METABOLISM OF LIPIDS: DIGESTION OF LIPIDS. TRANSPORT FORMS OF LIPIDS

PHYSIOLOGICAL ROLE OF LIPIDS  Energetic role (fuel molecules)  Components of membranes (structural role)  Precursors for many hormones (steroids)  Signal molecules (prostaglandins)  Protective role (lipids surround important organs)  Enzyme cofactors (vitamin K)  Electron carriers (ubiquinone)  Insulation against temperature extremes

TRIACYLGLYCEROLS ARE HIGHLY CONCENTRATED ENERGY STORES Triacylglycerols (TGs) and glycogen - two major forms of stored energy TGs which are more efficient energy stores because: (1) They are stored in an anhydrous form (2) Their fatty acids are more reduced than monosaccharides.

Fat breakdown about 50 % of energy in liver, kidney and skeletal muscles up to 95 % of energy cardiac muscle Fats are the major source of energy for: fasting animal organism in diabetes 1 g of triacylglycerols stores more than six times as much energy as a 1 g of glycogen Glycogen reserves are depleted in 12 to 24 hours after eating, triacylglycerols within several weeks.

Fatty acids and glycerol - substances that are directly used as a fuel by mammalian organisms. Fatty acids (FA) and glycerol for metabolic fuels are obtained from triacylglycerols: (1) In the diet (2) Stored in adipocytes (fat storage cells) Free fatty acids occur only in trace amounts in cells For supplying of fatty acids as a fuel for organism, the triacylglycerols have to be digested

DIGESTION OF DIETARY LIPIDS Lipids in diet:  triacylglycerols  phospholipids  cholesterol Digestion – in small intestine. Enzyme – pancreatic lipase. Lipase catalyzes hydrolysis at the C1 and C3 positions of TGs producing free fatty acids and 2-monoacylglycerol.

Colipase – protein which is present in the intestine and helps bind the water-soluble lipase to the lipid substrates. Colipase also activates lipase. Bile salts (salts of bile acids) are required for lipids digestion. Bile salts are synthesized in the liver from cholesterol. Taurocholate and glycocholate - the most abundant bile salts. Amphipathic: hydrophilic (blue) and hydrophobic (black)

TGs are water insoluble and lipase is water soluble. Digestion of TGs takes place at lipid-water interfaces. Rate of digestion depends on the surface area of the interface. Bile salts are amphipathic, they act as detergent emulsifying the lipid drops and increasing the surface area of the interface.

Bile salts also activates the lipase. Inadequate production of bile salts results in steatorrhea.

Dietary phospholipids are degraded by phospholipases Lysophospho- glycerides are absorbed and in the intestinal cells are reesterified back to glycero- phospholipids. Phospholipases are synthesized in the pancreas. Major phospholipase is phospholipase A 2 (catalyses the hydrolysis of ester bond at C2 of glycerophospholipids and lysophosphoglycerides are formed).

Lysophosphoglycerides can act as detergent and therefore in high concentration can disrupt cellular membranes. Lysophosphoglyceride is normally present in cells in low concentration. Snake venom contain phospholipase A 2 and causes the lysis of erythrocytes membranes.

Dietary cholesterol Most dietary cholesterol is unesterified Cholesteryl esters are hydrolyzed in the intestine by an intestinal esterase Free cholesterol is solublized by bile-salt micelles for absorption After absorption in the intestinal cells cholesterol react with acyl-CoA to form cholesteryl ester.

ABSORPTION OF DIETARY LIPIDS 2-monoacylglycerols, fatty acids, lysophosphoglycerides, free cholesterol form micelles with bile salts. Lipid absorption – passive diffusion process.

Micelles migrate to the microvilli and lipids diffuse into the cells. Bile acids are actively absorbed and transferred to the liver via portal vein. Bile salts can circulate through intestine and liver several time per day.

In the intestinal cells the fatty acids are converted to fatty acyl CoA molecules. Three of these molecules can combine with glycerol, or two with monoacylglycerol, to form a triacylglycerols st reaction is catalyzed by monoacylglycerol acyltransferase 2-nd reaction is catalyzed by diacylglycerol acyltransferase

These lipids assemble with phospholipids and apoproteins (apolipoproteins) to form spherical particles called lipoprotein Structure: Hydrophobic core: -TGs, -cholesteryl esters Hydrophilic surfaces: -cholesterol, -phospholipids, -apolipoproteins TRANSPORT FORMS OF LIPIDS TGs, cholesterol and cholesterol esters are insoluble in water and cannot be transported in blood or lymph as free molecules

The main classes of lipoproteins 1.Chylomicrons. 2.Very low density lipoproteins (VLDL). 3.Intermediate density lipoproteins (IDL). 4.Low density lipoproteins (LDL). 5.High density lipoproteins (HDL).

are the largest lipoproteins (180 to 500 nm in diameter) are synthesized in the ER of intestinal cells contain 85 % of TGs (it is the main transport form of dietary TGs). apoprotein B-48 (apo B-48) is the main protein component deliver TGs from the intestine (via lymph and blood) to tissues (muscle for energy, adipose for storage). bind to membrane-bound lipoprotein lipase (at adipose tissue and muscle), where the triacylglycerols are again degraded into free fatty acids and monoacylglycerol for transport into the tissue are present in blood only after feeding Chylomicrons exocytosis Lymphatic vessel

VLDL are formed in the liver contain 50 % of TGs and 22 % of cholesterol two lipoproteins — apo B-100 and apo E the main transport form of TGs synthesized in the organism (liver) deliver the TGs from liver to peripheral tissue (muscle for energy, adipose for storage) bind to membrane-bound lipoprotein lipases (triacylglycerols are again degraded into free fatty acids and monoacylglycerol) Apo B Apo E triacylglycerol cholesteryl esters phospholipids cholesterol

Lipoproteinlipase – enzyme which is located within capillaries of muscles and adipose tissue Function: hydrolyses of TGs of chylomicrons and VLDL. Formed free fatty acids and glycerol pass into the cells Chylomicrons and VLDL which gave up TGs are called remnants of chylomicrons and remnants of VLDL Remnants are rich in cholesterol esters Remnants of chylomicrons are captured by liver Remnants of VLDL are also called intermediate density lipoproteins (IDL) Fate of the IDL: - some are taken by the liver - others are degraded to the low density lipoproteins (LDL) (by the removal of more triacylglycerol)

LDL LDL are formed in the blood from IDL and in liver from IDL (enzyme – liver lipase) LDL are enriched in cholesterol and cholesteryl esters (contain about 50 % of cholesterol) Protein component - apo B-100 LDL is the major carrier of cholesterol (transport cholesterol to peripheral tissue)

Cells of all organs have LDL receptors Receptors for LDL are localized in specialized regions called coated pits, which contain a specialized protein called clathrin Apo B-100 on the surface of an LDL binds to the receptor Receptor-LDL complex enters the cell by endocytosis. Endocytic vesicle is formed

Vesicle fuse with lysosomes Lysosomal lipases and proteases degrade LDL LDL receptor itself returns to the plasma membrane Apo B-100 is hydrolyzed to amino acids Cholesteryl esters are hydrolyzed to free cholesterol and fatty acids Released free cholesterol: - is incorporated into the membranes or - is reesterified for storage inside the cell by the enzyme acyl CoA:cholesterol acyltransferase (ACAT) Feedback regulation: abundance of intracellular cholesterol suppresses the synthesis of LDL receptors and so the uptake of additional cholesterol from plasma LDL is blocked

LDL uptake by receptor-mediated endocytosis

 congenital disease when LDL receptor are not synthesized (mutation at a single autosomal locus)  the concentration of cholesterol in blood markedly increases  severe atherosclerosis is developed (deposition of cholesterol in arteries)  nodules of cholesterol called xanthomas are prominent in skin and tendons  most homozygotes die of coronary artery disease in childhood  the disease in heterozygotes (1 in 500 people) has a milder and more variable clinical course Familial hypercholesterolemia atherosclerosis xanthomas

HDL  are formed in the liver and partially in small intestine  contain the great amount of proteins (about 40 %)  pick up the cholesterol from peripheral tissue, chylomicrons and VLDL  enzyme acyltransferase in HDL esterifies cholesterols, convert it to cholesterol esters and transport to the liver

High serum levels of cholesterol cause disease and death by contributing to development of atherosclerosis Cholesterol which is present in the form of the LDL is so-called "bad cholesterol." Cholesterol in the form of HDL is referred to as "good cholesterol” HDL functions as a shuttle that moves cholesterol throughout the body

The ratio of cholesterol in the form of LDL to that in the form of HDL can be used to evaluate susceptibility to the development of atherosclerosis LDL/HDL Ratio For a healthy person, the LDL/HDL ratio is 3.5

Transport Forms of Lipids

Composition of the lipoproteins in plasma of humans