1. Low density lipoproteins, metabolism...Apolipoproteins: Role in Lipid Transport and Polymorphisms

2. Essay Introduction ▫What are apolipoproteins? And why are they important? Main text ▫Exogenous lipid transport ▫Endogenous lipid transport ▫Role of apoE in LP clearance ▫ApoE polymorphisms Clinical relevance ▫Hyperlipoproteinaemia  ApoE2 – type 3 hyperlipoprteinemia  ApoE3 – coronary heart disease, Alzheimer’s

3. Introduction What are apolipoproteins? ▫Structural proteins  With a hydrophobic region binding the lipid  And a hydrophilic region allowing overall particle to be soluble, to move in bloodstream Why are they important? ▫Metabolize lipoproteins (LPs)  Allow recognition of LPs by enzymes and receptors  LPs carry insoluble lipids in plasma (TG and CE)

4. dojfwoijef Large chylomicron (TG-rich)  chylomicron remnant (half chol, half TG)  [liver] [liver]  VLDL (TG-rich, little chol ~ chylo)  IDL (chol-rich, little TG)  LDL (chol, no TG)

5. Exogenous Lipid Transport

6. Apo-B48 ▫48% of the length of the full protein  so doesn’t act as a R, just a structural protein (for transport purpose) ▫Ligand-binding bit of apoB not there = so dependent on apoE Apo-CII ▫An ancillary protein  activates LPL Apo-E ▫To allow recognition of LP particle by CSRs  to be taken up by liver

7. dojfwoijef Large chylomicron (TG-rich)  chylomicron remnant (half chol, half TG)  [liver] [liver]  VLDL (TG-rich, little chol ~ chylo)  IDL (chol-rich, little TG)  LDL (chol, no TG)

8. Endogenous Lipid Transport

9. Apo-B100 is a full-length protein, with 2 fxns: ▫1) 38% structural – for VLDL to bind and be transported ▫2) other 62% is partly released, so can be recognized by receptors Apo-CIII ▫Inhibits LPL (remember CII activates it) ▫So, balance between CII and CIII determines how rapidly particle is degraded

10. Endogenous Lipid Transport LPL degrades VLDL (removes TG)  leaving IDL/VLDL remnant, then has two fates: ▫Apo-E mechanism: quicker, more efficient or Apo-B mechanism: slower, lower affinity OR ▫If IDL hangs around in blood long enough, acted on by hepatic lipase to leave  LDL LDL = no TG, so no Apo-E. Apo-E needs some TG present to bind the particle, so LDL relies on Apo-B-mediated-uptake = slow If LDL in blood for long enough, both protein+lipid = oxidized. No more LDLr recognition. Scavenged.

11. Endogenous Lipid Transport

12. Summary: Role of ApoE in LP Clearance The chylomicron remnants can only be taken up by ApoE (…since B48 = only structural!) VLDL and IDL can use ApoE But LDLs don’t have ApoE (…since comes off with TG) Type 3 hyperlipoproteinemia patients have a defect in ApoE ▫So  TGs and cholesterol (not just high LDL!)

13. rfw 112 mutation = E4 – AA 158 mutation = E2 – CC Box: R-binding region/LBD Normally, 1,2,3 sheets, with flexible linkages

14. ApoE2 and ApoE4 Polymorphisms Arg-158 and Asp-154 = salt bridge, with Arg-150 separate Arg-158  Cys-158, then Asp154 binds with Arg-150 instead Charge ratio in LBD changed (not negative anymore) Decreases ability of ApoE2 to bind to LDLr (5% of E3 efficiency) The two ends (C-terminal: lipid and N- terminal: R-binding-domain/LBD don’t normally interact Arg-61 too far to interact Cys-112  Arg-112 moves side chain of Arg-61 and pulls it in = so it interacts with Glu-255 N and C terminals now interacting

15. Clinical Relevance Hyperlipoproteinemia ▫High levels of LPs in blood (with lipid bound to them) ▫Diff classes, diff TG/CE levels ▫Type 3: most common  High CE and TG  Suffer from atherosclerosis from early age NB. Different from hyperlipidaemia = higher levels of LIPID attached to the same number of LP particles

16. ApoE and Associated Diseases ApoE2: type 3 hyperlipoproteinemia ▫  binding apoE2 activity to receptors   chylomicrons, VLDL and remnant particles in plasma ▫Also  hepatic LDLr activity (!)   LDL cholesterol concentration (since LDL only has ApoB)   HDL cholesterol concentration Most common if homozygous of E2 allele, but ~1% of these develop disorder, so other gen/env factors necessary for expression in E2/2 geneotype Also occurs with allele E3 or E4, but lower association

17. ApoE2 Homozygotes 95% of type 3 hyperlipoproteinemia = E2/E2 But only 1.5% actually have disease, due to… ▫High fat diet (sat fat) ▫Obesity (BMI>28) ▫Low oestrogen ▫Diabetes (T2) ▫Hypothyroidism

18. ApoE2 Homozygotes rely on LDLr for LP clearance Asymptomatic ▫Reduced LDL cholesterol who don’t have disease ▫Less intracellular cholesterol since  binding apoE2 activity to receptors  so increase in TG-rich particles ▫So since less cholesterol   hepatic LDLr activity/upregulation ▫So LOW LDL, HIGH HDL Type 3 Disease ▫Diabetes (e.g.) repress LDLr ▫So…  ApoE AND  LDLr = LDL and blood chol  s

19. ApoE and Associated Diseases ApoE4 : Greater risk of coronary heart disease, Increased risk and decreased onset age for Alzheimer’s ▫  ed total and LDL cholesterol ▫  ed apoB LDL receptor downregulation = common expl for increased cholesterol levels