lipid metabolism by kai

Major dietary lipids

• The major dietary lipids are Triacylglycerols (TAGs), cholesterol, and phospholipids.

• TAGs are the primary energy source in many diets; cholesterol and phospholipids have structural and signaling roles.

Digestion of lipids in the mouth and stomach

• Lingual lipase has optimum pH around 2–5, remains active in the stomach, and acts on short-chain triglycerides.

• Gastric lipase is secreted by chief cells; its optimum pH is around 4–5 and secretion is stimulated by gastrin.

• Up to ~30% of TAG digestion occurs in the stomach.

Emulsification and intestinal digestion

• Emulsification in the intestine is a prerequisite for lipid digestion.

  • Emulsification disperses lipids into smaller droplets, reducing surface tension and increasing surface area for enzymes.

  • This process is aided by bile salts, peristalsis, and phospholipids.

• Bile salts (e.g., sodium glycocholate and sodium taurocholate) lower surface tension and stabilize emulsions.

• Pancreatic enzymes involved:

  • Pancreatic lipase

  • Cholesterol esterase

  • Phospholipase A2

• Digestive products:

  • Pancreatic lipase hydrolyzes TAGs to 2-monoacylglycerol (2-MAG) and free fatty acids. Approximate yields: 2-MAG ~78%, 1-MAG ~6%, glycerol + fatty acids remaining.

  • Cholesterol esters are hydrolyzed to free cholesterol and fatty acids.

  • Phospholipase A2 produces lyso-phospholipid and a fatty acid.

• Emulsification and enzymatic digestion enable formation of mixed micelles, which are essential for the absorption of fat and fat-soluble vitamins.

Absorption and transport of lipids

• Long-chain fatty acids (>14 carbons) are absorbed into the lymphatic system via chylomicrons, not directly into blood.

• Bile salt micelles transport digestion products (2-MAG, long-chain fatty acids, cholesterol, phospholipids) to the intestinal mucosa.

• Micelle formation is essential for the absorption of fat-soluble vitamins A, D, E, and K.

• Bile salts are largely reabsorbed in the ileum and returned to the liver via the enterohepatic circulation.

• Inside mucosal cells, long-chain fatty acids are re-esterified to form TAGs.

• Glycerol is absorbed from the intestinal lumen directly into the bloodstream.

• TAGs, cholesterol esters, and phospholipids, along with apolipoproteins (e.g., ApoB-48 and ApoA), are packaged into chylomicrons.

• Chylomicrons are secreted into lymphatics → thoracic duct → systemic circulation.

• Short-chain fatty acids (SCFAs) and medium-chain fatty acids (MCFAs) do not require re-esterification; they can be absorbed directly into the portal blood and transported to the liver for immediate energy use.

• SCFAs are absorbed rapidly and preferentially used for energy; MCFAs are also rapidly utilized in energy metabolism.

• Summary of products:

  • Long-chain fatty acids, 2-MAGs, cholesterol, phospholipids → mixed micelles → absorbed into enterocytes → re-esterified to TAGs → chylomicrons export via lymphatics.

  • Glycerol absorbed into bloodstream directly.

  • SCFAs/MCFAs absorbed into portal circulation for immediate liver use.

Beta-oxidation of fatty acids

• Location: mitochondria of most tissues (including liver and muscle).

• Activation: fatty acids are activated to acyl-CoA by acyl-CoA synthetase using ATP.

• Transport: acyl-CoA is transported into mitochondria via the carnitine shuttle.

  • Carnitine palmitoyltransferase I (CPT I) transfers the acyl group to carnitine to form acyl-carnitine.

  • Translocase shuttles acyl-carnitine into the matrix.

  • Carnitine palmitoyltransferase II (CPT II) transfers the acyl group back to CoA inside the matrix.

• Beta-oxidation cycles:

  • Each cycle shortens the fatty acyl-CoA by two carbons and yields one FADH2 and one NADH, plus a released acetyl-CoA at the final cleavage.

  • For a saturated fatty acid with n carbons:

  • Number of cycles =
    $ext{cycles} = rac{n}{2} - 1$

  • Acetyl-CoA produced =
    $rac{n}{2}$

• Energy yield (net, after activation cost):

  • Activation to acyl-CoA costs 2 ATP equivalents.

  • Each acetyl-CoA yields ~10 ATP in the TCA and oxidative phosphorylation (3 NADH, 1 FADH2, 1 GTP per acetyl-CoA).

  • Each beta-oxidation cycle yields 1 FADH2 (~1.5 ATP) and 1 NADH (~2.5 ATP).

  • Net ATP for a saturated even-length fatty acid with n carbons:
    $ext{Net ATP} = 7n - 6$

  • Example: Palmitate (n = 16) yields
    $ext{Net ATP}_{C16} = 7(16) - 6 = 106 ext{ ATP}$

• Important notes:

  • SCFAs and MCFAs do not require the carnitine shuttle and can be oxidized directly in mitochondria after absorption; they provide rapid energy.

  • Beta-oxidation defects or carnitine deficiency affect long-chain fatty acid oxidation; MCFA/s to fatty acid oxidation can be less affected.

Ketone bodies and ketosis

• Ketone bodies are produced mainly in liver mitochondria from acetyl-CoA when carbohydrate supply is limited (starvation or diabetes) or when insulin is deficient.

• Major ketone bodies:

  • Acetoacetate (primary)

  • β-hydroxybutyrate (secondary; formed from acetoacetate using NADH)

  • Acetone (volatile, from spontaneous decarboxylation of acetoacetate)

• Biochemical pathway (simplified):

  • Two acetyl-CoA condense to form acetoacetyl-CoA:
    $2\, ext{acetyl-CoA} \rightarrow \text{acetoacetyl-CoA}$

  • Acetoacetyl-CoA combines with another acetyl-CoA to form HMG-CoA via HMG-CoA synthase:
    $\text{acetoacetyl-CoA} + \text{acetyl-CoA} \rightarrow \text{HMG-CoA}$

  • HMG-CoA is broken down by HMG-CoA lyase to acetoacetate and another acetyl-CoA:
    $\text{HMG-CoA} \rightarrow \text{acetoacetate} + \text{acetyl-CoA}$

  • Acetoacetate can be reduced to β-hydroxybutyrate using NADH:
    $\text{acetoacetate} + \text{NADH} + H^+ \rightarrow \beta\text{-hydroxybutyrate} + \text{NAD}^+$

  • Acetoacetate can spontaneously decarboxylate to form acetone.

• Transport and utilization:

  • Ketone bodies are released into blood and transported to extrahepatic tissues (brain, heart, skeletal muscle) where they are converted back to acetyl-CoA for entry into the TCA cycle.

  • In tissues, acetoacetate is converted to acetoacetyl-CoA (via SCOT: succinyl-CoA:acetoacetate transferase) and then split into two acetyl-CoA molecules by thiolase; acetyl-CoA then enters the TCA cycle.

  • The liver does not use ketone bodies for energy because it lacks the necessary enzyme (SCOT) in many contexts and primarily exports them.

• Ketosis and clinical features:

  • Ketosis occurs when ketone bodies accumulate in blood due to excess production or insufficient clearance.

  • Clinical signs include breath with acetone smell, ketonuria, osmotic diuresis, dehydration, and potential metabolic acidosis.

  • Physiological responses include reduced buffering capacity and compensatory Kussmaul respiration.

• Causes and clinical relevance:

  • Diabetes mellitus (especially untreated) is the most common cause of ketosis due to lack of insulin → increased lipolysis → elevated fatty acids → ketone body production.

  • Starvation also elevates ketone production as carbohydrate supply declines.

  • Hormonal milieu with high glucagon/insulin ratio promotes ketogenesis.

• Detection and treatment:

  • Detection: ketone bodies in urine (ketonuria) and acetone breath; a clinical sign is ketosis with acidosis.

  • Treatment: administration of insulin and glucose to suppress lipolysis, along with careful fluid and electrolyte management to correct dehydration and metabolic disturbances.

Connections to broader metabolism and clinical implications

• Carbohydrates are essential for the proper metabolism of fat; low carbohydrate availability shifts energy metabolism toward fat oxidation and ketone body production.

• Proper fat digestion and absorption require coordinated biliary and pancreatic functions and intact enterohepatic circulation.

• Disorders of lipid digestion, absorption, or metabolism (e.g., steatorrhea, malabsorption, insulin deficiency, carnitine deficiency) have systemic energy and electrolyte consequences.

Key equations and numerical references

• Beta-oxidation net ATP for a saturated fatty acid with n carbons:
  $ext{Net ATP} = 7n - 6 \, \text{ATP}$

• Palmitate example (n = 16):
  $ext{Net ATP}_{C16} = 7\times 16 - 6 = 106\, \text{ATP}$

• Ketogenesis simplified steps:

  • 2 Acetyl-CoA → Acetoacetyl-CoA

  • Acetoacetyl-CoA + Acetyl-CoA → HMG-CoA

  • HMG-CoA → Acetoacetate + Acetyl-CoA

  • Acetoacetate + NADH → β-hydroxybutyrate + NAD^+

  • Acetoacetate → Acetone (spontaneous decarboxylation)

• Ketolysis in extrahepatic tissues (example):
  $\text{Acetoacetate} + \text{Succinyl-CoA} \rightarrow \text{Acetoacetyl-CoA} + \text{Succinate}$
  $\text{Acetoacetyl-CoA} \rightarrow 2\text{Acetyl-CoA}$

Note on terminology from the transcript

• SCFAs = short-chain fatty acids; MCFA = medium-chain fatty acids.

• ApoB-48 and ApoA are apolipoproteins involved in chylomicron formation and lipid transport.

• Steatorrhea refers to increased fat in feces due to malabsorption.

• Rothera’s test (ketone bodies in urine) is used clinically to detect ketosis.