Fatty Acid Metabolism

• Fatty acid metabolism covers lipid transport, fatty acid catabolism, triglyceride synthesis, and glycerophospholipid synthesis

Minecraft Depiction

• Key fatty acid-containing biological molecules can be categorized into storage lipids and membrane lipids
• Storage lipids include triacylglycerols, which contain glycerol esterified with three fatty acids
• Membrane lipids include glycerophospholipids, sphingolipids, galactolipids, archaeal ether lipids, and sterols

Important Fats

• Triglycerides are a glycerol esterified to three fatty acids, and function for energy storage in adipocytes and muscle lipid droplets, as well as providing thermal insulation and physical protection
• Phospholipids consist of a glycerol esterified to two fatty acids and one head group, found in membranes and involved in intracellular signaling
• Cholesterol is found in membranes and produces steroid hormones for signaling

Important Facts

• Lipids come from the diet or can be synthesized from glucose or amino acids
• Essential fatty acids like linoleic acid and alpha-linolenic acid must be obtained through diet
• Lipid metabolism varies among cell types
• Fatty acids are the primary metabolic fuel for heart and skeletal muscle at rest
• The brain uses ketone bodies derived from fatty acid oxidation in the liver, not fat directly

Fatty Acid Nomenclature

• In fatty acid nomenclature, carbon atoms are numbered, with the carboxyl group carbon as number 1 and the methyl group carbon designated as ω
• Fatty acids are named based on the number of carbons, number of double bonds, and the position of the first double bond from the methyl end

Dietary fatty acids

• Dietary fatty acids include caprylic (C8:0), capric (C10:0), lauric (C12:0), myristic (C14:0), palmitic (C16:0), palmitoleic (C16:1), stearic (C18:0), oleic (C18:1), linoleic (C18:2), and α-linolenic (C18:3) acids
• Common sources of fatty acids include coconut oil, palm oil, milk fat, animal fats, marine oils, and seed oils

Lipid transport

• Fats can be synthesized or acquired through diet
• Bile salts emulsify dietary fats in the small intestine, forming mixed micelles
• Intestinal lipases degrade triacylglycerols

Transported Fats

• Fatty acids and other breakdown products are absorbed by the intestinal mucosa, and are converted into triacylglycerols
• Triacylglycerols, cholesterol, and apolipoproteins are incorporated into chylomicrons
• Chylomicrons move through the lymphatic system and bloodstream
• Lipoprotein lipase, activated by apoC-II in the capillary, converts triacylglycerols to fatty acids and monoacylglycerols
• Fatty acids enter cells and are oxidized or re-esterified for storage

Lipoproteins

• Lipoproteins transport lipids in the bloodstream and include chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL)
• Lipoproteins contain triacylglycerols, cholesterol, phospholipids, and apolipoproteins

Serum Albumin

• Serum albumin is a blood protein produced by hepatocytes that non-covalently binds and transports up to 7 FFAs (free fatty acids)
• Serum albumin accounts for about half of the total blood serum protein

Fatty acids in blood

• Fatty acids have low solubility in blood and cytosol, which necessitates transporter proteins
• Lipoproteins transport various lipids, while albumin carries non-esterified fatty acids

Fatty Acids from blood

• Bulk fluid flow from plasma to the interstitial space brings albumin close to muscle fibres
• Cellular fatty acid uptake involves dissociation from albumin, diffusion through interstitial fluid, insertion into the outer leaflet of the phospholipid bilayer, flip-flop to the inner leaflet, diffusion through cytosol, and binding to fatty acid binding protein (FABP) in cytosol

Fatty Acid Transporters

• Fatty acids enter cells via routes involving passive diffusion or facilitated transport by proteins like FABPpm, CD36, and FATP
• Intracellular fatty acids are bound to FABP and activated to fatty acyl-CoAs

Fatty acyl-CoA synthetase

• Fatty acyl-CoA synthetase isozymes are present in the outer mitochondrial membrane (and elsewhere), catalyzing both steps of the fatty acid activation reaction
• The overall reaction for fatty acyl-CoA formation involves three steps: two-step formation of the fatty acyl-CoA derivative, and the hydrolysis of the pyrophosphate created
• The overall reaction equation is: fatty acid + CoA + ATP ⇌ fatty acyl-CoA + AMP + 2Pi (∆G'° = −34 kJ/mol)

After Production

• Cytosolic fatty acyl-CoA can be used to make triglyceride for storage in adipocytes and muscle cells, used to make phospholipids, used to make cholesterol, or catabolized

Carnitine Shuttle

• Long-chain fatty acids (14+ carbons) are transported into mitochondria as carnitine esters via the carnitine shuttle, requiring activation to a fatty acyl-CoA and subsequent attachment to carnitine
• The carnitine shuttle involves carnitine acyl-transferase 1 (CAT1), an acylcarnitine/carnitine cotransporter, and carnitine acyl-transferase 2 (CAT2)
• CAT1 converts acyl-CoA to acyl-carnitine in the mitochondrial outer membrane and cytosol
• An Acylcarnitine/carnitine cotransporter exchanges a mitochondrial carnitine for a cytosolic (intermembrane space) acyl-carnitine
• CAT2 resynthesizes an acyl-CoA from an acyl-carnitine in the matrix
• Carnitine-mediated entry is a rate-limiting step and a major control point for fatty acid oxidation in mitochondria
• Carnitine acyltransferase 1 is inhibited by malonyl-CoA, preventing simultaneous synthesis and degradation of fatty acids

Fatty Acid Oxidation Stages

• Stage 1: β oxidation involves the oxidative removal of successive two-carbon units from fatty acids in the form of acetyl-CoA
• Stage 2: oxidation of acetyl-CoA groups to CO2 in the citric acid cycle
• Stage 3: electron transfer chain and oxidative phosphorylation

Stage 1: Enzymatic reactions

• Step 1 is catalyzed by acyl-CoA dehydrogenase, which involves dehydrogenation of fatty acyl-CoA to yield a trans-Δ2-enoyl-CoA
• Step 2 is catalysed by enoyl-CoA hydratase, where water is added across the double bond of the trans-Δ2-enoyl-CoA to form L-β-hydroxyacyl-CoA
• Step 3 is catalysed by β-hydroxyacyl-CoA dehydrogenase, where L-β-hydroxyacyl-CoA is dehydrogenated to form β-ketoacyl-CoA
• Step 4 is catalysed by acyl-CoA acetyl-transferase (thiolase), where β-ketoacyl-CoA reacts with free coenzyme A to yield acetyl CoA and a fatty acyl-CoA shortened by two carbons

ETF

• ETF:CoQ oxidoreductase is in the inner membrane, facing the matrix, catalyzing the reduction of ubiquinone (CoQ) and oxidising FADH2 generated in beta-oxidation

Products of Beta Oxidation

• The shortened fatty acyl-CoA reenters the β-oxidation sequence
• Each pass removes one molecule of acetyl-CoA, two pairs of electrons, and four protons (H+)
• For palmitoyl-CoA, the overall reaction for one round of shortening is: palmitoyl-CoA + CoA + FAD + NAD+ + H2O → myristoyl-CoA + acetyl-CoA + FADH2 + NADH + H+
• Overall reaction for complete conversion to acetyl-CoAs is: palmitoyl-CoA + 7CoA + 7FAD + 7NAD+ + 7H2O → 8 acetyl-CoA + 7FADH2 + 7NADH + 7H+
• In stage 2, acetyl-CoA groups are oxidised to CO2 in the citric acid cycle, and in stage 3 the electron transfer chain and oxidative phosphorylation occur

ATP Production

• Each FADH2 donates a pair of electrons to ETF, generating 1.5 molecules of ATP
• Each NADH donates a pair of electrons to mitochondrial NADH dehydrogenase, generating 2.5 molecules of ATP
• In total, 4 ATP are formed for each pass through β oxidation
• Acetyl-CoA oxidized to CO2 and H2O by the citric acid cycle
• Overall reaction for the second and third stages of fatty acid oxidation (in the oxidation of palmitoyl-CoA) is: 8 acetyl-CoA + 16O2 + 80Pi + 80ADP → 8CoA + 80ATP + 16CO2 + 16H2O
• The overall reaction for the complete oxidation is: palmitoyl-CoA + 23O2 + 108Pi + 108ADP → CoA + 108ATP + 16CO2 + 23H2O

Water

• The pair of electrons transferred from NADH or FADH2 to O2 yields one H2O ("metabolic water")
• Reduction of O2 by NADH also consumes one H+ per NADH molecule: NADH + H+ + ½O2 → NAD+ + H2O

ATP Yield

• The complete oxidation of palmitoyl-CoA can yield 108 ATP, depending on the efficiency of the electron transport chain

Fatty Acid oxidation

• Fatty acid oxidation is inhibited by fatty acid synthesis, with malonyl-CoA (first intermediate of fatty acid synthesis) inhibiting carnitine acyltransferase

Peroxisomes

• Beta oxidation also occurs in peroxisomes, which are organelles found in plants and animals
• ß oxidation in peroxisomes includes dehydrogenation, addition of water to the resulting double bond, oxidation of β-hydroxyacyl-CoA to a ketone, and thiolytic cleavage by coenzyme A
• The peroxisomal and mitochondrial pathways differ in that the peroxisomal flavoprotein acyl-CoA oxidase that introduces the double bond passes electrons directly to O2, producing H2O2, which is cleaved to H2O and O2 by catalase
• The peroxisomal system is more active on very-long-chain fatty acids and branched-chain fatty acids

Ketone bodies

• Ketone bodies include acetone, acetoacetate, and D-β-hydroxybutyrate, made from acetyl-CoA in the liver when glucose is unavailable
• Acetone is exhaled, whereas acetoacetate and D-β-hydroxybutyrate are transported to extrahepatic tissues and converted to acetyl-CoA to be oxidized in the citric acid cycle

Liver production

• Ketone bodies formed in the liver are exported to other organs as fuel
• Thiolase catalyzes condensation of two acetyl-CoA molecules to form acetoacetyl-CoA
• HMG-CoA synthase catalyzes the condensation of acetoacetyl-CoA with acetyl-CoA to form β-hydroxy-β-methylglutaryl-CoA (HMG-CoA)
• HMG-CoA lyase catalyzes the cleavage of HMG-CoA to free acetoacetate and acetyl-CoA
• Acetoacetate decarboxylase catalyzes the decarboxylation of acetoacetate to acetone
• D-β-hydroxybutyrate dehydrogenase catalyzes the reversible reduction of acetoacetate to D-β-hydroxybutyrate
• D-β-hydroxybutyrate dehydrogenase catalyzes the oxidation of D-β-hydroxybutyrate to acetoacetate in extrahepatic tissue
• β-ketoacyl-CoA transferase catalyzes the activation of acetoacetate, and acetyl-CoA enters the citric acid cycle

Liver functions

• The liver lacks β-ketoacyl-CoA transferase and produces ketone bodies, but does not consume them

Synthesis

• Fatty acyl-CoA is used to make triglycerides and glycerophospholipids and is stored in adipocytes and muscle cells as lipid droplets
• Lipid droplets are often physically attached to mitochondria

Triacylglycerols

• Triacylglycerols and glycerophospholipids share two precursors: fatty acyl-CoA and glycerol 3-phosphate

Glycerol usage

• Glycerol 3-phosphate is formed from DHAP (dihydroxyacetone) or Glycerol
• Glycerol 3-phosphate dehydrogenase catalyzes the formation of glycerol 3-phosphate from DHAP
• Glycerol kinase catalyzes formation of small amounts of glycerol 3-phosphate from glycerol in liver and kidney

Fatty acid activation

• Acyl-CoA synthetases catalyze the formation of fatty acyl-CoAs from fatty acids, and are the same enzymes responsible for the activation of fatty acids for β oxidation
• Acyl transferase catalyzes the acylation of the two free hydroxyl groups of L-glycerol 3-phosphate by two molecules of fatty acyl-CoA to yield diacylglycerol 3-phosphate (phosphatidic acid)

Triglycerides

• Phosphatidic acid can be converted into triglycerides and glycerophospholipids via subsequent phosphatase and acyl transferase reactions and reactions

Synthesis Location

• Glycerophospholipid synthesis primarily on the surfaces of the smooth ER and the inner mitochondrial and membranes in eukaryotic cells, with phospholipids are transported between subcellular locations

Glycerophospholipid Composition

• Glycerophospholipids contain fatty acids, glycerol, phosphate, and head-group substituent
• Glycerophospholipids include phosphatidic acid, phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, phosphatidylglycerol, phosphatidylinositol 4,5-bisphosphate, and cardiolipin

Types Of Bonds

• Cells attach Phospholipid Head Groups via two strategies: beginning with phosphatidic acid or diacylglycerols, and attaching a polar head group through a phosphodiester bond by each of two alcohol hydroxyls that form an ester with phosphoric acid
• The Phospholipid Head Group is attached after it is activated by CDP
• One of the hydroxyls is first activated by attachment of a cytidine diphosphate (CDP) nucleotide
• CMP is then displaced in a nucleophilic attack by the other hydroxyl
• There are Two Strategies for Forming the Phosphodiester Bond: strategy 1 links CDP to diacylglycerol, forming the CDP activated phosphatidic acid-diacylglycerol, or strategy 2 connects the CDP to the hydroxyl of the head group
• Many Pathways for Phospholipid Biosynthesis Are Interrelated and begin with CDP-diacylglycerol, including synthesis of phosphatidylglycerol, cardiolipin, and phosphatidylinositol

ER

• The ER and nucleus are used when synthesized phosphatidylserine and Phosphatidylcholine

Lysophospatidylcholine

• Lipids of mammals, phospholipids in membranes by Lands cycle thanks to lyosphospatidylcholine acyltransferases (LPCATs)

Membrane insertion

• Membrane lipids are insoluble in water and cannot diffuse to their point of insertion, instead travelling to their intracellular destinations via transport vesicles, specific proteins, and inter-organelle contact sites

The importance of lipids

• Membrane structure and function are affected by phospholipid class and fatty acyl composition, rendering them important, which is true for Cardiolipin, Polyunsaturated fatty acids and Monounsaturated fatty acids

Cardiolipin

• Cardiolipin is essential for function of respiratory complexes and is the 'glue' holding respiratory supercomplexes together

Final Remarks

• Lipid metabolism is broad and varied with key roles, especially in biological structures, for energy, and intracellular and hormonal signaling
• In some cases, lipid roles are specific like annular lipids that support structure and function, and lipid roles are very specific