LIPID DIGESTION AND ABSORPTION Dietary lipid components Triacylglycerols

LIPID DIGESTION AND ABSORPTION

Dietary lipid components Triacylglycerols (more than 90%) n Phospholipids n Cholesterol n Cholesteryl esters n Free fatty acids n

Structure of phospholipids

Digestion of phospholipids

Triacylglycerol digestion: stomach n n lingual lipase gastric lipase – hydrolysis of ester bonds in position 3 of TAGs formed by short-chain and medium-chain lenght fatty acids – products: 1, 2 -diacylglycerols and fatty acids – optimum p. H: 4 – 6 – important in neonates and in individuals with pancreatic insufficiency

Emulsification n n Duodenum Increases the surface area of the hydrophobic lipid droplets so that the digestive enzymes can act effectively Mechanisms: detergent properties of bile salts and mechanical mixing due to peristalsis Emulsifying agents stabilize lipid droplets as they become smaller and prevent them from coalescing

Emulsification

Triacylglycerol digestion: small intestine Colipase anchors lipase at the lipidaqueous interface and causes a conformational change in the lipase that exposes its active site n Pancreatic lipase – hydrolysis of ester bonds in position 1 and 3 of TAGs – products: 2 -monoacylglycerols and fatty acids n

Metabolism of 2 -monacylglycerols: small intestine 1. uptake by enterocytes and further metabolism within the cell (monoacylglycerol pathway) 2. conversion to 1 -monoacylglycerols by isomerase and their uptake by enterocytes 3. conversion to 1 -monoacylglycerols and further hydrolysis by pancreatic lipase to glycerol and fatty acids in the intestinal lumen

Monoacylglycerol pathway: enterocytes conversion of 2 -monoacylglycerols to triacylglycerols n fatty acids in the form of acyl-Co. A are used for esterification of 2 -monoacylglycerols n

Metabolism of 1 -monoacylglycerols: enterocyte n n hydrolysis to glycerol and fatty acid by intestinal lipase (intracellular enzyme) phosphorylation of glycerol by glycerol kinase glycerol-3 -phosphate is esterified by acyl. Co. A to triacylglycerols (phosphatidic acid pathway) acyl-Co. A are formed by acyl-Co. A synthetase from fatty acids released by intestinal lipase

GEOMETRIC ISOMERS OF FATTY ACID C 18: 1 CH 3 ELAIDIC ACID - trans H H C C H COOH OLEIC ACID - cis H COOH