4. Lipid transport Flashcards
Name the various classes of lipids and their concentration in blood.
- triacylglycerols (+ diacylglycerols and monoacylglycerols) - 1.0 mmol/L
- phospholipids - 2.5 mmol/L
- cholesterol - 1.5 mmol/L, and cholesterol esters (cholesterol + fatty acid) - 3.5 mmol/L
- free fatty acids - 0.4 mmol/L
- vitamins A, D, E and K
Total lipids = 4,000-8,500 mg/L
How are lipids transported in blood?
Hydrophobic molecules insoluble in water so transported bound to carrier proteins:
~2% (mostly fatty acids released from adipose tissue by lipolysis to use as fuel in tissues) carried non-covalently bound to albumin (but has limited capacity, <3 mmol/L)
~98% carried as lipoprotein particles (consisting of phospholipids, cholesterol, cholesterol esters, proteins and TAGs)
Describe the structure of a phospholipid. How are these classified?
- Hydrophilic head consisting of: polar head group + phosphate + glycerol
- Hydrophobic non-polar tail consisting of 2 saturated or unsaturated fatty acids
Classified according to polar head group, e.g. choline = phosphatidylcholine, inositol = phosphatidylinositol
Where to we obtain cholesterol?
- some obtained from diet
- most synthesised in liver (300mg/day)
Why is cholesterol essential for life?
- essential component of cell membranes (modulates fluidity)
- precursor of steroid hormones - cortisol, aldosterone, testosterone and oestrogen
- precursor of bile acids
How is cholesterol transported around the body?
As cholesterol ester - esterified with a fatty acid by LCAT or acyl-coenzyme A
What are plasma lipoprotein particles?
Spherical multi-molecular complexes containing variable amounts of different lipids in non-covalent (mostly hydrophobic) association with specific apoproteins.
Primary function is to transport water-insoluble lipid molecules in the bloodstream.
Describe the structure of a lipoprotein particle.
- Spherical surface coat consisting of:
- phospholipid monolayer
- small amount of cholesterol
- peripheral (apoC, apoE) and integral (apoA, apoB) apolipoproteins - Hydrophobic core/cargo:
- traglycerol
- cholesterol ester
- fat-soluble vitamins (A, D, E and K)
What are the 5 classes of plasma lipoproteins? How do these differ?
- chylomicrons
- VLDL (very low density lipoproteins)
- IDL (intermediate density lipoproteins)
- LDL (low density lipoproteins)
- HDL (high density lipoproteins)
Differ in relative amount of lipid type and apoprotein composition - different physical properties, e.g. density.
What does the removal of lipids from chylomicrons or VLDLs form?
Lipid removal from:
- chylomicrons forms chylomicron remnants
- VLDL leads to sequential generation of IDL and LDL particles
How is lipoprotein density determined? What is the appearance of centrifuged serum containing chylomicrons?
- Flotation ultracentrifugation
- Presence of chylomicrons (only present in blood 4-6 hrs after meal) = “creamy” appearance
What is lipoprotein density determined by? Which type has the highest density?
Surface to volume ratio - particle diameter inversely proportional to density.
From most dense (high % protein):
- HDL (5-15 nM)
- LDL (18-28 nM)
- IDL (25-30 nM)
- VLDL (30-80 nM)
- chylomicrons (100-1000 nM)
What are the different structures and types of apolipoproteins?
2 structures:
- integral (pass through phospholipid bilayer)
- peripheral (rest on top of bilayer)
6 major classes: A, B, C, D, E and H.
apoB (VLDL, IDL, LDL) and apoAI (HDL) are important.
What are the roles of apolipoproteins?
- Structural
- packaging water-insoluble lipid - Functional
- co-factor for enzymes (activating)
- ligands for cell surface receptors (e.g. LDL Rs)
What are the transport functions of the different lipoprotein types?
- chylomicrons - transport dietary TAG from intestine to tissues such as adipose tissue
- VLDL - transport TAG synthesised in liver to adipose tissue for storage
- IDL - short-liver precursor for LDL. transport of cholesterol synthesised in the liver to tissues.
- LDL - transport of cholesterol synthesised in liver to tissues.
- transport of excess tissue cholesterol to liver for disposal as bile salts and to cells requiring additional cholesterol