Triacylglycerol Rich Lipoprotiens: Metabolism And Associated Dyslipodaemias

Question 0

The LDL receptor

Answer 0

Binds apo E and B
Cell surface glycoprotein
839 AA
Post translation modification leading to molecular weight increase
2 asparagine linked digosaccharide chains and 18 serene linked digosaccharides

Question 1

Familial hypercholesterolemia

Answer 1

Hyperlipideamia
5% of all coronary artery deaths
Highly variable phenotype-> cholesterol levels spread over a wide range 11-25mmol
Majority of cholesterol carried on LDL
Response to treatment varies
-> heterozygous respond well to statins and other cholesterol lowering therapies
-> homozygotes are often resistant to treatment -> LDL receptor expression increased when cellular cholesterol is low
Poor patient compliance

Question 2

Domains of the LDL R

Answer 2

Exon 1-> sequence signal-> encodes the 21 amino acid sequence that is cleaved from the protein during translocation into endoplasmic reticulum
Exon 2-6-> encodes the ligand binding domain-> 7 repeats of 40 amino acids containing 6 cysteine residues -> COOH terminal on each contains a negatively charged triplet, SDE, important for ligand binding
Exons 7-14-> three growth factor repeats with a 280 AA between 2 and 3-> required for dissociation of lipoproteins from the receptor in the endosome during receptor recycling
Exon 15-> carb chain domain-> 58 AA enriched serine and there online residues-> attachment site for O linked sugar chains-> absence has no significance
Exon 16 and 5’ of 17-> transmembrane domain-> 22 hydrophobic AA
3’ of 17-> cytoplasmic domain-> 50 AA-> localisation of R in endocytosis

Question 3

Synthesis of the LDL R

Answer 3

Precursor protein is produced in the ER
Modified by Golgi
Transported to cell membrane
Binds LDL
LDL-R complex is internalised by endocytosis in a coated pit
R is endocytosed whether it is bound to LDL or not
pH decreased to 5 in endosome triggers receptor release of LDL
R is recycled to membrane and LDL is hydrolysed

Question 4

Mutational functional classes of LDLr

Answer 4

1) random mutations that disrupt receptor synthesis in ER-> don’t make any
2) ligand binding and EGF domain mutations that block transport to Golgi -> 2a completely blocked, 2b reduced transport
3) ligand binding domain mutations that produce proteins that don’t bind LDL properly
4) cytoplasmic domain mutations that effect cytoplasmic domain -> 4a only cytoplasmic, 4b cytoplasmic domain and membrane spanning region-> can’t internalise
5) EGF precursor domain mutations that block acid dependent dissociation of receptor and ligand-> receptors not recycled

Question 5

Ligand binding

Answer 5

Repeats 2-7 are required for apo B binding
Repeat 5 for apo e binding
Can bind different ligands-> repeats form a lattice around Ca2+ -> ligand binding domains can fold independently -> form 7 binding domains on 1 R

Question 6

Internalisation

Answer 6

Only one mutation ever found
JD mutation-> tyrosine residues in NPXY sequence replaced by cystine
Tyrosine sticks out

Question 7

Ligand release and recycling

Answer 7

All recycling mutations found in EGF domain
Deleting entire EGF domain-> can do everything but release
B properlar structure
Normally when pH deceased-> decreased folding in EGF domain-> displaces LDL

Question 8

Diagnosis of familial hypercholesterolemia

Answer 8

Males and females
>18 years
Presence of documented LDL R mutation of LDL cholesterol level about 95th percentile for sex and age
Tendon xanthomas in them or 1st degree relative
LDL cholesterol above 95th percentile in first degree relative
Proven CAD in patient or first degree R
Exclude secondary causes -> renal, liver, thyroid
Can have the disease without the mutation