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 Condensation of 3 acetate Acetate (C2) èmevalonate (C6) isopentenyl pyrophosphate (C5) è squalene (C30 ) è Cholesterol (C27) 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! FIGURE 21-34 Formation of mevalonate from acetyl-CoA. The origin of C-1 and C-2 of mevalonate from acetyl-CoA is shown in pink. Committed and rate-limiting step Enzyme is an integral protein in the ER

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

Formation of squalene in the synthesis of cholesterol Isopentenyl-pyrophosphate (C5) Geranyl pyrophosphate (C10) Farnesyl-pyrophosphate (C15) squalene (C30) + Farnesyl PP squalene

Formation of cholesterol from squalene Contains rings multistep

Formation of cholesterol esters 3 FIGURE 21-38 Synthesis of cholesteryl esters. Esterification converts cholesterol to an even more hydrophobic form for storage and transport. 3 CH3 - (CH2)n

Cholesteryl esters In tissues: Acyl-CoA + cholesterol cholesteryl esters + CoASH acyl-CoA – 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 pH -in the endoplasmic reticulum - neutral pH in the pancreatic juice

high level of cholesterol inhibits de novo synthesis of cholesterol Regulation of cholesterol synthesis high level of cholesterol inhibits de novo synthesis of cholesterol FIGURE 21-34 (part 2) Formation of mevalonate from acetyl-CoA. The origin of C-1 and C-2 of mevalonate from acetyl-CoA is shown in pink.

Regulation of cholesterol synthesis The gene of HMG-CoA 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 cholesterol binds to SCAP (SREBP cleavage-activating protein) -cholesterol sensor- SREBP is bound to SCAP – SREBP is inactive low cholesterol SREBP – SCAP complex migrates to Golgi in vesicles - cleavage by two proteases - release of amino-terminal domain into the cytosol -entry into the nucleus- activation of transcription of target genes FIGURE 21-43 SREBP activation. Sterol regulatory element-binding proteins (SREBPs, shown in green) are embedded in the ER when first synthesized, in a complex with the protein SREBP cleavage-activating protein (SCAP, red). (N and C represent the amino and carboxyl termini of the proteins.) When bound to SCAP, SREBPs are inactive. When sterol levels decline, the complex migrates to the Golgi complex, and SREBP is cleaved by two different proteases in succession. The liberated amino-terminal domain of SREBP migrates to the nucleus, where it activates transcription of sterol-regulated genes.

Covalent modification of the enzyme (phosphorylation-dephosphorylation Regulation of cholesterol synthesis Covalent modification of the enzyme (phosphorylation-dephosphorylation FIGURE 21-44 Regulation of cholesterol formation balances synthesis with dietary uptake. Glucagon promotes phosphorylation (inactivation) of HMG-CoA reductase; insulin promotes dephosphorylation (activation). X represents unidentified metabolites of cholesterol that stimulate proteolysis of HMG-CoA reductase. 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 Pancreatic lipase - major enzyme for TG hydrolysis - products are fatty acids and 2-monoacyl-glycerol - colipase (12kDa) binds to lipase and prevents inhibition of lipase by bile acids - acts on bile acid micelles 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 enzymes for digestion of lipids Lipase -Triacylgycerol Products: fatty acids (R1-COOH R3-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 Primary bile acids Cholesterol -Side chain oxidation Tauro ~ Glyco ~ -Side chain oxidation -double bound reduced - +OH Tauro ~ Glyco ~ Cholic acid Primary bile acids Chenodeoxycholic acid taurine glycine Glycocholic acid Taurocholic acid Conjugation to glycine or taurine Glycochenodeoxycholic acid Taurochenodeoxycholic acid

From liver to gall bladder upon hormonal stimulus Glycocholic acid Taurocholic acid Glycochenodeoxycholic acid Taurochenodeoxycholic acid Na, K salts From liver to gall bladder upon hormonal stimulus Small intestine - detergent *Deconjugation *OH - 7’ Primary bile salts Secondary bile acids Deoxycholic acid 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 FFA / ALBUMIN Liver Extrahepatic tissues OXYDATION (ENERGY) SYNTHESIS OF TISSUE LIPIDS 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/mL 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 Nonpolar groups Interact with lipids in the central core Polar groups Interact with water and ions of the plasma Amphipathic molecules LDL receptor: recognize B-100 apoprotein (or apo E)

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

TABLE 21-2 Apolipoproteins of the Human Plasma Lipoproteins

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

VLDL VLDL released from the liver (Apo B100) - 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 B100 - endocytosis by the liver rest in the circulation apo E Converted to LDL

HDL2 HDL3 Extrahepatic tissues Cholesterol Cholesterol esters TG VLDL Cholesterol Nascent HDL Cholesterol esters TG LCAT LCAT PL ApoA ApoC Lipids Lipids HDL2 HDL3 Apoproteins Apoproteins TG/Cholesterol á Uptake by the liver E, C VLDL Chylomicron

Lecithin-cholesterol acyl-transferase (LCAT) 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

FIGURE 21-40a Lipoproteins and lipid transport FIGURE 21-40a Lipoproteins and lipid transport. (a) Lipids are transported in the bloodstream as lipoproteins, which exist as several variants that have different functions, different protein and lipid compositions (see Tables 21-1, 21-2), and thus different densities. Dietary lipids are packaged into chylomicrons; much of their triacylglycerol content is released by lipoprotein lipase to adipose and muscle tissues during transport through capillaries. Chylomicron remnants (containing largely protein and cholesterol) are taken up by the liver. Endogenous lipids and cholesterol from the liver are delivered to adipose and muscle tissue by VLDL. Extraction of lipid from VLDL (along with loss of some apolipoproteins) gradually converts some of it to LDL, which delivers cholesterol to extrahepatic tissues or returns to the liver. The liver takes up LDL, VLDL remnants (called intermediate density lipoprotein, or IDL), and chylomicron remnants by receptormediated endocytosis. Excess cholesterol in extrahepatic tissues is transported back to the liver as HDL. In the liver, some cholesterol is converted to bile salts.

LDL receptor Recognizes Apo B-100 Apo E - the clearance of LDL from the plasma (also that of IDL - recognition factor is ApoE) (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 HMG-CoA 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 more LDL is formed * Atherosclerosis at an early age 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 mRNA for LDL receptor rises & more receptor is synthesized * Therapy for homozygotes: liver transplantation