Biochemistry in Tables, Schemes, and Graphs - S.D. Zhamsaranova 2009

Lipid Metabolism and Functions
Stages of Lipid Breakdown in the Gastrointestinal Tract

Lipid Digestion takes place in the duodenum, which receives lipase via Pancreatic juice and conjugated Bile acids as part of bile.

1. Fat emulsification is a prerequisite for digestion, as it makes the hydrophobic substrate more accessible to the action of hydrolytic Enzymes—lipases. Emulsification occurs with the participation of bile acids, which, due to their amphiphilic nature, surround fat droplets and reduce surface tension, leading to droplet fragmentation.

2. Fat Hydrolysis is carried out by pancreatic lipase, which adsorbs onto The surface of fat droplets and cleaves ester bonds in triacylglycerols (TAG). Fatty acids are cleaved primarily from the a-position. As a result, diacylglycerol is formed, followed by ß-monoacylglycerol, which is the main product of hydrolysis:

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The following conditions are necessary for the active function of pancreatic lipase:

- pH close to neutral environment;

- bile acids that emulsify fats;

- colipase protein, synthesized in the Pancreas and secreted along with pancreatic lipase.

3. Formation of mixed micelles and absorption of hydrolysis products. Lipid hydrolysis products—fatty acids with a long hydrocarbon radical, ß-monoacylglycerols, Cholesterol, as well as bile acid salts—form structures in the intestinal lumen called mixed micelles. Mixed micelles are structured such that the hydrophobic PARTS OF THE molecules face inward, while the hydrophilic parts face outward; therefore, micelles dissolve well in the aqueous phase of the Small Intestine contents. Micelle stability is ensured mainly by bile acid salts. Micelles approach the brush border of small intestine mucosal Cells, and the Lipid Components of the micelles diffuse through the membranes into the cells. Fat-soluble Vitamins A, D, E, K and bile acid salts are absorbed along with lipid hydrolysis products.

BILE ACIDS

bile acids perform 2 main Functions:

- fat emulsification

- fatty acid absorption.

Bile acids enter the intestine as part of bile in the form of conjugates with Glycine and taurine. After DIGESTION AND ABSORPTION, bile acids return to the Liver via the portal vein, completing this cycle up to 10 times a day. This cycle is called the enterohepatic Circulation of bile acids.

Cholesterol is a constant component of bile. Like bile acids, it undergoes reabsorption, but some amount of bile acids and cholesterol is lost with feces. To replenish the loss of bile acids excreted with feces, the synthesis of bile acids from cholesterol occurs continuously.

4. Resynthesis of fats in the mucosa of the small intestine.

The First stage of fat resynthesis is the fatty acid activation reaction: HSKoA + RCOOH + АТФ → R-СO∼КоА + АМФ + Н₄Р₂О₇.

The reaction is catalyzed by the enzyme acyl~CoA synthetase. The resynthesis of fats involves not only fatty acids absorbed from the intestine, but also fatty acids synthesized within the body; therefore, resynthesized fats differ in composition from dietary fats.

In the epithelial Cells of the small intestine, lipoprotein complexes known as chylomicrons (CM) are formed from fats produced through resynthesis, as well as from dietary cholesterol esters and fat-soluble vitamins. CMs then deliver fats to peripheral Tissues.

LIPID TRANSPORT

Lipids, particularly fats, are insoluble in the aqueous Phases of the body; therefore, lipid transport through the Blood AND Lymph is carried out in the form of complexes with Proteins and Phospholipids called Lipoproteins.

Lipoproteins are spherical particles comprising a Hydrophobic core, which consists of triacylglycerols (TAG) and cholesterol esters (CE), and an amphiphilic shell containing phospholipids, Glycolipids, and proteins (apoproteins).