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Overview of Lipid Metabolism Through Fed State: Lipid Metabolism

1. Differences Between Omega 3s and Omega 6s

Omega 3:
  • Definition: Essential fatty acids that must be obtained from dietary sources.
  • Sources: Found in cold water fish such as salmon, tuna, and halibut.
  • Conversion: The body converts alpha-linolenic acid (ALA) into eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), the two forms of omega-3 fatty acids that are utilized more efficiently by the body.
  • Health Benefits:
    • Reduces inflammation.
    • Supports heart health.
    • Crucial for brain functions.
    • Aids in recovery, beneficial for athletes.
  • Examples: Fatty fish, chia seeds, flaxseeds, walnuts.
  • Metaphor: Think of omega-3s as the “calm down” fats.
Omega 6:
  • Definition: Essential fatty acids that also must be obtained from the diet.
  • Function: Important for growth and development, and supports immune function.
  • Inflammatory Response: Can promote inflammation, particularly when consumed in excess.
  • Sources: Found in vegetable oils, fried or processed foods, and many packaged snacks.
  • Metaphor: Think of omega-6s as the “alert” fats.
Key Differences:
  • Both omega-3 and omega-6 fatty acids are essential for health.
  • Most people consume enough omega-6, whereas many do not get sufficient omega-3.
  • An imbalance with excessive omega-6 relative to omega-3 can lead to increased inflammation.

2. Transamination and Deamination

Transamination:
  • Definition: The process of transferring an amino group (-NH2) from one amino acid to a keto acid. No nitrogen is lost during this process, just transferred.
  • Equation: ext{Amino Acid}_1 + ext{α-Ketoglutarate} ightarrow ext{α-Keto Acid}_1 + ext{Glutamate}
    • Example: ext{Alanine} + ext{α-Ketoglutarate}
      ightarrow ext{Pyruvate} + ext{Glutamate}
  • Enzyme: Aminotransferase (ALT or AST).
  • Cofactor: Vitamin B6 (PLP). The nitrogen is collected onto glutamate.
Deamination:
  • Definition: The removal of an amino group as free ammonia (NH3). This is the step where nitrogen is released.
  • Main Site: Primarily occurs in the liver.
  • Equation:
    ext{Glutamate} + ext{(NAD+)} + ext{H}_2 ext{O}
    ightarrow ext{α-Ketoglutarate} + ext{NH}_3 + ext{NADH} + ext{(H+)}
  • Enzyme: Glutamate dehydrogenase.
  • Location: Mitochondria in the liver. Glutamate releases its nitrogen as ammonia, which enters the urea cycle.

3. Locations of Carbohydrate, Protein, and Lipid Digestion

Carbohydrates:
  1. Mouth:
    • Salivary amylase begins breaking down starch into smaller polysaccharides.
  2. Stomach:
    • Minimal activity; acid inactivates amylase.
  3. Small Intestine:
    • Pancreatic amylase breaks starch into disaccharides.
    • Brush-border enzymes (maltase, sucrase, lactase) convert disaccharides into monosaccharides (glucose, galactose, fructose) for absorption.
Proteins:
  1. Starts in Stomach:
    • Hydrochloric acid (HCl) denatures proteins.
    • Pepsin cleaves proteins into polypeptides.
  2. Small Intestine (Main Site):
    • Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) further digest polypeptides.
    • Absorbed as amino acids and small peptides.
Lipids (Fats):
  1. Mouth:
    • Minor digestion occurs through lingual lipase.
  2. Stomach:
    • Minor digestion through gastric lipase.
  3. Small Intestine (Most Important):
    • Bile (from liver/gallbladder) emulsifies fats.
    • Pancreatic lipase breaks triglycerides down into monoglycerides and free fatty acids.

4. Emulsification:

Definition:
  • The process of breaking large fat globules into smaller droplets to facilitate easier digestion.
  • Metaphor: Think of it as turning one big oil blob into numerous tiny droplets.
Location:
  • Occurs in the small intestine.
  • Bile, which is produced in the liver and released from the gallbladder, plays a crucial role.
Mechanism:
  • Bile Salts:
    • Have a dual nature; one side is hydrophobic (attracted to fat) and the other is hydrophilic (attracted to water).
    • They surround fat droplets, breaking them apart.
Importance:
  • Essential because fat and water do not mix, and digestive enzymes like pancreatic lipase are water-soluble.
  • Without emulsification, these enzymes cannot act effectively on large fat globules.

5. Lipoprotein Transport System

Chylomicrons:
  • Production: Formed in the gut from dietary fats.
  • Function: Primary lipoprotein in a fed state, transporting triglycerides to adipose tissues for storage.
  • Size: Largest size among lipoproteins.
  • Characteristics:
    • Nascent chylomicrons contain Apo48 and ApoA.
    • Mature chylomicrons contain ApoCII (activates lipoprotein lipase, allowing the uptake of free fatty acids) and ApoE (enables liver uptake of chylomicron remnants).
    • Upon uptake, remnants are converted into VLDLs in the liver.
VLDLs:
  • Production: Synthesized in the liver.
  • Function: Transport triglycerides to adipose tissue.
  • Size: Next largest after chylomicrons.
  • Characteristics: Contains ApoB-100 protein.
  • Circulation: Found in the fasted state.
LDLs (Low-Density Lipoproteins):
  • Origin: Produced from VLDLs after triglyceride removal.
  • Function: Deliver cholesterol to tissues.
  • Characteristics: Also contains ApoB-100 protein; known as 'bad cholesterol'.
  • Circulation: Typically present in a fasted state.
HDLs (High-Density Lipoproteins):
  • Function: Remove excess cholesterol from tissues for degradation.
  • Production: Synthesized in the liver.
  • Size: Smallest among lipoproteins, dense composition.
  • Characteristics: Composed mainly of proteins and contains ApoA-1.
  • Circulation: Present in both fed and fasted states.

6. Transport and Distribution

In Fed State:
  • Chylomicrons distribute free fatty acids for storage following meal digestion.
  • Involves conversion of glucose carbons to fatty acids and triacylglycerols.
In Fasted State:
  • Distribution of fatty acids from adipose tissues for oxidation.
  • VLDL distribution from the liver to provide fatty acids and cholesterol.
ATP Production from Fatty Acid Oxidation:
  • For a 16-carbon fatty acid:
    8 ext{ Acetyl-CoA}
    ightarrow 8 imes 10 = 80 ext{ ATP}

7. Enzymes for Digestion

Carbohydrates:
  • Mouth: Salivary amylase breaks down starch into maltose.
  • Small Intestine:
    • Pancreatic amylase continues starch breakdown.
    • Brush-border enzymes (maltase, sucrase, lactase) convert disaccharides into monosaccharides.
Proteins:
  • Stomach: Pepsin hydrolyzes proteins into smaller polypeptides.
  • Small Intestine:
    • Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) further digest polypeptides into smaller peptides.
    • Brush border peptidases convert peptides into amino acids.
Fats (Lipids):
  • Mouth: Lingual lipase initiates fat digestion.
  • Stomach: Gastric lipase continues the process.
  • Small Intestine:
    • Bile emulsifies fat (not an enzyme).
    • Pancreatic lipase converts triglycerides into monoglycerides and free fatty acids.

8. Hormones of the Gut

Secretin and CCK

Cholecystokinin (CCK):

  • Stimulates the pancreas to release enzymes in response to fats and proteins.
  • Triggers gallbladder contraction (bile release) and pancreatic enzyme release.

Secretin:

  • Stimulates the pancreas to elevate pH through the release of bicarbonate.
  • CCK and secretin are produced in enteroendocrine cells of the intestine.

9. Steps in Beta Cell for Insulin Release

  1. GLUCOSE ENTRY: Glucose enters beta cells via GLUT2 transporter (passive transport).
  2. METABOLISM: Glucose is metabolized through glycolysis, TCA cycle, and oxidative phosphorylation, which increases ATP production.
  3. POTASSIUM CHANNELS: Rise in ATP causes ATP-sensitive K+ channels to close, preventing K+ exit, leading to membrane depolarization.
  4. CALCIUM ENTRY: Depolarization opens voltage-gated Ca2+ channels; Ca2+ rushes in.
  5. INSULIN RELEASE: Ca2+ triggers insulin vesicle fusion with the membrane, leading to insulin exocytosis.
  • Initiating Substrate: The primary substrate leading to insulin production is glucose.

10. Regulation of PFK1 and PFK2

PFK1
  • Definition: Controls the rate of glycolysis, catalyzing the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, representing the rate-limiting step of glycolysis.
  • Regulation:
    • Activation: Turns on when energy is low; stimulated by high AMP, ADP, and fructose-2,6-bisphosphate.
    • Inhibition: Turns off when energy is high; inhibited by elevated ATP and citrate levels.
PFK2
  • Definition: Does not directly control glycolysis but produces fructose-2,6-bisphosphate, a strong activator of PFK1.
  • Regulation:
    • Controlled by hormones (insulin and glucagon).
    • In the fed state (high insulin): activates PFK-2, increasing fructose-2,6-bisphosphate, thus promoting glycolysis.
    • In the fasting state (high glucagon): inhibits PFK-2, reducing fructose-2,6-bisphosphate, thereby inhibiting glycolysis.

11. Regulation of Glycogen Synthase

Regulation Mechanisms:
  • Kinases: Enzymes that phosphorylate glycogen synthase, inactivating it.
  • Phosphatases: Dephosphorylate glycogen synthase, activating it.
  • Allosteric Regulation: Glycogen synthase is also regulated by glucose-6-phosphate, which acts as an allosteric activator.

12. Brush Border Enzymes

For Carbohydrates:
  1. Lactase
  2. Sucrase
  3. Maltase
  4. Isomaltase
For Proteins:
  1. Aminopeptidase
  2. Dipeptidyl aminopeptidase
  3. Tripeptidases

13. Glucogenic vs. Ketogenic Amino Acids

Glucogenic Amino Acids:
  • Can be converted into glucose through gluconeogenesis.
Ketogenic Amino Acids:
  • Convert into ketone bodies; cannot be converted into glucose.

14. De Novo Fatty Acid Synthesis

Activation Conditions:
  • Primarily activated when carbohydrate intake exceeds energy expenditure, leading to excess glucose being converted into fatty acids.

15. Omega and Delta Nomenclature for Fatty Acids

Delta System:
  • Counts from the carboxyl end (COOH).
  • Example: 18:1(riangle9)18:1 \big( riangle^9\big)
    • 18 carbons, 1 double bond starting at carbon 9 from the carboxyl end.
Omega System:
  • Counts from the methyl end (CH3).
  • Example: 18:3(extω−3)18:3 \big( ext{ω-3}\big)
    • 18 carbons, 3 double bonds, with the first double bond occurring 3 carbons from the methyl end.

16. Classification of Amino Acids

  • Net Electrical Charge: Positive, negative, or neutral.
  • Structure: Aliphatic, aromatic, etc.
  • Polarity: Polar, nonpolar.
  • Carbon Fate: Glucogenic or ketogenic.
  • Essentiality: Essential (must be obtained through diet) vs. non-essential.

17. Monosaccharide and Fatty Acid Transporters

Transporters:
  • GLUT1: Widely distributed across the body.
  • GLUT2: Found in enterocytes, liver, and pancreatic beta cells.
  • GLUT3: Specific to the brain.
  • GLUT4: Present in muscle and adipose tissues, regulated by insulin.
  • GLUT5: Fructose transporter.
Km Values:
  • Understanding of Km values is crucial to determine transporter affinity for glucose.

18. Hexokinase vs. Glucokinase Regulation

Hexokinase:
  • Location: Primarily in most tissues (muscle, brain).
  • Km: Low: functions effectively at low glucose levels.
  • Inhibition: Inhibited by its product, glucose-6-phosphate (G6P).
Glucokinase:
  • Location: Found in the liver and pancreatic beta cells.
  • Km: High; only active when glucose levels are high (after meals).
  • Regulation: Not inhibited by G6P; regulated through glucokinase regulatory protein (GKRP).

19. Essential Amino Acids

  1. Histidine
  2. Isoleucine
  3. Leucine
  4. Lysine
  5. Methionine
  6. Phenylalanine
  7. Threonine
  8. Tryptophan
  9. Valine

20. Pathways in a Fed State

Hormones:
  • Insulin is the primary hormone present after meals, signaling an anabolic state.
Active Pathways:
  1. Glycolysis: Utilization of glucose for energy.
  2. Glycogenesis: Storage of glucose as glycogen.
  3. Fatty Acid Synthesis (Lipogenesis): Conversion of excess carbohydrates into fat.
  4. Protein Synthesis: Building tissue and muscle from amino acids.
Enzyme Activation:
  • Enzymes in the fed state are typically dephosphorylated, making them active.
  1. Glycogen Synthase: Active when dephosphorylated; promotes glycogen synthesis.
  2. Glycogen Phosphorylase: Inactive when phosphorylated; prevents glycogen breakdown.
  3. Pyruvate Kinase: Liver version active in the fed state.
  4. PFK-2: Insulin-induced dephosphorylation activates it, promoting glycolysis.
Summary Rule:
  • Fed State = Insulin = Dephosphorylation = Storage and Anabolism.

21. Respiratory Exchange Ratio (RER) Values

Zymogens:
  1. Pepsinogen (activated to pepsin in stomach by HCl).
  2. Trypsinogen (activated to trypsin in small intestine by enteropeptidase).
  3. Chymotrypsinogen (activated to chymotrypsin in small intestine by trypsin).

22. Steps in Insulin Release from Beta Cells

  1. Glucose enters through GLUT2 transporter.
  2. Undergoes glycolysis, TCA cycle, and ETC, producing ATP.
  3. Increased ATP closes potassium (K+) channels.
  4. Depolarization leads to calcium (Ca2+) influx.
  5. Calcium triggers insulin vesicle fusion, resulting in insulin release into the bloodstream.
Process Summary:
  • Glucose → Increased ATP → K+ Channels Close → Ca2+ Influx → Insulin Release.