Chapter 20: Lipid Metabolism
20.1 — Lipid Digestion, Absorption, & Transport
Triacylglycerols
Constitute approximately 90% of dietary lipids and are the primary form of metabolic energy storage in humans.
Mechanisms for digesting, absorbing, and transporting fats must accommodate their hydrophobic nature.
Fats are water-insoluble, whereas digestive enzymes are soluble in water, resulting in digestion occurring at lipid-water interfaces.
The rate of fat digestion is influenced by the surface area of the lipid-water interface.
Bile Acids
Also known as bile salts, bile acids are amphipathic cholesterol derivatives that solubilize fat globules.
Synthesized in the liver and stored in the gallbladder as glycine or taurine conjugates, they are secreted into the small intestine.
Pancreatic Lipase
Known as triacylglycerol lipase, it catalyzes the hydrolysis of triglycerides at positions 1 and 3 to yield 1,2-diacylglycerols and subsequently 2-acylglycerols.
Requires pancreatic colipase for binding to the lipid-water interface and necessitates the formation of mixed micelles with bile acids and phosphatidylcholine.
The active site of pancreatic lipase resembles the catalytic triad of serine proteases; its lid is closed in the absence of micelles, and colipase promotes binding to lipid surfaces.
The products of lipid digestion are absorbed in the small intestine with the assistance of bile acids and then bind to intestinal fatty acid-binding protein (I-FABP) within intestinal cells.
Lipid Transport via Lipoproteins
Lipoproteins are spherical particles comprising a core of triglycerides and cholesteryl esters, surrounded by a coat of proteins, phospholipids, and cholesterol.
Chylomicrons
Formed by intestinal cells to package triglycerides, they transport lipids throughout the bloodstream.
High-Density Lipoproteins (HDL)
Transport lipids from peripheral tissues back to the liver.
VLDL, IDL, LDL
The liver synthesizes very low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), and low-density lipoproteins (LDL) to facilitate lipid transport to various tissues.
Note that density is inversely related to particle size.
Apolipoproteins (Apoproteins)
The protein components of lipoproteins.
Chylomicrons interact with capillary beds, where triglycerides are hydrolyzed by lipoprotein lipase, allowing tissues to uptake free fatty acids.
Chylomicrons that are cholesterol-enriched dissociate and are taken up by the liver after becoming chylomicron remnants.
Further LDL Functions
VLDLs are also hydrolyzed by lipoprotein lipase, sending remaining glycerol back to either the liver or kidneys for oxidation.
LDL, through receptor-mediated endocytosis, delivers cholesterol or cholesteryl esters into other cells.
LDL receptors facilitate the uptake of LDL via clathrin-coated pits that cluster endocytic receptors that invaginate to form vesicles.
After fusion with endosomes, LDL dissociates and is delivered to lysosomes, where it is hydrolyzed to yield free cholesterol, amino acids, and fatty acids.
HDL Functions
Assembled from recycled lipoprotein components, HDL acquires cholesterol from the plasma membrane; lecithin–cholesterol acyltransferase (LCAT) converts cholesterol to cholesteryl esters.
Binds to the liver through the SR-BI receptor, facilitating lipid transfer to hepatic cells, with cholesterol ultimately converted to bile acids.
20.2 — Fatty Acid Oxidation
Activation of Fatty Acids
Triggered by hormone-sensitive lipase during metabolic needs, triglycerides mobilize free fatty acids (FFAs), which bind to serum albumin for transport.
Fatty acids are converted to acyl-CoA in a cytosolic, ATP-dependent acylation reaction, releasing PPi.
This activation is catalyzed by acyl-CoA synthetases (thiokinases) tailored to various chain lengths.
Transport Across Mitochondrial Membranes
The acyl-CoA component of acyl-CoA transfers to carnitine via two carnitine palmitoyl transferases (I and II) for movement across the mitochondrial inner membrane, via a transport carrier protein.
β-Oxidation Process
β-oxidation comprises four enzymatic reactions
Acyl-CoA Dehydrogenase (AD) - dehydrogenates to form a trans-α,β double bond.
Enoyl-CoA Hydratase (EH) - hydrates the double bond at the formed compound to create 3-L-hydroxyacyl-CoA.
3-L-Hydroxyacyl-CoA Dehydrogenase (HAD) - NAD+-dependent reaction converts the β-hydroxyacyl-CoA into β-ketoacyl-CoA.
β-Ketoacyl-CoA Thiolase (KT or Thiolase) - cleaves the Cα—Cβ bond yielding acetyl-CoA and a new acyl-CoA.
Mitochondria possess four ADs, specific to very short (C4–C6), medium (C6–C10), long (C12–C18) chain fatty acyl-CoAs.
Each cycle of β-oxidation yields 1 NADH, 1 FADH2, and 1 acetyl-CoA.
FADH2 is subsequently transferred to electron-transfer flavoprotein (ETF) and then to CoQ.
Oxidation of Unsaturated Fatty Acids
Natural unsaturated fatty acids predominantly possess cis double bonds at C9 and exhibit further bonds every three carbon atoms.
Issues with double bonds in unsaturated fatty acids include:
Enoyl-CoA formed with cis double bond cannot bind to EH; Enoyl-CoA isomerase transforms this into trans double bond for EH binding.
Creating a double bond at an even-numbered carbon leads to a poor substrate (2,4-dienoyl-CoA) for EH; NADPH-dependent 2,4-dienoyl-CoA reductase reduces the bond into trans-3-enoyl-CoA.
If converted into 3,5-enoyl-CoA, the subsequent isomerization of 3,2-enoyl-CoA is done by 3,2-enoyl-CoA isomerase to trans-2-enoyl-CoA, allowing oxidation to resume.
20.3 — Ketone Bodies
Ketogenesis in the Liver
Acetyl-CoA is converted into ketone bodies (e.g. acetoacetate, acetone, D-β-hydroxybutyrate) in liver mitochondria, which are utilized as metabolic fuel by tissues such as the heart and skeletal muscle.
Key Enzymes in Ketogenesis
Thiolase (Acetyl-CoA Transferase) - condenses two molecules of acetyl-CoA into acetoacetyl-CoA.
HMG-CoA Synthase - adds a third acetyl-CoA to form β-hydroxy-β-methylglutaryl-CoA (HMG-CoA).
HMG-CoA Lyase - cleaves HMG-CoA to yield acetoacetate and acetyl-CoA via mixed aldol-Claisen ester cleavage.
β-Hydroxybutyrate Dehydrogenase - reduces acetoacetate to β-hydroxybutyrate using NADH.
Nonenzymatic decarboxylation of acetoacetate can yield acetone and CO2, with ketone bodies released by the liver being reconverted into two acetyl-CoA in tissues through 3-ketoacyl-CoA transferase and thiolase.
20.4 — Fatty Acid Biosynthesis
Transport of Acetyl-CoA into the Cytosol
When ATP requirements are low, acetyl-CoA is integrated into fat storage.
Acetyl-CoA is converted into citrate and transported into the cytosol via the tricarboxylate transport system.
ATP-citrate lyase catalyzes the conversion of citrate, CoA, and ATP into acetyl-CoA, oxaloacetate, ADP, and Pi.
Oxaloacetate is converted to malate by malate dehydrogenase, which then transforms into pyruvate, CO2, and NADPH via the malic enzyme.
Pyruvate returns to mitochondria, while NADPH is utilized for fatty acid biosynthesis.
Acetyl-CoA Carboxylase (ACC)
Catalyzes the first and rate-limiting step of fatty acid synthesis, involving the reaction of HCO3- and ATP with biotin.
CO2 attaches to acetyl-CoA, forming malonyl-CoA.
Regulation of ACC
Stimulated by citrate.
Inhibited by long-chain fatty acyl-CoAs.
AMP-dependent protein kinase (AMPK) phosphorylates ACC at Ser79, inactivating it.
Glucagon and epinephrine promote phosphorylation (inactivation), whereas insulin fosters dephosphorylation (activation).
Fatty Acid Synthase
A cytosolic multifunctional enzyme responsible for the conversion of acetyl-CoA into fatty acids.
The growing fatty acid attaches to the acyl-carrier protein (ACP) of fatty acid synthase, which structurally resembles CoA but binds a Ser residue instead of AMP.
Reaction Steps of Fatty Acid Synthase
Malonyl/Acetyl-CoA-ACP Transacylase (MAT) - Transfers acyl and malonyl groups to ACP.
β-Ketoacyl-ACP Synthase (KS)
Transfers the acetyl group from ACP to Cys residue (condensation) where malonyl-ACP attacks in a decarboxylation reaction to yield acetoacetyl-ACP.
Reduction and Dehydration
Converts acetoacetyl-ACP to butyryl-ACP, from which the butyryl group is transferred, allowing for chain elongation.
Each cycle lengthens the chain by 2 carbons from the thioester end, reaching palmitoyl-ACP after seven cycles.
Palmitoyl thioesterase hydrolyzes the thioester bond, yielding palmitate.
Fatty Acid Elongation & Desaturation
Achieved by elongases and desaturases; mitochondrial elongation resembles the reverse of fatty acid oxidation, with NADPH replacing FADH2.
Endoplasmic reticulum elongation involves acyl-CoA and malonyl-CoA additions followed by reductions via NADPH.
Desaturases generate unsaturated fatty acids by oxidizing carbons prior to C9. - Essential fatty acids, which possess double bonds beyond C9, must be obtained through diet.
Formation of Triacylglycerols (Triglycerides)
Fatty acyl-CoA esters, glycerol-3-phosphate, and dihydroxyacetone phosphate (DHAP) entities serve as precursors.
Enzymes involved:
Glycerol-3-phosphate acyltransferase (from mitochondria/ER) forms lysophosphatidic acid.
DHAP acyltransferase (from peroxisomes or ER) yields a product that reduces into lysophosphatidic acid.
Sequential reactions convert lysophosphatidic acid into triacylglycerols through several specific enzymes.
DHAP for glycerol-3-phosphate synthesis originates from glycolysis or a shortened gluconeogenesis form termed glyceroneogenesis, wherein DHAP is reduced by NADH-dependent reductases.
20.5 — Regulation of Fatty Acid Metabolism
Hormonal Control
Insulin and glucagon have regulatory roles on lipid metabolism rates, affecting fatty acid oxidation versus synthesis.
Blood fatty acid concentrations are determined by triacylglycerol hydrolysis in adipose tissue through hormone-sensitive triacylglycerol lipase, activated by phosphorylation via glucagon and inactivated by insulin.
The rise in blood lipids propels ketone body production in the liver.
Protein kinase A (PKA) inactivates acetyl-CoA carboxylase to inhibit fatty acid synthesis, whereas phosphatase-1 (PP1) activates the enzyme.
Malonyl-CoA, a fatty acid synthesis product, inhibits carnitine palmitoyl transferase I, restricting new fatty acids from entering mitochondria for oxidation.
Heart cells do not synthesize fats; they possess enzymes for malonyl-CoA synthesis to modulate fatty acid oxidation.
AMP-dependent Protein Kinase (AMPK)
Phosphorylation of acetyl-CoA carboxylase, inhibiting fatty acid synthesis.
AMPK is activated by AMP and inhibited by ATP, allowing for fat synthesis only when energy levels are high.
20.6 — Synthesis of Other Lipids
Glycerophospholipid Biosynthesis
1,2-Diacylglycerol and phosphatidic acid serve as glycerophospholipid precursors, linking polar head groups to C3 of glycerol via phosphodiester bonds.
In mammals, ethanolamine and choline head groups are activated prior to attaching.
ATP phosphorylates the hydroxyl group of choline or ethanolamine, with the phosphoryl group reacting with cytosine triphosphate (CTP) to yield a CDP derivative, releasing PPi.
The C3 OH of diacylglycerol attacks the phosphoryl group of activated head group (CDP), forming the glycerophospholipid.
Phosphatidylserine Formation
Through the action of phosphatidylethanolamine serine transferase, the ethanolamine head group can be substituted for serine.
During the formation of phosphatidylinositol and phosphatidylglycerol, phosphatidic acid reacts with CTP to form CDP-diacylglycerol, while:
Inositol condenses with CDP-diacylglycerol to yield phosphatidylinositol.
Glycerol-3-phosphate attacks CDP to form phosphatidylglycerol phosphate, which is subsequently hydrolyzed to yield phosphatidylglycerol.
Fatty Acid Preferences
Enzymes forming phosphatidic acid generally favor saturated fatty acids at C1 and unsaturated at C2.
Plasmalogens
Eukaryotic membranes can also have plasmalogens, characterized by hydrocarbon chains linked through a vinyl ether bond and alkylacylglycerophospholipids with ether bonds at C1.
Sphingolipid Biosynthesis
Building Blocks
Sphingoglycolipids, including cerebrosides and gangliosides, involve the attachment of sugar units to ceramide (N-acylsphingosine).
Synthesis Steps
3-Ketosphinganine Synthase - Condenses palmitoyl-CoA and serine to form 3-ketosphinganine.
3-Ketosphinganine Reductase - Reduces the ketone to create sphinganine via NADPH.
Acyl-CoA Transferase - Forms an amide bond between acyl-CoA and sphinganine's amino group, forming dihydroceramide.
Dihydroceramide Dehydrogenase - Converts dihydroceramide to ceramide with the help of FAD.
Prostaglandin Formation from C20 Fatty Acids
Various functions between tissues include triggering pain, fever, or inflammation.
Cyclooxygenase (COX)
Enzyme prostaglandin H2 synthase catalyzes the formation of a cyclopentane ring in arachidonic acid, performing two reactions:
Adding two O2 molecules (cyclooxygenase).
Converting hydroperoxy group into a hydroxyl group (peroxidase), resulting in prostaglandin and thromboxane synthesis.
Lipoxygenase generates leukotrienes that spur histamine synthesis and immune responses.
NSAIDs
Nonsteroidal anti-inflammatory drugs (e.g., ibuprofen, acetaminophen) noncovalently inhibit COX isoforms.
Specific isoforms include COX-1 (homeostasis), COX-2 (inflammation), and COX-3 (pain and fever).
20.7 — Cholesterol Metabolism
Synthesis from Acetyl-CoA
Cholesterol synthesis is a lengthy pathway in which acetyl-CoA transforms into isoprene units that condense into a linear 30-C molecule.
The linear molecule cyclizes to manifest the cholesterol four-ring structure.
Key steps include:
The first two ketogenesis enzymes converting three acetyl-CoA molecules to HMG-CoA.
HMG-CoA reductase reduces HMG-CoA to mevalonate with two NADPH.
Mevalonate undergoes multiple phosphorylation reactions catalyzed by mevalonate-5-phosphotransferase and mevalonate kinase, leading to the formation of pyrophosphomevalonate.
Pyrophosphomevalonate is decarboxylated (ATP-dependent) leading to the creation of isopentenyl pyrophosphate.
Isopentenyl pyrophosphate isomerizes into dimethylallyl pyrophosphate.
Four isopentenyl and two dimethylallyl pyrophosphates finalize into squalene through three reactions with the help of two prenyltransferases.
Squalene Formation and Further Processing
Squalene synthase catalyzes the condensation of two farnesyl-PPi molecules creating squalene through a head-to-tail reaction.
Squalene undergoes cyclization initiated by squalene epoxidase introducing oxygen to form the epoxide 2,3-oxidosqualene, which is processed by oxidosqualene cyclase into lanosterol via cationic intermediates.
The conversion of lanosterol to cholesterol is a 19-step enzyme-catalyzed process occurring in the endoplasmic reticulum.
Cholesterol acts as a precursor to steroid hormones, bile acids, and cholesteryl esters.
HMG-CoA Reductase Regulation
HMG-CoA reductase catalyzes the rate-limiting step in cholesterol biosynthesis subject to competitive inhibition, allosteric effects, and covalent modification.
Phosphorylation by AMPK results in inactivation.
Long-term control relates to enzyme expression rates and protein degradation changes in response to cholesterol levels:
High cholesterol levels trigger the binding of sterol regulatory element binding protein (SREBP) cleavage-activating protein (SCAP) to SRE, which leads to SREBP's nuclear translocation and upregulation of HMG-CoA reductase gene transcription.
Statins and Cholesterol Regulation
Statins therapeutically inhibit HMG-CoA reductase through competitive binding, promoting LDL receptor synthesis, reducing cellular cholesterol by increasing LDL plasma levels while decreasing IDL and LDL cholesterol circulation.
Abnormal Cholesterol Transport
Individuals with LDL receptor deficiencies experience elevated LDL cholesterol levels, leading to xanthomas and increased atherosclerosis risk due to cholesterol deposits in skin and tendons.
Cholesterol synthesis is similarly regulated; elevated cellular cholesterol levels incoherently suppress LDL receptor synthesis, causing LDL to remain in circulation.
Excess cholesterol is stored in the cytosol as cholesteryl esters, facilitating transport away from the plasma membrane via ATP-binding cassette transporter A1 (ABCA1) and subsequent HDL formation with apolipoprotein A-I.