Lipid 3 Metabolism - Part 3 Notes

LIPIDS - PART 3

Learning Objectives

  • Understand the physiological importance of lipid metabolic pathways:
    • De novo synthesis of fatty acids
    • Beta-oxidation
    • Ketolysis
  • Describe the De novo synthesis of fatty acids
  • Understand the pathway for mobilization of fats - lipolysis (from adipose tissue)
  • Describe the beta-oxidation process (including the role of the carnitine shuttle)
  • Name the ketone bodies and their metabolic relevance
  • Describe ketolysis

Terminology/Abbreviations

  • Acetyl CoA Carboxylase (ACC)
  • Acetyl coenzyme A (ACoA)
  • Adipocyte
  • Biotin Carboxyl group
  • Carnitine
  • Carnitine shuttle
  • Cis double bonds
  • Citrate (in TCA cycle)
  • De novo synthesis
  • Epinephrine
  • FA = Fatty Acid
  • FAD+/FADH2FAD^+/FADH_2
  • Fatty acid synthase (FAS)
  • Glucagon
  • Hepatocyte
  • Insulin
  • Lipases
  • Lipolysis
  • Malonyl CoA
  • NAD+/NADHNAD^+/NADH
  • NADP+/NADPHNADP^+/NADPH
  • Oxaloacetate (OAA) (in TCA cycle)
  • Oxidation
  • Palmitate
  • Palmitoyl CoA
  • PUFA = polyunsaturated fatty acids
  • Pyruvate (in glycolysis)
  • sER = smooth endoplasmic reticulum
  • β-carbon

LIPIDS – Fatty Acid Metabolism

  • Fatty acids are synthesized from and oxidized to a common compound → ACETYL-COA
  • Fatty acids are oxidized in the mitochondria and synthesized in the cytoplasm.
  • Physiological conditions that promote FA synthesis largely inhibits oxidation (and vice-versa).
  • Cellular compartmentalization is essential for preventing futile cycling!

LIPIDS – De Novo Synthesis of Fatty Acids

  • Fatty acid synthesis (de novo) occurs mainly in the cytosol of:
    • Liver cells (hepatocyte)
    • Mammary glands (lactating)
    • Adipose tissue cells (adipocyte)
  • Other tissues can synthesize FA in small quantities → kidneys, brain, and lungs.
  • Animals can synthesize all FAs they need except for the essential FAs, which must be supplied through the diet.
  • Short FAs are produced in lactating mammary glands (butyric acid, caproic acid → milk fat).
  • Substrates:
    • Excess carbohydrates and proteins from the diet that exceed the body‘s needs for these nutrients during the feeding period.
    • Acetyl Coenzyme A (ACoA) from mitochondria is key.
    • The process requires ATP and NADPH.
  • Primary product:
    • Palmitate or palmitic acid (16 C) is the primary end product of De novo FA Synthesis.
      • Can be further elongated in smooth endoplasmic reticulum (sER).
      • Brain cells can produce very long FA required for synthesis of brain phospholipids.
    • Certain enzymes present in sER can cause desaturation of LCFA by adding cis double bonds.
    • A variety of polyunsaturated FA (PUFA) can be produced by desaturation + elongation.

LIPIDS - De Novo Synthesis of FA

1. Cytosolic Acetyl CoA Production
  • Move acetate units from mitochondrial acetyl CoA to the cytosol (from the mitochondrial matrix).
  • Mitochondrial ACoA is produced mainly by oxidation of pyruvate.
  • Since CoA portion of ACoA cannot cross the inner mitochondrial membrane, the acetyl group must be incorporated into citrate for membrane transport.
  • Citrate is produced by condensation of ACoA with oxaloacetate (OAA).
  • Citrate in the cytosol is then cleaved to OAA and ACoA by ATP-citrate lyase.
  • This process is stimulated when mitochondrial [citrate] is high → this happens when [ATP] is high → high energy signal.
2. ACoA Carboxylation to Malonyl CoA
  • Carboxylation of ACoA to malonyl CoA is catalyzed by acetyl CoA carboxylase (ACC).
  • Biotin (Vit H or B7) and ATP are required in the carboxylation process.
  • This is the rate-limiting step and the regulated step in FA synthesis.
  • ACC is allosterically activated by citrate and inactivated by palmitoyl CoA (pathway end product-negative feedback).
  • ACC synthesis is also stimulated by high-calorie and high-carbohydrate diets (nutrient availability) and hormonally (insulin).
3. Synthesis of Palmitate 16:0
  • All other reactions of fatty acid synthesis (in eukaryotes) are driven by the enzyme fatty acid synthase (FAS).
  • This involves the addition of two carbons from malonyl CoA to the carboxyl end of a series of acyl acceptors (amino acid such as cysteine).
  • NADPH is required in this process (NADPH from the Pentose Phosphate Pathway).
  • The result is the production of palmitate (a fully saturated fatty acid, 16:0).
  • Carbons provided directly by ACoA (via malonyl CoA) are red, carbons released as CO2CO_2 are blue.

STORAGE OF LIPIDS (lipogenesis)

  • Newly synthesized fatty acids can be stored as mono- (one), di- (two) or triacylglycerols (three fatty acids).
  • The three FA in TAG can vary: C-1 often saturated, C-2 unsaturated, C-3 either.
  • The presence of unsaturated fatty acids lowers the melting temperature of the lipid.
  • Lipids caloric value per unit mass is over twice as great as carbs and proteins (i.e., 9 kCal/gm for TG compared to about 4 kCal/gm for carbohydrate and protein).
  • Lipogenesis → TAG is stored as lipid droplets in adipocytes (fat depot).
  • A small part is stored in the liver and released into the blood as VLDL.
  • MOBILIZATION of fat (from adipose tissue) requires release from their TAG form → lipolysis.
    • With help of Hormone-Sensitive-Lipase (HSL) (stimulated by epinephrine and glucagon).
    • TAG → glycerol + Free fatty acids.

FATTY ACID β-OXIDATION

  • Major pathway for FA catabolism → Occurs in the mitochondria.
  • Long-chain fatty acids must form an active intermediate (fatty acyl CoA) before being oxidized inside the mitochondria.
  • The carnitine shuttle is required to transport fatty-acyl-CoA into the mitochondria (rate-limiting transport).
  • Carnitine:
    • Carnitine is a compound synthesized from amino acids lysine and methionine in the liver and kidneys (abundant in mitochondrial membranes of muscle tissue).
    • Carnitine also can be absorbed from the diet mostly from animal products (red meat, poultry, dairy).
    • Carnitine deficiencies cause decreased ability of tissues to use LCFA as fuel
      • Can be caused by cellular defects, genetic or medical conditions, or due to liver or kidney pathology.
    • Carnitine shuttle (CPT1) can be inhibited by malonyl CoA, so newly synthesized FA cannot be transferred into mitochondria to be degraded.
  • It is a cyclic process, each cycle is catalyzed by enzymes with chain- length specificity.
  • Each cycle produces: 1 acetyl-CoA + 1 NADH + 1 FADH2
  • First cycle of β-oxidation : A sequence of four reactions that involve the β-carbon and cause the shortening of the FA by two carbons at the carboxyl end.
    • A reduction that produces FADH2
    • A hydration
    • A second reduction that produces NADH
    • A CoA-dependent thiolytic cleavage that frees a molecule of acetyl CoA

SUMMARY OF THE ENERGY YIELD FROM THE OXIDATION OF 1 PALMITOYL CoA (16 CARBONS)

  • 2 ATP: activation of palmitate to palmitoyl CoA (fatty acyl CoA).
  • Oxidation of 1 palmitoyl CoA:
    • 8 ACoA
    • 7 NADH
    • 7 FADH2
  • Final products after full oxidation (beta-oxidation, TCA cycle, and OxPhos)
    • ATP
    • CO2CO_2
    • H2OH_2O

Fasted-state Metabolism

  • Fasted-state metabolism must maintain plasma glucose homeostasis for the brain.
    1. Liver glycogen becomes glucose.
    2. Adipose lipids become free fatty acids and glycerol that enter blood.
    3. Muscle glycogen can be used for energy. Muscles also use fatty acids and break down their proteins to amino acids that enter the blood.
    4. Brain can use only glucose and ketones for energy.

LIPIDS – KETONE BODIES

  • Alternative fuel for cells
    • The adult liver mitochondria can convert ACoA from fatty acid oxidation (beta oxidation) into ketone bodies → acetoacetate, β-hydroxybutyrate, and acetone.
  • Acetone is metabolically inert and can cause a fruity smell on the breath and urine of ketotic patients.
  • Acetoacetate and β-hydroxybutyrate are free soluble lipids
    • Transported in the blood plasma to peripheral tissues (muscle, brain, kidney, mammary gland, small intestine, fetal liver).
    • In peripheral tissue cells, it can be converted back into acetyl CoA, which enters the TCA cycle for ATP production.
    • Can be used in the biosynthesis of glycerophospholipids, sphingolipids, and sterols.
    • Negative feedback on hormone sensitive lipase (HSL) activity in adipocytes.
  • Important energy supply for peripheral tissues:
    • Are water-soluble, can be transported without albumin or lipoproteins.
    • Can cross blood-brain barrier and placental barrier.
    • Are used proportionally to their concentration in the blood by extrahepatic tissue.
    • If concentrations are high enough cardiac and skeletal muscle, intestinal mucosa cells, renal cortex, brain, and fetus can use ketone bodies.
  • As a result, ketone bodies save glucose (important during fasting and prolonged exercise).
  • During fasting, fatty acids mobilized from adipose tissue move to the liver.
  • Fatty acid oxidation produces high amounts of NADH (exceeding oxidative capacity of TCA/OxPhos in the liver)  ACoA goes into ketogenesis.
  • Lipolysis of triglycerides in adipocytes stimulates the production of ketone bodies
    • Increased Acetyl CoA (due to beta-oxidation of FFAs) → exceeds the oxidative capacity of the liver → stimulating the production of ketone bodies.

LIPIDS- KETOLYSIS

  • KETOLYSIS: KETONE BODIES USED BY THE PERIPHERAL TISSUES
  • In normal conditions, the liver constantly produces low levels of ketone bodies.
    • However, it increases during fasting (or pathologic conditions such as diabetes mellitus) when ketone bodies are required as a source of energy to peripheral tissues.
      • Ketone bodies (KB) synthesis occur in the liver → KB used in peripheral tissues.
      • KB are hydrophilic → quickly transported via plasma.
      • Mammalian RBC and liver cannot use KB as a source of energy
        • RBC lack mitochondria hepatocyte lack thiophorase
    • KETOLYSIS in peripheral tissue
      • 3-hydroxybutyrate is oxidized to acetoacetate.
      • Acetoacetate + CoA molecule → Acetoacetyl CoA
      • Acetoacetyl CoA → 2 Acetyl-CoA
  • When the rate of ketone body formation exceeds the rate of their use:
    • Ketone body levels begin to rise in the blood  ketonemia.
    • And eventually in the urine  ketonuria.
      • This is most frequently encountered in cases of uncontrolled type 1 diabetes mellitus (T1D).
      • Blood concentration of ketone bodies can reach 90 mg/dl (<3 mg/dl in normal individuals).
    • Elevation of ketone bodies in the blood can cause acidemia.
    • Urinary loss of glucose and ketone bodies can also lead to dehydration, thus increased thirst (PUPD – polyuria and polydipsia).

Excessive Ketone Body Production in Diabetes Mellitus

  • The elevated amount of hydrogen ions (from the ketone bodies) and decreased plasma volume can cause diabetic ketoacidosis (DKA).
  • A frequent symptom is a fruity odor on the breath (acetone).
  • Ketoacidosis may also result from prolonged fasting and excessive ethanol consumption.