Lipid metabolism 3 4 internet Cholesterol Essential molecule

Lipid metabolism 3 -4 internet

Cholesterol Essential molecule *Modulates membrane fluidity *Precursor of -steroid hormones - bile acids All cells can synthesize it

Synthesis of cholesterol Acetate (C 2) è mevalonate (C 6) Condensation of 3 acetate è isopentenyl pyrophosphate (C 5) èsqualene (C 30 ) èCholesterol (C 27) Polymerization of 6 isoprene units Cyclization plus oxidation and methyl removal

Formation of mevalonate in the synthesis of cholesterol Enzyme is cytosolic distinct from the mitochondrial isoenzyme! Enzyme is an integral protein in the ER Committed and rate-limiting step

Formation of activated isoprenes in the synthesis of cholesterol DIMETHYL-ALLYL PYROPHOSPHATE

Formation of squalene in the synthesis of cholesterol Isopentenylpyrophosphate (C 5) Geranyl pyrophosphate (C 10) Farnesyl-pyrophosphate (C 15) squalene (C 30) + Farnesyl PP squalene

Formation of cholesterol from squalene Contains rings multistep

Formation of cholesterol esters 3 CH 3 - (CH 2)n 3

Cholesteryl esters In tissues: Acyl-Co. A + cholesterol cholesteryl esters + Co. ASH acyl-Co. A – cholesterol acyltransferase (ACAT) (localized in the endoplasmic reticulum) Liver Intestine Adrenal cortex Arterial wall – accumulate in atherosclerosis

Cholesteryl esters In the plasma: Phosphatidylcholine + cholesterol cholesteryl ester + lysophosphatidyl choline Lecithin-cholesterol acyltransferase (LCAT) LCAT is produced in the liver - secreted into the plasma in a lipoprotein – acts on the surface of HDL – cholesterol esters are trapped within the lipoprotein

Hydrolysis of cholesteryl esters cholesteryl esterase Present: in many tissues -in lysosomes –acid p. H -in the endoplasmic reticulum - neutral p. H in the pancreatic juice

Regulation of cholesterol synthesis high level of cholesterol inhibits de novo synthesis of cholesterol

Regulation of cholesterol synthesis The gene of HMG-Co. A reductase and genes of other enzymes in the uptake and synthesis of cholesterol and unsaturated fatty acids are controlled by sterol regulatory element-binding protein (SREBPs) -gene regulation only by the amino-terminal domain of SREBP high cholesterol low cholesterol binds to SCAP SREBP – SCAP complex (SREBP cleavage-activating protein) migrates to Golgi in vesicles -cholesterol sensor- cleavage by two proteases SREBP is bound to SCAP – SREBP is inactive release of amino-terminal domain into the cytosol -entry into the nucleusactivation of transcription of target genes

Regulation of cholesterol synthesis Covalent modification of the enzyme (phosphorylationdephosphorylation unknown mediator

Digestion and absorption of lipids Adult human digests: ~60 -150 g lipid/day 90 % of the ingested lipids: triacylglycerols Problem: poor water solubility limited accessibility to digestive enzymes

Digestion and absorption of TG involve: -enzymatic hydrolysis -emulsification -uptake of the products by epithelial cells -TG resynthesis and packaging into chylomicrons -chylomicron release into the lymphatic system

Hydrolysis of triacylglicerol Lingual and gastric lipase - acts in the stomach - 30 % of total TG hydrolysis - slow – surface is limited - some dispersion of lipids -aided by stomach peristaltics Pancreatic lipase - major enzyme for TG hydrolysis - products are fatty acids and 2 -monoacyl-glycerol - colipase (12 k. Da) binds to lipase and prevents inhibition of lipase by bile acids - acts on bile acid micelles

Pancreatic enzymes for digestion of lipids Lipase -Triacylgycerol Products: fatty acids (R 1 -COOH R 3 -COOH) 2 -monoacyl-glycerol Lipid esterase – requires bile acids -Cholesterol esters, monoacylglycerol Phospholipases – requires bile acids

Bile salts • Biological detergents • Polar derivatives of cholesterol • Synthesized in the liver, secreted into the bile with phospholipids and cholesterol and released into the small intestine • Form micelles accommodating phospholipids and fatty acids – mixed micelles water-insoluble cholesterol is accommodated (“solubilized”) excess cholesterol – stone formation in the gallbladder Effects of bile acids: -Detergents- solubilize dietary lipids

Bile acids Cholesterol Tauro ~ Glyco ~ Cholic acid Tauro ~ Glyco ~ -Side chain oxidation -double bound reduced - +OH Primary bile acids Chenodeoxycholic acid taurine glycine Glycocholic acid Taurocholic acid Conjugation to glycine or taurine Glycochenodeoxycholic acid Taurochenodeoxycholic acid

Glycocholic acid Taurocholic acid Glycochenodeoxycholic acid Taurochenodeoxycholic acid Na, K salts Primary bile salts From liver to gall bladder upon hormonal stimulus Small intestine - detergent *Deconjugation *OH - 7’ Deoxycholic acid Secondary bile acids Litocholic acid Secondary bile salts Na, K salts

The enterohepatic circulation of bile salts

Transporters for the enterohepatic circulation of bile salts

Absorption fatty acids – diffusion transporter (for long chain fatty acids) cholesterol – channel ABC transporters to pump cholesterol back to the lumen Medium and short chain fatty acids enter capillaries, long chain fatty acids are used to TG synthesis and chylomicron formation in intestinal epithelial cells Chylomicrons enter lymph vessels – travel to the systemic venous system – TG (long chain fatty acids) reach adipose tissue and muscle before coming into contact with the liver – FA metabolized and stored (liver is protected from lipid overload)

Transport of lipids Problem: hydrophobic lipids in an aqueous environment Solution: More insoluble lipids (triacylglycerols & cholesterol esters) associated with more polar ones (phospholipids, cholesterol) and proteins LIPOPROTEIN COMPLEX

Transport of lipids • FFA – free fatty acids – Unesterified long-chain fatty acids (less than 5 % of the total lipids) TRANSPORT - NOT BY LIPOPROTEINS

Metabolism of plasma FFA Adipocytes Triglicerides HORMONE SENSITIVE LIPASE FFA Extrahepatic tissues OXYDATION (ENERGY) SYNTHESIS OF TISSUE LIPIDS FFA / ALBUMIN Liver SYNTHESIS OF TISSUE LIPIDS KETONE BODIES

FFA – FREE FATTY ACIDS Source: *Lipolysis of TG in adipose tissue *Lipoprotein lipase during uptake of TG from plasma into tissues Transported in albumin binding 0. 1 - 2 meq/m. L 6 -16 mg/100 ml

FFA – FREE FATTY ACIDS Level: Low – fully fed condition High –fully fasting state vigorous excersise uncontrolled diabetes Between meal Falls - after eating Rises - prior to the next meal

Lipoproteins Generally: in the central core: nonpolar lipids (triglicerides and cholesterol ester) surrounded by phospholipids, unesterified cholesterol and apoproteins Amphipathic molecules Nonpolar groups Polar groups Interact with lipids in Interact with water and ions of the central core the plasma LDL receptor: recognize B-100 apoprotein (or apo E)

Fraction Source Chylomicrons Intestine VLDL (Apo B-100, C, E ) Liver Intestine Density Protein % Lipid Predominant lipid 1 -2 98 -99 TG 7 -10 90 -93 TG IDL VLDL (Apo. B-100, E) ~11 89 TG, Cholesteryl esters LDL (B-100) VLDL ~20 79 Pl, Cholesteryl esters HDL (Apo A; C; E) Liver ~60 ~ 40 PL, cholesterol FFA Adipose tissue ~99 1

Transport of triglycerides from the intestine to the extrahepatic tissues and the liver. CHYLOMICRONS Adipose, Muscle, Heart

Chylomicron & VLDL Chilomicron clearance from the blood is rapid Half time < 1 hour Adipose tissue Heart Muscle LIPOPROTEIN LIPASE Bound to heparan sulfate on the wall of capillaries

LIPOPROTEIN LIPASE -Phospholipids & apo C-II are cofactors Chylomicrons & VLDL provide both the substrate (TG) and cofactors (PL, apo C-II) – in heart • Km for TG low – in adipose tissue (activated by insulin) • Km is 10 times greater (in starvation TG heart enzyme remains saturated; the same during lactation in mammary gland

Transport of triglycerides from the liver to the extrahepatic tissues. VLDL Transport of cholesterol. LDL, HDL

VLDL • VLDL released from the liver (Apo B 100) precursor of LDL too – in tissues lipoprotein lipase • IDL (relatively rich in cholesterol) • IDL disappears from the blood within 2 -6 hours – In the liver - it binds to LDL receptors -affinity for apo E is higher than for B 100 - endocytosis by the liver – rest in the circulation apo E Converted to LDL

Extrahepatic tissues VLDL Nascent HDL Cholesterol esters TG LCAT PL LCAT Apo. A Apo. C Lipids HDL 2 HDL 3 Apoproteins E, C VLDL Chylomicron Apoproteins TG/Cholesterol á Uptake by the liver

HDL TRANSPORTS OF CHOLESTEROL FROM TISSUES TO THE LIVER Nascent HDL – secreted by the liver (and the intestine) PL bilayer + apo C, apo A Lecithin-cholesterol acyl-transferase (LCAT) - Cholesterol esters are formed move into the hydrophobic interior of PL bilayer pushes apart until spherical pseudomicellar HDL is formed Esterified cholesterols transferred from HDL to chylomicron, VLDL, IDL by cholesterol ester transfer protein

TRANSPORT OF CHOLESTEROL BY HDL TG TG TG

LDL receptor Recognizes Apo B-100 Apo E - the clearance of LDL from the plasma (also that of IDL - recognition factor is Apo. E) (VLDL - no binding to B-100 or E receptor, as apo C-III inhibits binding) On binding to the receptor endocytotic vesicles are formed

LDL receptor The expression of the LDL receptor is regulated by the need of the cell for cholesterol Down-regulated – when sufficient cholesterol is available Up-regulated – when the cell requires additional cholesterol

LDL receptor • LDL receptors – Liver – Ovary – Adrenal cortex • Cholesterol metabolism is controlled by cholesterol released from LDL

Regulation of cholesterol level in humans • Plasma cholesterol – in lipoproteins Normal cholesterol concentration: 3. 1 – 5. 7 mmol/L (120 -220 mg/100 ml) (65 % is esterified) After an overnight fasting no CHYLOMICRONS 70 % of cholesterol in LDL

Regulation of cholesterol level in humans Cholesterol free diet → 10 to 25 % decrease in plasma cholesterol concentration (larger decrease only through inhibition of cholesterol biosynthesis) Dietary cholesterol restriction is recommended for everyone especially for patients with hypercholesterolemia > 5. 2 mmol/L greater tendency to atherosclerosis Saturated fatty acids → plasma cholesterol concentration increases

Regulation of cholesterol level in humans STATINS – inhibitors of HMGCo. A reductase * Reduce the plasma cholesterol concentration by 30 % - 50 % * Minimum toxicity * Cardioprotective effect

Regulation of cholesterol level in humans • The main route of cholesterol metabolism is the conversion to bile acids – 0. 8 mmol/day of bile acids are lost (constant level in the body: 15 -30 g) Bile acid – binding resin – Decreases bile acid reabsorption →increases the loss of cholesterol

Regulation of cholesterol level in humans – Dietary restriction of cholesterol – Bile acid – binding resin – Cholesterol synthesis inhibitor Cholesterol for bile acid synthesis is provided by LDL →Plasma cholesterol concentration decreases

Familial hypercholesterolemia • The absence or deficiency of functional receptors for LDL • High concentration of LDL-cholesterol in the plasma • Mutation at a single autosomal locus

Familial hypercholesterolemia • Heterozygotes * One gene * Half of the normal receptor number entry of LDL into liver & other cells is impaired increased plasma LDL level (~ 300 mg/dl) entry of IDL is also impaired formed * Atherosclerosis at an early age more LDL is • Homozygotes – lack of LDL receptors * One mutant gene from both parents * ~ 700 mg/dl LDL in the plasma * Coronary artery disease in childhood

Familial hypercholesterolemia * Therapy for heterozygotes: – stimulation of the single normal gene to produce more LDL receptors – when cholesterol is required in cells the amount of m. RNA for LDL receptor rises & more receptor is synthesized * Therapy for homozygotes: liver transplantation