Unit 6 - Blood Lipoproteins Flashcards
what is the main job of lipoproteins?
protect hydrophobic cargo from aqueous environment while shuttling them from tissue to tissue
overall structure of lipoproteins
inner hydrophobic core of TAG and cholesterol esters, surrounded by shell with amphipathic PL, unesterified cholesterol, and apoproteins
-TAG and cholesterol are from diet or de novo
how do size and density of blood lipoproteins differ?
largest chylomicron (lowest density from TAGs)
smallest HDLs (highest density)
-density is PRO:lipid ratio
-also separated based on electrophoretic mobility, or based on their density by ultracentrifugation
how do lipoproteins separate in ultracentrifugation and electrophoresis?
top (+) to bottom (-)
- chylomicron
- LDL (beta-lipoprotein)
- VLDL (pre-beta-lipoprotein), chylomicron remnants
- IDL
- LDL
- HDL2 (alpha-lipoprotein)
- HDL3
what are the functions of apolipoproteins?
- provide recognition sites for cell surface receptors
- serve as activators for enzymes involved in lipoprotein metabolism
- required structural components of lipoprotein
- transferred between lipoproteins
- divided based on structures and function, into classes denoted by letters, and subclasses of roman numerals
Apo AI
- where is it synthesized?
- what does it do?
most abundant (structural) apo-LP in HDL
- made by liver and intestine
- activates LCAT
- involved in RCT (reverse cholesterol transport)
- ligand for ABCA1 and SR-B1
- considered an antiatherogenic PRO, but genetic defects not always associated with CVD
Apo AII
- where is it synthesized?
- what does it do?
made mostly in liver
- present with AI in some HDL (especially HDL3)
- activates LPL and inhibits LCAT
- may be proatherogenic
what does LCAT do? what is it activated by?
lecithin cholesterol acyltransferase activated by apo AI
- transfers FA from C2 of phosphotidylcholine/lecithin to cholesterol to make CE and lysoPC
- involved in maturation of HDL
what is RCT?
reverse cholesterol transport
- transfer of peripheral tissue cholesterol by ABCA1 to nascent HDL
- esterified by LCAT
- delivery of CE and free cholesterol on HDL-2 to hepatocytes thru SR-B1 (selective for CE from HDL) for hormone/bile salt/acid synthesis or disposal via bile
- release of HDL-3 (depleted of lipids)
Apo B-100
- where is it synthesized?
- what does it do?
made in liver
-binds to lipids made by MTP (microsomal triglyceride transport protein) to make VLDL, and stays for IDL and LDL
what do measurements of Apo B-100 mean?
in plasma, it reflects particle number
-higher levels associated with CVD
Apo B-48
- where is it synthesized?
- what does it do?
made in intestine (48% of B-100 from N-terminal; truncated)
- involved in chylomicron metabolism
- not recognized by LDL receptor
- post-transcriptional editing creats a stop codon
Apo CI - III
- where is it synthesized?
- what does it do?
all made in liver
- exchanged freely among lipoproteins
- important for TG metabolism, b/c interfere w/ recognition of apo E by lipoprotein receptors, or displace apo E from lipoproteins
- I activates LCAT
- II activates LPL
- III inhibits LPL
apo E
- where is it synthesized?
- what does it do?
made in liver, and associated with all LPs except LDL
- yet somehow recognized by LDL receptor and LRP (LDL receptor-related PRO, also remnant receptor) to mediate hepatic uptake of chylomicrons and VLDL remnants, plus IDL
- primarily responsible for clearance of intestinal-derived LPs after a meal, and clearance of VLDL and IDL before converted to LDL
what are the 3 isoforms of apo E?
2 - least common; risk factor for dysbetalipoproteinemia (type III hyperlipidemia) characterized by elevated chylomicron and VLDL remnants in plasma
-binds poorly to receptors
3 - most common, and “normal” variant
4 - associated with Alzheimer’s disease (increased susceptibility and lower age of onset)
where does chylomicron metabolism occur and what does it do?
assembled in intestinal mucosal cells
-carries dietary TAG, choelsterol, ADEK, and cholesterol esters to peripheral tissues
what are the 5 steps of chylomicron metabolism?
- B-48 (unique to chylo) made and glycosylated in intestinal mucosal cells, and MTP loads it with dietary lipid (TAG)
- transferred from ER to Golgi, and packaged in secretory vesicles, that fuse with plasma membrane to release nascent chylomicron, which enters lymphatic system, then blood - when nascent chylomicron reaches blood, it gets apo E (recognizes hepatic receptors) and apo C (including II from HDL)
- LPL (attached by heparin sulfate to capillary walls) is activated by CII to hydrolyze TAG and make FA and glycerol
- FA stored in adipose tissue, or used for energy in muscle
- glycerol used by liver for lipid synthesis or gluconeogenesis - particle decreases size, increases density and returns apo C(II) to HDL to make chylomicron remnant
- CR taken up by liver by apo E binding to specific LPR, followed by endocytosis
- lysosomal hydrolytic enzymes degrade to cholesterol, AA, and FA, and receptors are recycled
what is lipoprotein lipase? how does it work?
LPL is anti-parallel homodimer
- each subunit has N-terminal domain with lipolytic site, and C-terminal domain that binds to particle with substrate specificity to CII
- after binding to CII, a lid moves away from lipoprotein so that TAG can be degraded