Dietary Lipids and Digestion

• Dietary lipids consist of triacylglycerols (99%), cholesterol ester, phospholipids, and fat-soluble vitamins.

Biological Importance of Lipids

• Lipids are one of the three main food stuffs
• Lipids are palatable and provide more energy than carbohydrates and protein.
• Carbohydrates supply 4.10 k Cal/g and provide energy for 24 hours of fasting
• Fat provides 9.30 k Cal/g and energy for several days of fasting
• Proteins supply 4.10 k Cal/g
• Lipids provide essential fatty acids and fat-soluble vitamins (E, D, A, K).
• They enrich milk with fat content and are important for brain activities in the form of compound lipids.
• Lipids are structural components of cell membranes as phospholipids and glycolipids.
• In the body, lipids exist as tissue fat and depot fat.

Digestion of Neutral Fat

• In the mouth, lingual lipase is secreted from Ebner's gland on the dorsum of the mouth and is active.
• In the mouth, lingual lipase has optimal pH between 4-4.5, the substrate that it acts on are triacylglycerols with short chain fatty acids, and the end products are glycerol and free fatty acids
• Lingual lipase is active only in infants due to suitable stomach pH and emulsified milk fat droplets.
• In the stomach: Gastric lipase is produced by gastric mucosa, similar to lingual lipase, and is actively secreted with a pH of 7.
• Gastric lipase acts on: Triacylglycerols, producing glycerol and free fatty acids after ester bond hydrolysis.
• Gastric lipase has no effect in adult stomach, except in cases of gastric HCl deficiency
• Gastric lipase is active in infants' stomachs, after consumption of a heavy protein or fat meal where HCl is deficient and has a pH of 5.
• The presence of fat in the stomach stimulates enterogasterone hormone secretion, delaying gastric emptying and prolonging satiety.
• Maximal neutral fat digestion occurs in the small intestine via pancreatic lipase.
• Bile salts emulsify fat by breaking down globule size, increasing surface area for enzyme action.

Pancreatic Lipase

• Pancreatic lipase source is pancreatic acini
• Pancreatic lipase activation requires bile salts, co-lipase, phospholipids, and Ca++ ions
• Pancreatic lipase, in obstructive jaundice, there is no fat digestion due to the absence of bile salts
• Pancreatic Lipase substrate is triacylglycerols
• Pancreatic Lipase breaks the primary ester bonds of triacylglycerols (containing C12-C18 saturated or unsaturated fatty acids) to form β monoacyl glycerol and 2 fatty acids
• 28% of β monoacyl glycerol is converted to α Monoacyl glycerol by Isomerase enzyme
• Lipase enzyme catalyzes hydrolysis of primary ester bond of α monoacyl glycerol forming Glycerol and free fatty acid
• 72% of triacylglycerols are hydrolyzed into β monoacyl glycerol + 2 fatty acids
• 6% hydrolyzed into α monoacyl glycerol + 2 fatty acids
• 22% hydrolyzed into glycerol + 3 fatty acids due to slow action of isomerase enzyme

Intestinal Lipase

• Act on 1 monoacylglycerol in intestinal mucosa forming glycerol + free fatty acid.

Digestion of Cholesterol

• Cholesterol digestion; no digestion occurs, it is absorbed as it is

Cholesterol Esterase Enzyme

• Cholesterol Esterase Enzyme source is pancreatic acini
• The mode of secretion of Cholesterol Esterase Enzyme is active
• Cholesterol Esterase Enzyme pH is 8
• Cholesterol Esterase Enzyme: acts on cholesterol ester, producing cholesterol and free fatty acid after ester bond hydrolysis

Digestion of Phospholipids

• Phospholipids are compound lipids made of alcohol (glycerol), 2 fatty acids, phosphoric acid, and a base

Phospholipases

• The source is the pancreas and the intestine
• There are 4 main types including phospholipase A1, A2 (B): that are present in pancreatic juices, phospholipase A2: that are present in snake venom, phospholipase C that are present in intestinal and pancreatic juices, phospholipase D that are found only in plants
• Phospholipase A2 catalyzes hydrolysis of ester bond between glycerol and unsaturated fatty acid (2nd position) in phospholipids forming Lysophospholipid + free fatty acid
• Phospholipase A₁ catalyzes hydrolysis of ester bond between glycerol and saturated fatty acid (1st position) in lysophospholipid
• Phospholipase C catalyzes hydrolysis of ester bond between glycerol and phosphoric acid (3rd position) in phospholipid forming Diacylglycerol + phosphoryl choline
• Phospholipase D catalyzes hydrolysis of ester bond between phosphoric acid and choline in phosphoryl choline

Absorption

• Site of absorption: Jejunum and Ileum.
• Short chain fatty acids and glycerol are water-soluble and pass directly to the liver via portal circulation.
• Other lipids combine with bile salts to form water-soluble molecules (Micelles) due to their hydrotropic properties.
• Micelles have a hydrophobic portion directed inward and a hydrophilic portion directed outward.
• In intestinal mucosa, micelles split, and bile salts are reabsorbed back to the liver via enterohepatic circulation.
• Long chain fatty acids are activated in the intestinal mucosa by Thiokinase enzyme to Acyl CoA.
• Acyl CoA then combines with monoacylglycerols, diacylglycerols, lysophospholipid, and cholesterol.
• Triacylglycerols, phospholipids, and cholesterol bind with Apolipoprotein B to form Chylomicrons.
• Chylomicrons enter lacteals, pass through lymphatic drainage to the thoracic duct, and reach systemic circulation, binding to Apolipoprotein C.

Chylomicrons

• Formed of triacylglycerols (core) and phospholipids + protein (outer shell).
• Give serum a milky appearance after a fatty meal.
• They are removed by lipoprotein lipase.

Chyluria

• Presence of chylomicrons in urine after a fatty meal.
• Chyluria cause is due to an abnormal connection between the lymphatic drainage of the intestine and the urinary tract.

Steatorrhea

• Condition characterized by abnormally increased fat content in the stool (normally less than 5 g/day).
• Deficiency of pancreatic lipase: Caused by pancreatitis or obstruction of the pancreatic duct, resulting in undigested fat and no loss of fat-soluble vitamins.
• Deficiency of bile salts: Usually caused by liver disease or bile duct obstruction, leading to digested fat but loss of fat-soluble vitamins.
• Healthy intestinal mucosa deficiencies

Fate of Triacylglycerols

• Triacylglycerols, after digestion can undergo: storage as depot fat or Tissue fat, oxidation for energy, synthesis of new molecules of neutral fat or excretion in stools
• Milk of a lactating female contain neutral fat, phospholipids, cholesterol and free fatty acids
• Free fatty acids in milk are mainly polyunsaturated, with a small amount of short-chain fatty acids.

Storage and Mobilization of Fat

• Lipids inside the body are divided into tissue fat and depot fat.

Tissue Fat

• Found in cell membranes and the nervous system (neurolemma).
• Consists mainly of phospholipids, cholesterol, and glycolipids, with little triacylglycerols.
• Fatty acids are mainly unsaturated.
• Not affected by starvation and Not oxidized to produce energy
• Tissue fat is an essential component of cell organelles.
• Tissue fat acts as an electrical insulator, facilitating rapid nerve depolarization along myelinated nerves.

Depot Fat

• Stored form of fat inside the body, found in cytoplasm of adipose tissue cells as yellow droplets and is found under the skin, around the breast, liver and kidney or in the omentum and mesentry
• Composed mainly of triacylglycerols and small amounts of phospholipids and cholesterol.
• Sources are absorbed fat and carbohydrates via lipogenesis.
• Affected by starvation and is used to store of energy.
• Also functions as pads for internal organs.
• Depot fat provides protection to body prominences.
• Depot fat contains 7-dehydrocholesterol, which converts to vitamin D upon UV exposure.
• Depot fat is a thermal insulator in subcutaneous tissues, preventing heat loss.

Mobilization and Metabolism of Fat

• In Caloric balance, stored triacylglycerols are in continuous state of lipolysis and re-esterification.

Lipolysis

• It is the breakdown of stored triacylglycerols in adipose tissue into glycerol and free fatty acids.
• It occurs when energy needs rise, such as during starvation, diabetes mellitus, growth, low carbohydrate diets, or certain infections.
• Lipolysis involves intracellular lipase enzymes, including hormone-sensitive triacylglycerol lipase, diacylglycerol lipase, and monoacylglycerol lipase.
• The end products of lipolysis are glycerol and free fatty acids.

Fate of Free Fatty Acids

• Re-esterification in adipose tissue to form triacylglycerol again.
• Diffusion into the plasma, then transported to tissues for oxidation.

Fate of Glycerol

• Glycerol cannot readily re-esterify in adipose tissue due to low glycerokinase activity.
• Glycerokinase converts glycerol to glycerol-3-phosphate, essential for re-esterification.
• Glycerol diffuses into the plasma and is taken up by the liver and kidney.
• Glycerol in the liver and kidney may undergo gluconeogenesis (give glucose), glycolysis (give pyruvate), or lipogenesis (give triglycerides).
• If lipolysis exceeds lipogenesis: Accumulation of it in the form of free fatty acids, which diffuse into plasma and bind to albumin for transport to tissues for oxidation.

Lipogenesis (Re-esterification of Glycerol)

• The resynthesis of triacylglycerols involving Acyl CoA (fatty acids) and Glycerol 3-Phosphate.
• Lipogenesis site is mainly adipose tissue, liver, and lactating mammary glands.
• Lipogenesis occurs in steps:
  • Biosynthesis of active glycerol.
  • Biosynthesis of fatty acids.
  • Biosynthesis of triacylglycerols.

Biosynthesis of active glycerol:

• Synthesis from glucose via glycolysis produces dihydroxyacetone phosphate, which is then reduced to glycerol 3-phosphate.

Biosynthesis of fatty acids:

• Process occurs through extramitochondrial (cytoplasmic), microsomal, and mitochondrial systems.
• Extramitochondrial system (cytoplasmic - De novo synthesis of fatty acids):
  • Occurs mainly in the cytoplasm of tissues like liver, kidney, brain, lung, mammary gland, and adipose tissue.
  • Mainly produces palmitic acid (16 C), but can also produce short-chain fatty acids, It's the rate determining enyzme is acetyl CoA

Sources of Acetyl CoA

• Acetyl CoA, the main building block for fatty acids, comes from oxidative decarboxylation of pyruvic acid within mitochondria.
• Since Acetyl CoA cannot diffuse through the mitochondrial membrane to cytoplasm, it must be transported via the citrate transport system
• The fatty acid synthase system is a multienzyme complex
  • It is a dimmer (Formed of 2 subunits).
  • Each subunit (Monomer) contains 7 separate enzymes & an Acyl Carrier Protein (ACP).
  • Only the dimmer is active.
  • Has two active sites: One (-SH) group carries Malonyl CoA and the other (-SH) carries the Acetyl CoA.

Steps of extramitochondrial pathway:

• Synthesis of Malonyl CoA:
  • It is synthesized from Acetyl CoA by the action of Acetyl CoA Carboxylase in the presence of CO2 + Biotin + ATP.
• Synthesis of Palmitate:
  • Acetyl CoA (2C) is elongated by (2C) to become Butyryl CoA (4C).
  • Another molecule of Malonyl CoA binds to free SH group.
  • The process is repeated and every time (2 C) are added by a new molecule of malonyl CoA till Palmityl enzyme complex (16 C) is formed.
  • Energy required for synthesis of Palmitic acid is 7 ATP.
• Free palmitic acid made must be activated to Palmitoyl CoA by Thiokinase or acyl CoA Synthase enzyme.

Fate of Palmitic Acid

• The free palmitic acid must be activated to Palmitoyl CoA by Thiokinase or acyl CoA Synthase enzyme before it can proceed in any other pathway.

Esterification

• Palmitate + Glycerol yields→ Acylglycerol.
• Palmitate + Cholesterol yields→ Cholesterol ester.

Chain Elongation

• Form fatty acids with more than 16 carbon atoms.

Desaturation

• Elongation of palmitate to stearic acid before desaturation at (C9-C10) to produce oleic acid.

Sphingosine Formation

• Formed from Palmitoyl CoA + Serine amino acid.

Regulation of Fatty Acid Synthesis

• The rate-limiting reaction is Malonyl CoA formation, catalyzed by Acetyl CoA Carboxylase.
• Factors regulating fatty acid synthesis include:
  • Long-chain Acyl CoA inhibits fatty acid synthesis through Acetyl CoA Carboxylase inactivation.
  • Insulin stimulates fatty acid synthesis, glucose transport, glycolysis, pyruvic acid production, and Acetyl CoA synthesis.
• Stimulates glucose transport into the cell as in adipose tissue → stimulate Glycolysis → ↑ production of Pyrovic acid → ↑ Acetyl CoA → stimulate fatty acid synthesis.
• Activates inactive (Phosphorylated) Pyrovate Dehydrogenase & Acetyl CoA Carboxylase to active form (Dephosphorylated).

Sources of NADPH+H*

• HMP shunt is the main source of NADPH+H*.
  • HMP shunt occurs in tissues where Lipogenesis is active as adipose tissue & mammary gland and occurs in cytosol.
• Action of malic enzyme on malate to produce pyruvate which occurs in cytosol

Microsomal System

• A system utilized in the biosynthesis of fatty acids and it is the main site for fatty acid elongation 10-16 C atoms to 22 & 24 C atoms
• Fasting inhibits chain elongation
• 2 C atoms are added using Malonyl CoA as acetyl donor and NADPH+H+ as a coenzyme.
• This system is utilized during myelination to provide C22 - C24 fatty acids that are present in sphingolipids

Mitochondrial System

• Occurs in mitochondria under anaerobic conditions.
• Elongation of fatty acids differs from the Microsomal system where the source of 2 C atoms is Acetyl CoA instead of Malonyl CoA and as a by product, tissues can get rid of NADPH+H⁺ in anaerobic conditions producing oxidized NADP.

Biosynthesis of Triacylglycerols

• Consists of combining glycerol with 3 fatty acids.
• The first step involves activation of glycerol to glycerol 3-phosphate, catalyzed by glycerokinase, which is present in liver, kidney, lactating mammary glands, and intestine.
• The activation of fatty acids, creating CoA
• The final step synthesizes triacylglycerols utilizing enzymes mainly localized in microsomes and mitochondria.

Regulation of Lipogenesis

• During the fed state: insulin secretion stimulates phosphodiesterase that lead to increase glycolysis and glycerol 3-p, consequently increase lipogenesis
• During The fasted state: anti-insulin hormones secreted inhibits lipogenesis and stimulates the action of adenyl cyclase increasing cAMP stimulate lipolysis.