormones
• Substrates for:
  • Cell membrane synthesis
  • Steroid hormone synthesis
  • Vitamin D synthesis
  • Bile production
  • Energy storage and utilization (see “Sources of ATP synthesis”)

Lipid metabolism

• Digestion and absorption: Ingested fats (lipids) are cleaved by enzymes (e.g., pancreatic lipase) and absorbed in the small intestine.
• Lipid transport
  • Absorbed lipids are transported in chylomicrons via the lymphatic system into the bloodstream, where they reach the liver, peripheral tissues (which have LDL receptors) and adipose tissue (storage).
  • Lipids circulating in the bloodstream are transported in lipoproteins (contain hydrophilic apolipoproteins) because the hydrophobic lipids are insoluble in plasma.

Lipid digestion

• Dietary lipids: TAGs, phospholipids, and cholesterol esters
  • Not readily absorbed by enterocytes due to:
    • Hydrophobic properties
    • Large molecular size
  • First broken down by lipases; in the mouth, stomach, and intestinal lumen and packaged into micelles
  • Micelles
    • A spherical-shaped aggregation of surfactant molecules in a colloid
    • Composed of an outer layer of hydrophilic (polar) heads and an inner hydrophobic (nonpolar) core
    • Enclose lipids and fat-soluble vitamins, forming a package
    • Readily absorbed by the membranes of enterocytes → deliver internal contents to the cell
• Bile release: Once a lipid enters the intestinal lumen, bile is secreted into the lumen to emulsify the lipid contents.
• Pancreas secretion: The pancreas secretes pancreatic lipase, colipase, and cholesterol esterase, which hydrolyze the lipid into cholesterol, fatty acids, and 2-monoglyceride molecules.
• Absorption of short-chain and medium-chain fatty acids: pass the enterocytes → released to the hepatic portal vein → liver → enter the general circulation and bind albumin
• Absorption of long-chain fatty acids: absorbed by enterocytes and activated → re-esterified to triglycerides and cholesterol esters
  1. Activation
     • Takes place at the cytosolic side of the outer mitochondrial membrane
     • Acyl-CoA-synthetase transfers fatty acid to coenzyme A → activated fatty acid (acyl-CoA)
  2. Esterification
     • Takes place in ER
     • Monoglyceride + acyl-CoA → TG
     • Transport of TG to Golgi apparatus
  3. Formation of chylomicrons
     • Takes place in Golgi apparatus
     • Apolipoproteins B-48 and phospholipids are added.
  4. Secretion of chylomicrons into lymph → thoracic duct → bloodstream

Acyl-CoA and acetyl-CoA should not be confused with each other. Acyl-CoA is a collective name for all activated fatty acids. Acetyl-CoA is the acyl-CoA of acetic acid (also known as acetate).

There is a very small amount of lipid in the stool of healthy individuals. Defects in lipid digestion result in steatorrhea (i.e., fatty stool).

Enzymes in lipid digestion

Lipases are enzymes that catalyze the breakdown of fats into glycerol and fatty acids.

Lipid resorption

The decomposition products of lipid digestion form mixed micelles with bile acids.

Lipoproteins ^[1]

• Structure
  • Consist of a hydrophobic core and a hydrophilic shell
  • Core and shell are composed of different lipids, depending on the specific lipoprotein
    • Core: hydrophobic lipids (cholesterol esters, triglycerides)
    • Shell: hydrophilic lipid components (free cholesterol, phospholipids) and hydrophilic proteins, which are known as apolipoproteins
• Main function:
  • Transport of hydrophobic lipids in blood
  • Of all lipoproteins, HDL and LDL transport the largest portion of cholesterol.

Abnormalities in the structure or metabolism of lipoproteins increase the risk of atherosclerosis.

The lipoproteins in order of increasing triglycerides are HDL, LDL, IDL, VLDL, and chylomicrons.

Free fatty acids in the blood are not transported by lipoproteins but are instead bound to albumin.

HDL is Healthy (protective against atherosclerosis) and LDL is Lethal (cholesterol plaque formation in peripheral arteries increases risk of cardiac disease and stroke).

Apolipoproteins

Particles originating from the LIVer: LDL, IDL, VLDL.

A-I is Activates LCAT and is only present on Alpha-lipoproteins (only HDL)

Two Cs of apolipoprotein C-II (two) function: Catalyzes Cleavage.

Enzymes in lipid transport

Lipoprotein lipase is activated by binding to its cofactor apo C-II.

Fatty acids and triacylglycerols (TAGs) are important energy carriers. They are stored in the adipose tissue and can be mobilized from there if necessary and degraded (via beta oxidation) while releasing energy in the form of ATP. TAGs are the storage form of fatty acids in the body. They consist of one molecule of glycerine esterified with three fatty acids. TAG metabolism is subject to strict regulation by the hormone-sensitive lipase of adipose tissue.

Fatty acids

Overview

• Carboxylic acid with an unbranched chain of carbon atoms differing in length (from 1–24 carbon atoms)
• Typically found as esters, including:
  • Triglyceride (TAGs)
    • A lipid composed of one glycerol molecule esterified with three fatty acid molecules
    • Storage form of fatty acids, mainly stored in adipose tissue
    • Fatty acids and TAGs are important energy carriers
    • Monoglyceride and diglyceride: Only one or two of the hydroxyl groups of glycerol are esterified with fatty acids.
  • Phospholipids
  • Cholesterol esters
• Degradation via beta oxidation releases energy in the form of ATP

An increased concentration of triglycerides in the blood is called hypertriglyceridemia. It can be hereditary (lack of lipoprotein lipase), acquired (obesity, chronic alcohol use), or a combination of both. Like hyperlipoproteinemia, hypertriglyceridemia increases the risk of vascular disease (atherosclerosis, coronary heart disease, peripheral vascular disease).

Characteristics

• Fatty acid length
  • Short-chain fatty acid (SCFA): total carbon-chain length between 1–6
  • Medium-chain fatty acid (MCFA): total carbon-chain length between 7–12
  • Long-chain fatty acids (LCFA): total carbon-chain length between 13–20
  • Very long-chain fatty acid (VLCFA): total carbon-chain length 20
  • Odd-chain fatty acid: contains an odd number of carbon atoms
• Fatty acid saturation
  • Saturated (without C=C double bonds)
  • Unsaturated fatty acids (with C=C double bonds)
    • Monounsaturated fatty acids: have one double bond
    • Polyunsaturated fatty acids: have > 1 double bonds
• Nomenclature: Unsaturated fatty acids are named using the omega system.
  • The last carbon in the fatty acid molecule (i.e., the furthest from the carboxyl group) is labeled as ω (omega).
  • A fatty acid is given a name that reflects the position of the first double bond relative to omega-carbon

Excessive dietary intake of trans-unsaturated fatty acids (e.g., fried foods, margarine, and shortening) raises LDL levels and lowers HDL levels, thereby increasing the risk of cardiovascular events.

Essential fatty acids ^[4][5]

• Definition: polyunsaturated fatty acids that cannot be synthesized by humans and need to be ingested
• Types
  • Omega-3 fatty acids
    • Examples: alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA)
    • Sources: nuts, seeds, cold-water fish, algae
    • Function
      • Replace arachidonic acid in platelet membranes, leading to decreased thromboxane production and platelet aggregation
      • Associated with decreased plasma triglycerides levels and decreased cardiovascular risk
  • Omega-6 fatty acids
    • Examples: linoleic acid
    • Sources: cereals, soybeans, seeds, and seed oils
    • Function: Metabolized to arachidonic acid, which is a precursor for prostaglandins and leukotrienes
    • Clinical significance: Excessive intake, which often occurs in Western diets, promotes inflammation and is associated with an increased risk of cardiovascular events. ^[5]

Overview of fatty acid metabolism

The breakdown of fatty acids is not simply a reversal of fatty acid synthesis; there are a number of differences between the two processes.

The Sytrate (citrate) shuttle is essential for fatty acid Synthesis.

Fatty acids travel to their site of degradation by CARnitine.

Fatty acid synthesis

Overview

• Definition: the creation of fatty acids from acetyl-CoA and NADPH through the action of fatty acid synthases
• Metabolism site: cytoplasm of liver (mainly), adipose tissue, and lactating mammary glands
• Primary end-product: palmitic acid (palmitate), a 16-carbon fatty acid (only fatty acid that humans can synthesize de novo)
  • The synthesis of one palmitic acid requires 8 acetyl-CoA, 7 ATP, and 14 NADPH.
• Rate-limiting enzyme: acetyl-CoA carboxylase

Procedure

1. Acetyl-CoA groups (from glycolysis) are transported from the mitochondria to the cytoplasm through the citrate shuttle.
   • In mitochondria, acetyl-CoA combines with oxaloacetate to form citrate.
   • Citrate is then shuttled to the cytoplasm (instead of continuing in the citric acid cycle).
2. In the cytoplasm, ATP citrate lyase hydrolyzes citrate back into acetyl-CoA and oxaloacetate.
3. Acetyl CoA carboxylase activates acetyl-CoA and converts it into malonyl-CoA.
   • Acetyl-CoA carboxylase requires biotin, ATP, and CO₂.
   • Fatty acid synthase uses the carbons from acetyl-CoA to synthesize a new fatty acid (i.e., palmitate).

Regulation

Fatty acid synthesis is regulated via phosphorylation of acetyl-CoA carboxylase.

• Upregulation
  • Insulin, ↑ glucose, ATP, and citrate activate acetyl-CoA carboxylase (these substances indicate an energy excess).
    • Excess glucose can be converted into fatty acids in the liver and then transported to the adipose tissue for storage.
    • Insulin also enhances the synthesis of acetyl-CoA carboxylase via the transcription factor SREBP-1c.
• Downregulation
  • Glucagon, ↓ glucose, epinephrine, AMP, catecholamines, inhibit acetyl CoA-carboxylase (these substances indicate an energy deficit).
  • Acyl-CoA and palmitoyl-CoA inhibit acetyl-CoA carboxylase via negative feedback (indicating adequate supply of free fatty acids).

Fatty acid degradation ^[6]

Overview

• Definition: : the process by which fatty acids are catabolized by the pathway of beta oxidation
• Aims: energy generation
  • ATP production from NADH and FADH₂ creation
  • During fasting: ketone body production from gained acetyl-CoA
• Metabolism site: mitochondria; of several tissues, including liver, muscle, and adipose tissue; erythrocytes and the brain are not able to utilize fatty acid
• Primary end-products: acetyl-CoA, ketone bodies, NADH, and FADH₂
• Rate-limiting enzyme: carnitine palmitoyltransferase 1 (carnitine shuttle)

Procedure

1. Fatty acid transport (into the mitochondria)
   • LCFA: enter via carnitine-dependent shuttle
     • Fatty acyl-CoA synthetase; on the outer mitochondrial membrane activates the fatty acid by attaching CoA → forming a fatty acyl group
     • Carnitine palmitoyltransferase 1 (also known as carnitine acyltransferase I) transfers the fatty acyl group to carnitine on the outer mitochondrial membrane → forming a fatty acylcarnitine
     • Fatty acylcarnitine is then shuttled into the mitochondria.
   • SCFA and MCFA: diffuse freely into mitochondria
2. Beta oxidation (in mitochondrial matrix): a catabolic process in which a fatty acid chain is cleaved (oxidized) at the beta carbon (every second carbon) by dehydrogenase enzymes in several cycles.
   • Breaks down odd-chain fatty acids into acetyl-CoA and propionyl CoA (see propionic acid pathway) and even-chain fatty acids into acetyl-CoA only.
   • Beta oxidation of VLCFA occurs in peroxisomes (see degradation of VLCFA).
   • For every cleaved acetyl-CoA, one molecule of FAD, H₂O, and NAD⁺ each are required.
   • Acetyl-CoA enters the citric cycle.
   • Acyl-CoA dehydrogenase catalyzes the initial step of beta oxidation: fatty acyl-CoA + NAD⁺ + FAD⁺ → acetyl-CoA + NADH + FADH₂
     • Two types of acyl-CoA dehydrogenase
       • Long-chain acyl-CoA dehydrogenase (LCAD)
       • Medium-chain acyl-CoA dehydrogenase (MCAD)

Regulation

• Upregulation: glucagon indirectly upregulates beta oxidation by inhibiting acetyl CoA-carboxylase and decreasing the malonyl-CoA concentration.
• Downregulation
  • Malonyl-CoA inhibits carnitine palmitoyltransferase I (the rate-limiting step in beta oxidation). ; ; ;
  • Insulin indirectly downregulates beta oxidation by activating fatty acid synthesis and increasing the malonyl-CoA concentration.

Clinical significance

• Primary carnitine deficiency
• Medium-chain acyl-CoA dehydrogenase deficiency
• Jamaican vomiting sickness
  • Acyl-CoA dehydrogenase is inhibited by the toxin hypoglycin, contained in unripe ackee, the fruit of the ackee tree, which is found in West Africa and Jamaica.
  • Intoxication results in severe vomiting, coma, and possibly death.

Carnitine deficiency results in toxic accumulation of LCFA in the cytoplasm of myocytes and other cells. Patients present with hypoketotic hypoglycemia, fatty liver, myopathy, hypotonia, and fatigue. Treatment consists of oral supplementation of the amino acid carnitine.

MCAD deficiency is characterized by the defective breakdown of MCFA, which renders FAs an unusable alternative energy source in the case of carbohydrate deficiency. Because the liver cannot degrade FAs beyond C8–C10, acetyl-CoA and NADH are missing for ketone body production and gluconeogenesis. This deficiency results in nonketotic hypoglycemia, encephalopathy, and lethargy in fasting states. C8–C10 acylcarnitines can be found in the blood.

Degradation of very long-chain fatty acids (20 carbons)

• Beta oxidation occurs in both mitochondria and peroxisomes.
  • Peroxisomes: beta oxidation until octanoyl-CoA (C8) is formed
  • Mitochondria: further oxidation of C8

Degradation of fatty acids with an odd number of carbon atoms (propionic acid pathway)

• Final cycle of fatty acid oxidation
  • Degradation of fatty acids with an even number of carbon atoms yields two acetyl-CoA (out of a 4-carbon fragment).
  • Degradation of fatty acids with an odd number of carbon atoms yields one acetyl-CoA and one propionyl-CoA (out of a 5-carbon fragment).
• In the mitochondria, propionyl-CoA is converted into succinyl-CoA (a citric acid cycle intermediate) in a two-step pathway.
  • Propionyl-CoA carboxylase converts propionyl-CoA into methylmalonyl-CoA.
    • Requires biotin as a cofactor
  • Methylmalonyl-CoA mutase converts methylmalonyl-CoA into succinyl-CoA.
    • Requires vitamin B₁₂ as a cofactor: deficiency of vitamin B₁₂ leads to elevation of methylmalonyl-CoA level (differentiating it from folate deficiency, which also causes megaloblastic anemia)

Triglyceride synthesis

Synthesized triglycerides are either stored in adipose tissue or transported to the muscle for energy utilization.

• Triglyceride synthesis occurs mainly in the liver and adipose tissue.
• Both glycerol and fatty acids have to be activated for triglyceride synthesis.
  • Glycerol → glycerol-3-phosphate (catalyzed by glycerol kinase, which is found in the liver but not adipose tissue)
  • Fatty acids → acyl-CoA
• Rate-determining enzyme: glycerol-3-phosphate acyltransferase (links glycerol-3-phosphate with two acyl-CoA molecules)

Triglyceride degradation

• Lipases split triglycerides into glycerol and three fatty acids.
  • Glycerol is converted to glyceraldehyde-3-phosphate (GAP), which undergoes glycolysis or gluconeogenesis.
  • The fatty acids begin beta oxidation.

Ketone bodies

• Water-soluble molecules that are produced by the liver and used by peripheral tissues (e.g., heart, brain, skeletal muscle) as an energy source when glucose is not readily available (e.g., during prolonged starvation, in diabetic ketoacidosis, or chronic heavy drinking)
• Three ketone bodies: acetoacetate, β-hydroxybutyrate, and acetone (acetone is a breakdown product of acetoacetate and β-hydroxybutyrate; has a fruity smell)

Ketogenesis

• Metabolism site: : mitochondria of hepatocytes
• Starting substance: acetyl-CoA
• Steps
  1. Two molecules of acetyl-CoA, catalyzed by thiolase, condense to form acetoacetyl-CoA.
  2. Acetoacetyl-CoA combines with another molecule of acetyl-CoA, catalyzed by rate-determining enzyme HMG-CoA synthase, to produce HMG-CoA.
  3. HMG-CoA lyase subsequently breaks down HMG-CoA into ketone bodies; acetoacetate and acetyl-CoA
  4. Acetoacetate can then be reduced to β-hydroxybutyrate or undergo spontaneous decarboxylation to form acetone (which gives the breath a fruity smell).
• Regulation:
  • In prolonged fasting and diabetic ketoacidosis
    • Oxaloacetate is depleted for gluconeogenesis.
    • This results in a buildup of acetyl-CoA molecules → shunts the pathway toward the production of ketone bodies.
  • In alcoholism
    • Excess NADH shunts oxaloacetate to malate.
    • This results in a buildup of acetyl-CoA molecules → shunts the pathway toward the production of ketone bodies.

Ketone body synthesis takes place exclusively in the mitochondria of hepatocytes. Ketone bodies are then released into the blood and transported to their target tissues (mainly the brain and muscle).

Two molecules of acetyl-CoA → acetoacetyl-CoA → HMG-CoA → acetoacetate → β-hydroxybutyrate. Acetone is formed by spontaneous decarboxylation of acetoacetate. The body has no use for acetone, which is excreted primarily via the lungs (gives breath a fruity odor). A small fraction is also exerted in the urine.

Ketogenolysis

• Metabolism site: : Mitochondria of extrahepatic tissues (cardiac and skeletal muscle, the brain, and the renal cortex) can metabolize acetoacetate and β-hydroxybutyrate, producing acetyl-CoA, which means that RBCs are unable to use ketone bodies for energy (due to lack of mitochondria).
• Process
  1. β-hydroxybutyrate → acetoacetate, catalyzed by β-hydroxybutyrate dehydrogenase
  2. Acetoacetate plus succinyl-CoA → acetoacetyl-CoA plus succinate, catalyzed by thiophorase (succinyl-CoA:3-ketoacid CoA transferase)
  3. Acetoacetyl-CoA → two molecules of acetyl-CoA, catalyzed by thiolase
  4. Finally, acetyl-CoA can be metabolized in the TCA cycle.
• Clinical relevance
  • See “Diabetic ketoacidosis.”
  • The urine test for ketone bodies can only screen for acetoacetate and, therefore, not detect β-hydroxybutyrate.

RBCs do not have mitochondria and hepatocytes lack the thiophorase enzyme. Therefore, neither of them can utilize ketone bodies for energy.

Cholesterol

• Definition: a polycyclic steroid alcohol absorbed through food but also synthesized in the body
• Function:
  • Incorporated in all cell membranes in the lipid bilayer: increases membrane fluidity and stability
  • Precursor for synthesis of:
    • Steroid hormones (e.g., androgens, estrogens, mineralocorticoids, glucocorticoids)
    • Bile acids
    • Vitamin D
• Digestion:
  • Dietary cholesterol is a mixture of free and esterified cholesterol (i.e., bound to fatty acids).
  • Esterified cholesterol is broken down by cholesterol esterase into cholesterol and free fatty acids.
• Resorption: Cholesterol combines with bile salts to form absorbable bile salt micelles.
  • Further processing: reesterification in the cytosol of enterocytes and incorporation into chylomicrons
• Transport: Since cholesterol is apolar, it must be rendered into a water-soluble form for its transport within the body.
  • Transport in bile
    • As cholesterol: together with lecithin and bile acids in the form of water-soluble micelles
    • As bile acids; : In the liver, cholesterol is partially converted to bile acids, which enter the enterohepatic circulation.
  • Transport in blood via lipoproteins
    • From the intestine to the liver: together with TAGs in chylomicrons
    • From the liver to peripheral tissues: via VLDL and LDL
    • From peripheral tissues to the liver (reverse cholesterol transport): via HDL and IDL
• Excretion: via bile as a whole molecule or modified in the form of bile acids

Excess cholesterol secretion into bile (e.g., in pregnancy, obesity) can lead to precipitation of cholesterol crystals and gallstone formation (cholelithiasis).

There is no intestinal absorption of cholesterol without bile salts. Bile salt deficiency can be caused by gallstones or a tumor of the biliary tract.

Cholesterol synthesis

• Starting substance: acetyl-CoA
• Metabolism site: cytoplasm of hepatocytes and intestinal epithelial cells
• Rate-determining enzyme: HMG-CoA reductase in the membrane of smooth ER (catalyzes the reaction: HMG-CoA → mevalonate; requires two NADPH)
• Regulation: of HMG-CoA reductase
  • Stimulated by: insulin, thyroxine, estrogen
  • Inhibited by: glucagon, cholesterol (feedback inhibition via SREBP)
  • Sterol regulatory element-binding proteins (SREBPs): transcription factors that can bind to intracellular cholesterol via the SCAP protein
    • Cholesterol deficiency: SREBP does not bind intracellular cholesterol but migrates to the nucleus and binds to the sterol regulatory element (SRE) of the LDL receptor gene and to enzymes of cholesterol synthesis → increased transcription of LDL receptors and enzymes → increased uptake and synthesis of cholesterol
    • Cholesterol excess: SCAP-SREBP complex binds to cholesterol → no transportation to the nucleus

Simplified cholesterol synthesis: acetyl-CoA → acetoacetyl-CoA → HMG-CoA → mevalonate → squalene → cholesterol

The enzyme HMG-CoA reductase is clinically important because it is the target for drugs that are designed to reduce the plasma concentration of cholesterol (i.e., HMG-CoA reductase inhibitors, which have a structure similar to that of mevalonate). They are also referred to as statins.

Laboratory considerations

• In laboratory tests, total cholesterol, triglycerides, HDL, and LDL are usually measured.
• If levels are elevated or reduced, testing should be repeated after at least 2 weeks.
• See “Parameters of fat metabolism” for optimal and pathological levels.

Associated conditions

• Atherosclerosis
  • Cholesterol embolization syndrome
  • Carotid artery stenosis
  • Peripheral arterial disease
  • Stroke
  • Myocardial infarction
  • Coronary heart disease
• Skin manifestations
  • Xanthoma (e.g., tendinous xanthoma)
  • Xanthelasma
• Eye manifestations
  • Lipemia retinalis
  • Arcus lipoides corneae
  • Arcus senilis
• Disorders of fatty acid metabolism
  • MCAD deficiency
  • Carnitine transporter deficiency
• Abetalipoproteinemia
• Fatty liver

Other

• Dyslipidemia classification according to Frederickson
• Statins
• Second-line lipid-lowering agents