1. Lipid metabolism 3-4 internet

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

3. 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

4. Formation of mevalonate in the synthesis of cholesterol
Enzyme is cytosolic distinct from the mitochondrial isoenzyme!
FIGURE 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

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

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

7. Formation of cholesterol from squalene
Contains rings
multistep

8. Formation of cholesterol esters3
FIGURE Synthesis of cholesteryl esters. Esterification converts cholesterol to an even more hydrophobic form for storage and transport. 3
CH3 - (CH2)n

9. 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

10. 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

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

12. 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 (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.

13. 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 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.

14. Covalent modification of the enzyme (phosphorylation-dephosphorylation
Regulation of cholesterol synthesis
Covalent modification of the enzyme (phosphorylation-dephosphorylation
FIGURE 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

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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. The enterohepatic circulation of bile salts

23. Transporters for the enterohepatic circulation of bile salts

24. 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)

25. 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

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

27. 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

28. 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
meq/mL
6-16 mg/100 ml

29. 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

30. 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)

31. 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

32. TABLE 21-2 Apolipoproteins of the Human Plasma Lipoproteins

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

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. TRANSPORT OF CHOLESTEROL BY HDLTG TG TG

41. 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.

42. 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

43. 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

45. LDL receptor LDL receptors
Liver
Ovary
Adrenal cortex
Cholesterol metabolism is controlled by cholesterol released from LDL

46. Regulation of cholesterol level in humans
Plasma cholesterol – in lipoproteins
Normal cholesterol concentration:
3.1 – 5.7 mmol/L ( mg/100 ml)
(65 % is esterified)
After an overnight fasting no CHYLOMICRONS
70 % of cholesterol in LDL

47. 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

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

49. 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: g)
Bile acid – binding resin
Decreases bile acid reabsorption
→increases the loss of cholesterol

50. 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

51. 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

52. 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

53. 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