Metabolism of lipids: digestion, Metabolism of lipids: digestion, absorption, resynthesis in the absorption, resynthesis in the

intestinal wall.intestinal wall.

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 energyTGs 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 A2

(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 A2 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.

CH2

CH

CH2

OH

OH

O C

O

R2 R1 CO SCoA

CH2

CH

CH2

O

OH

O C

O

R2

C

O

R1

HSCoA

CH2

CH

CH2

O

OH

O C

O

R2

C

O

R1

HSCoAR3 CO SCoA

CH2

CH

CH2

O

O

O C

O

R2

C

O

R1

C

O

R3

+ +

+ +

1.

2.

1-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 estersHydrophilic 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

exocytosisLymphatic

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 BApo E

triacylglycerol

cholesteryl esters

phospholipidscholesterol

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

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

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

•TGs are delivered to adipose tissue in the form of chylomicrones and VLDL, hydrolyzed by lipoprotein lipase into fatty acids and glycerol, which are taken up by adipocytes.

•Then fatty acids are reesterified to TGs.

•TGs are stored in adipocytes.

•To supply energy demands fatty acids and glycerol are released – mobilisation of TGs.

Storage and Mobilization of Fatty Acids (FA)

adipocyte

TG hydro-lysis is inhibited by insulin in fed state

At low carbohydrate and insulin concentrations (during fasting), TG hydrolysis is stimulated by epinephrine, norepinephrine, glucagon, and adrenocorticotropic hormone.

•Lipolysis - hydrolysis of triacylglycerols by lipases. •A hormone-sensitive lipase converts TGs to free fatty acids and monoacylglycerol•Monoacylglycerol is hydrolyzed to fatty acid and glycerol or by a hormone-sensitive lipase or by more specific and more active monoacylglycerol lipase