Fatty Acid Metabolism

Adipose Tissue Function

  • Major storage site for lipids: Triglycerides.
  • Adipocytes: Major cell-type of adipose tissue (lipid storage).
  • Acts as an endocrine organ.

Physiology of Triglyceride Absorption and Delivery

  • Triglycerides are absorbed and delivered to target tissues for storage and utilization.

Liberation of Fatty Acids from Adipose Tissue

  • Fatty acids are liberated from adipose tissue through lipolysis.

β-oxidation of Fatty Acids (Fatty Acid Oxidation)

  • Process by which fatty acids are broken down in the mitochondria to produce energy.

Ketone Body Biosynthesis and Utilization

  • Ketone bodies are synthesized from acetyl-CoA in the liver during periods of low glucose availability and utilized by other tissues for energy.

Classes of Lipids

  • Free Fatty Acid (FFA): also known as nonesterified fatty acid (NEFA).
    • Example structure shown.
  • Cholesterol: A type of lipid with a specific ring structure.
    • Example structure shown.
  • Cholesteryl Ester: Cholesterol with a fatty acid attached.
    • Structure: R−COO−CholesterolR-COO-\text{Cholesterol}
  • Triglyceride: The major lipid in the diet and in storage.
    • Structure: Glycerol + 3 fatty acids.

Fatty Acids

  • Long alkyl chains with a carboxylic acid at one end (head).
    • General formula: CH3−(CH2)n−COO−CH3-(CH2)n-COO^-
  • Numbering starts from the carboxylate end: α, β, γ, ω.
  • Examples:
    • Stearic acid: Saturated C18 (18:0).
    • Oleic acid: Unsaturated C18 (18:1 ∆9).
    • Linoleic acid: C18 with two unsaturations (18:2 ∆9, ∆12).
    • Linolenic acid: C18 with three unsaturations (18:3 ∆9, ∆12, ∆15).

Adipose Tissue

  • Major storage site for lipids in the form of triglycerides.
  • Adipocytes are the major cell type responsible for lipid storage.
  • Functions as an endocrine organ.

Fasting and Fed States

  • Fasting State:
    • Hormone: Glucagon.
    • Processes: Glycogenolysis, fatty acid oxidation, gluconeogenesis.
  • Fed State:
    • Hormone: Insulin.
    • Processes: Glycolysis, fatty acid biosynthesis, glycogenesis.

Control of Food Intake (Satiety/Hunger)

  • Hormones and peptides involved in regulating food intake:
    • Ghrelin: Stimulates food intake.
    • Leptin: Promotes satiety.
    • CCK, Amylin, Enterostatin, Glucagon, Insulin, PP, OEA, APO AIV, Endocannabinoids, GLP1, Oxyntomodulin, PYY, GRP, NMB, etc.

Ghrelin

  • Peptide hormone released by the stomach.
  • Acts in the hypothalamus to stimulate food intake.

Lipid Digestion

  • Organs involved:
    • Mouth (salivary glands).
    • Stomach.
    • Liver (bile).
    • Gallbladder.
    • Pancreas (pancreatic lipase).
    • Small intestine.
    • Large intestine.
    • Rectum.

Lipid Digestion and Transport Pathways

  • Exogenous Pathway: Dietary lipids.
    • Dietary fat is emulsified, forming micelles.
    • Micelles transport lipids to intestinal cells.
    • Lipids are assembled into chylomicrons.
    • Chylomicrons enter the lymphatic system and then the bloodstream.
  • Endogenous Pathway: Lipids from the liver.
    • Liver synthesizes VLDL (very low-density lipoproteins).
    • VLDL is processed into IDL and LDL.
    • LDL delivers cholesterol to extrahepatic cells.
    • HDL transports cholesterol from extrahepatic cells back to the liver.

Steps of Lipid Metabolism

  • Triglycerides are broken down into monoacylglycerol (MAG) and free fatty acids (FFA) by pancreatic lipase in the small intestine.
  • These are absorbed into intestinal cells and re-synthesized into triglycerides and cholesterol, then packaged into chylomicrons.
  • Chylomicrons transport triglycerides and cholesterol through the blood to adipocytes and muscle cells.
  • Lipoprotein lipase (LPL) breaks down triglycerides in chylomicrons into MAG and FFA, which are taken up by the cells.
  • In adipocytes, fatty acids are stored as triglycerides in lipid droplets.
  • In muscle cells, fatty acids undergo β-oxidation to produce energy.

Triglycerides/Triacylglycerol (TAG)

  • Structure:
    • Example: 1-Palmitoyl-2,3-dioleoyl-glycerol.
  • Highly hydrophobic.
  • Pack into large lipid globules.

Bile Salts

  • Emulsify lipids to increase surface area for enzymatic digestion.
  • Synthesized by the liver, stored in the gall bladder, and released into the small intestine.

Triglyceride Hydrolysis by Pancreatic Lipase

  • Pancreatic lipase is secreted by the pancreas.
  • Hydrolyzes triglycerides into 2 fatty acids + 2-monoacylglycerol (MAG) in the small intestine.
  • Cleaves triglycerides at the 1 and 3 positions.

Summary of Triglyceride Metabolism and Absorption

  • Large lipid droplets are mixed with bile salts to form micelles.
  • Pancreatic lipase acts on triglycerides to produce fatty acids and monoglycerides.
  • These are absorbed into mucosal cells.

Triglyceride and Cholesteryl Ester Synthesis in Intestinal Cells

  • Enzymes involved:
    • LPAAT, MGAT, DGAT, ACAT.
  • Triglycerides and cholesteryl esters are highly hydrophobic.

Chylomicron Synthesis

  • Triglycerides are packaged with cholesterol and cholesteryl esters into chylomicrons.
  • Chylomicron surface is decorated with proteins (e.g., apoC-II).

Chylomicrons

  • Released into systemic circulation via the intestinal lymph.

Lipoprotein Lipase (LPL)

  • Enzyme that lines the endothelium of capillaries surrounding adipose and muscle tissue.
  • Cleaves triglycerides within chylomicrons at the 1 and 3 positions to generate free fatty acids (FFA) and 2-monoacylglycerols.

GPIHBP1

  • Anchors lipoprotein lipase to the endothelium of capillaries surrounding adipose and muscle tissue.
  • Apolipoprotein C-II (apoC-II) activates LPL to increase triglyceride hydrolysis.

Chylomicron Remnants

  • Taken up by the liver.
  • Contain TAG, CE, apoB48, and apoE.

Lipoprotein Lipase Deficiency

  • Rare: 1-2 cases/million.
  • Elevated chylomicrons, triglycerides, and cholesterol.
  • Symptoms: Abdominal pain, loss of appetite, nausea, vomiting.

Lipid Droplets

  • Storage depots for cellular lipids: triglycerides and cholesteryl esters.
  • Dynamic structures that grow and shrink depending on lipid turnover.

Stimulation of Lipolysis

  • Occurs during fasting (glucagon) or “Fight or Flight” (epinephrine/adrenaline).
  • Hydrolysis of stored triglycerides into fatty acids within adipose tissue, which are subsequently released for use by other tissues (e.g., muscle).

Enzymatic Conversion of Triglycerides Into Fatty Acids

  • ATGL (Adipocyte triglyceride lipase): converts TAG to DAG.
  • HSL (Hormone sensitive lipase): converts DAG to MAG.
  • MAGL/MGL (Monoacylglycerol lipase): converts MAG to glycerol + FFA.

Regulation of Lipolysis

  • Protein Kinase A (PKA) phosphorylates proteins involved in lipolysis.
  • Hormonal Regulation: Glucagon and epinephrine stimulate lipolysis, while insulin inhibits it.

ABHD5/CGI-58

  • Sequestered by perilipin under “basal” state.
  • Phosphorylation of perilipin releases ABHD5, which then activates ATGL by recruiting it to the lipid droplet surface to stimulate triglyceride hydrolysis.

Chanarin–Dorfman Syndrome

  • Rare autosomal recessive neutral lipid storage disease.
  • Caused by mutations resulting in nonfunctional ABHD5.

Glycerol

  • Converted into glycolytic intermediates in the liver.
  • Released into blood, taken up by the liver, and used for gluconeogenesis.

Fatty Acid Oxidation/β-Oxidation

  • Mechanism through which cells utilize energy stored in fatty acids.
  • Series of enzymatic reactions within the mitochondrial matrix.
    • Fatty Acids -> Acetyl-CoA.

Overview of Fatty Acid Metabolism

  • Fatty acids are activated in the cytosol and transported into the mitochondrial matrix for β-oxidation.
  • β-oxidation generates acetyl-CoA, NADH, and FADH2.
  • Acetyl-CoA enters the Krebs cycle, while NADH and FADH2 enter the electron transport chain.
  • Ketone bodies are synthesized in the liver and transported to other tissues for energy.

Acyl-CoA Synthetase Enzymes (ACS)

  • Convert fatty acids into Acyl-CoAs.
  • Reaction: Fatty Acid + CoA + ATP -> Acyl-CoA + AMP + PPi.

Activation of Fatty Acids

  • Two ATP equivalents are used.
  • Mechanism:
    • Formation of acyladenylate intermediate.

Transport of Long Chain Fatty Acids Into Mitochondria

  • Inner mitochondrial membrane is not permeable to long chain Acyl-CoAs.
  • Carnitine palmitoyltransferase (CPT) system is required for transport.

Carnitine Shuttle

  • Carnitine acyltransferase I (CAT I) converts Acyl-CoA to Acyl-carnitine in the outer mitochondrial membrane.
  • Acyl-carnitine is transported across the inner mitochondrial membrane by a translocase.
  • Carnitine acyltransferase II (CAT II) converts Acyl-carnitine back to Acyl-CoA in the matrix.

Fatty Acid Oxidation/ẞ-Oxidation

  • Series of enzymatic reactions within the mitochondrial matrix.
    • Acyl-CoA -> Acetyl-CoA.
  • Palmitate (16:0) is an example.

Steps in Fatty Acid Oxidation/ẞ-Oxidation

  • Acyl-CoA dehydrogenase (AD): Formation of a trans-α,β double bond.
    • Reaction 1: Acyl-CoA Dehydrogenase.
    • Glutamate abstracts proton.
    • Hydride transferred to FAD.
  • Enoyl-CoA hydratase (EH): Hydration of the double bond.
    • Reaction 2: Enoyl-CoA Hydratase.
    • The water to be added is coordinated by 2 Glu via H-bonds.
  • 3-L-hydroxyacyl-CoA dehydrogenase (HAD): Oxidation of the hydroxyl group.
    • Reaction 3: NAD+-dependent Dehydrogenation.
    • Oxidizes a 2° alcohol using NAD+.
  • β-ketoacyl-CoA thiolase (KT): Cleavage of the β-ketoacyl-CoA.
    • Reaction 4: Cα-Cβ cleavage in a Thiolysis Reaction.
    • Generates Acetyl-CoA and Acyl-CoA 2C shorter.
  • Products of each round of β-oxidation:
    • 1 FADH2
    • 1 NADH
    • 1 Acetyl-CoA

Acyl-CoA Dehydrogenase (AD)

  • Multiple forms for different chain lengths.
    • VLCAD: very long-chain acyl-CoA DH (12-20C).
    • LCAD: long chain acyl-CoA DH (8-20C).
    • MCAD: medium-chain acyl-CoA DH (6-10C).
    • SCAD: short-chain acyl-CoA DH (4C).

Enoyl-CoA Hydratase

  • The water to be added is coordinated by 2 Glu via H-bonds

3-L-hydroxyacyl-CoA dehydrogenase (HAD)

  • Oxidizes a 2° alcohol using NAD+.
  • Multiple forms for different chain lengths

β-ketoacyl-CoA thiolase (KT)

  • Cα-Cβ cleavage in a Thiolysis Reaction.
  • Generates Acetyl-CoA and Acyl-CoA 2C shorter.
  • Multiple forms for different chain lengths.

Trifunctional Protein (TFP)

  • Single enzyme that contains EH, HAD, and KT activities.
  • Essential for the metabolism of highly hydrophobic long chain fatty acyl-CoAs.

Products of ẞ-oxidation

  • Shuttled to the Citric Acid Cycle and Oxidative Phosphorylation.

ATP Yield from Palmitate (16:0) Oxidation

  • Requires 7 rounds of β-oxidation.
    • 7 FADH2 (1.5 ATP/FADH2) = 10.5 ATP
    • 7 NADH (2.5 ATP/NADH) = 17.5 ATP
    • 8 Acetyl-CoA (10 ATP/Acetyl-CoA) = 80 ATP
  • Total: 108 - 2 ATP (for activation) = 106 ATPs/Fatty acid molecule.

β-oxidation of Unsaturated Fatty Acids

  • Requires additional enzymes to handle double bonds.
    • Enoyl-CoA isomerase
    • 2,4-dienoyl CoA reductase.

Beta-oxidation of Unsaturated Fatty Acids

  • Shift double bonds to generate trans-Δ2-Enoyl-CoA which serve as a substrate for enoyl-CoA hydratase (EH)

β-oxidation of Odd Chain Fatty Acids

  • Generates propionyl-CoA, which is converted to succinyl-CoA and enters the citric acid cycle.

Genetic Disorders of Beta Oxidation

  • Acyl-CoA Dehydrogenases.
    • VLCAD: Cardiomyopathy and muscle weakness.
    • LCAD: Pulmonary surfactant dysfunction.
    • MCAD: Most common beta-oxidation defect (1:15000).
      • Hypoketotic hypoglycemia with lethargy.
    • SCAD: Relatively mild.
      • Leads to elevated levels of butyrate.

Screened Disorders in New York State Newborn Screening Program

  • Acyl-CoA Dehydrogenases.
    • VLCAD: Cardiomyopathy and muscle weakness.
    • LCAD: Pulmonary surfactant dysfunction.
    • MCAD: Most common beta-oxidation defect (1:15000).
      • Hypoketotic hypoglycemia with lethargy.
    • SCAD: Relatively mild.
      • Leads to elevated levels of butyrate.
  • HAD
    • Lethal cardiomyopathy, infant-onset hepatic form (lethargy), OR peripheral neuropathy.

Disorders Identified by the New York State Newborn Screening Program

  • Inborn Errors of Metabolism.
    • Endocrinology
      • Congenital adrenal hyperplasia (CAH)
      • Congenital hypothyroidism (CH)
    • Hematology, Hemoglobinopathies
      • Hb SS disease (Sickle cell anemia)
      • Hb SC disease
      • Hb CC disease
      • Other hemoglobinopathies
    • Infectious Diseases
      • HIV-1 infection (HIV-1)
    • Amino Acid Disorders
      • Homocystinuria (HCY)
      • Hypermethioninemia (HMET)
      • Maple syrup urine disease (MSUD)
      • Phenylketonuria (PKU) and Hyperphenylalaninemia (HyperPhe)
      • Tyrosinemia (TYR-I, TYR-II, TYR-III)
    • Fatty Acid Oxidation Disorders
      • Carnitine-acylcarnitine translocase deficiency (CAT)
      • Carnitine palmitoyltransferase I (CPT-1) and II (CPT-II) deficiencies
      • Carnitine uptake defect (CUD)
      • 2,4-Dienoyl-CoA reductase deficiency (2,4Di)
      • Long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHAD)
      • Medium-chain acyl-CoA dehydrogenase deficiency (MCAD)
      • Medium-chain ketoacyl-CoA thiolase deficiency (MCKAT)
      • Medium/short-chain hydroxyacyl-CoA dehydrogenase deficiency (M/SCHAD)
      • Mitochondrial trifunctional protein deficiency (TFP)
      • Multiple acyl-CoA dehydrogenase deficiency (MADD) (also known as Glutaric acidemia type II (GA-II))
      • Short-chain acyl-CoA dehydrogenase deficiency (SCAD)
      • Very long-chain acyl-CoA dehydrogenase deficiency (VLCAD)
    • Organic Acid Disorders
      • Glutaric acidemia type I (GA-I)
      • 3-Hydroxy-3-methylglutaryl-CoA lyase deficiency (HMG)
      • Isobutyryl-CoA dehydrogenase deficiency (IBCD)
      • Isovaleric acidemia (IVA)
      • Malonic acidemia (MA)
      • 2-Methylbutyryl-CoA dehydrogenase deficiency (2-MBCD)
      • 3-Methylcrotonyl-CoA carboxylase deficiency (3-MCC)
      • 3-Methylglutaconic acidemia (3-MGA)
      • 2-Methyl-3-hydroxybutyryl-Co-A dehydrogenase deficiency (MHBD)
      • Methylmalonyl-CoA mutase deficiency (MUT), Cobalamin A,B (Cbl A,B) and Cobalamin C.D (Cbl C,D) cofactor deficiencies and other Methylmalonic acidemias (MMA)
      • Mitochondrial acetoacetyl-CoA thiolase deficiency (beta-ketothiolase deficiency) (BKT)
      • Multiple carboxylase deficiency (MCD)
      • Propionic acidemia (PA)
    • Urea Cycle Disorders
      • Argininemia (ARG)
      • Argininosuccinic acidemia (ASA)
      • Citrullinemia (CIT)
    • Other Genetic Conditions
      • Biotinidase deficiency (BIOT)
      • Cystic Fibrosis (CF)
      • Galactosemia (GALT)
      • Krabbe Disease
      • Severe Combined Immunodeficiency Disease (SCID)

Summary of β-oxidation

  • Acyl CoA (cytosolic)-> carnitine shuttle across inner mitochondrial membrane->Acyl CoA (mitochondrial matrix)-> 1st oxidation-\, 2nd oxidation-\,thiolysis

Fatty Acids and the Brain

  • Fatty acids do not readily diffuse into the brain.
    • During fasting, the brain cannot use fatty acids released by adipose tissue as a source of energy.
    • Alternate source is needed to transfer energy stored in fatty acids (i.e., in adipose tissue or liver) to the brain.

Ketone Bodies

  • Released by the liver into the bloodstream.
  • Used for energy during fasting/low blood glucose.
  • Can diffuse into the brain.

Ketone Bodies

  • Acetyl-CoA cannot be used as a substrate for gluconeogenesis.
  • Produced from acetyl-CoA in the liver.
  • Produced at low levels under normal conditions.
  • Ketone body levels increase when blood fatty acid concentrations are high:
    • Starvation conditions (>24 hour fast, long-term low blood glucose).
    • Untreated diabetes.
  • Fatty acid metabolism is a major source of acetyl-CoA for ketone body biosynthesis.

Energy Use by the Brain

  • Brain comprises ~2% of body weight but uses ~20% of glucose.
  • Brain (i.e., neurons) is heavily reliant on glucose metabolism.
    • This can be supplemented by ketone bodies during fasting/starvation.
  • During prolonged fasting/starvation, it is imperative that the brain continues to receive glucose as an energy source.
    • Liver and muscle shift to fatty acid metabolism to preserve glucose.

Products of Beta Oxidation

  • Inhibit Glycolysis in Liver and Muscle [NADH] [Acetyl-CoA].
  • Acetyl-CoA also stimulates pyruvate carboxylase to increase gluconeogenesis in the liver.

Ketogenesis

  • Formation of ketone bodies from acetyl-CoA.
  • Occurs in the liver.
  • Key enzymes:
    • thiolase (acetyl-CoA acetyltransferase)
    • hydroxymethylglutaryl-CoA synthase (HMG-CoA synthase)
    • hydroxymethylglutaryl-CoA lyase (HMG-CoA lyase)

Metabolism of Ketone Bodies

  • Ketone bodies are converted back to acetyl-CoA in other tissues.

Advantages of Ketone Bodies

  • Ketone bodies provide energy to other tissues.
    • Provide a way to transport acetyl-CoA between tissues.
  • Lower demand for glucose by brain during starvation.
  • Reduce amount of protein (i.e., amino acids) that must be broken down for gluconeogenesis.

Excess of Ketone Bodies

  • Diabetic ketoacidosis
    • Tissues unable to take up and utilize glucose
    • Excess ketone body production
    • Acetone can be smelled in breath
    • Blood pH decreased.
  • Alcoholic ketoacidosis
    • Found in alcoholics
    • High [NADH] and [lactate], depletion of pyruvate required for gluconeogenesis
    • Elevated ketone body production
    • Blood pH decreased.

Glut1 Deficiency and the Ketogenic Diet

  • Glut1 is a glucose transporter at the blood brain barrier.
  • Mutations in Glut1 reduce glucose uptake by the brain.
  • Children with Glut1 deficiency (heterozygous) frequently develop seizures that are poorly controlled by anti-epileptic medications.
  • Ketogenic diet reduces seizures in these patients.