Lipid Transport Flashcards
Describe free fatty acids.
- they’re formed from triacylglycerides stored in adipose tissue
- it circulates bound to protein as a Na+ salt, particularly the albumin protein
- saturation occurs at about 2 mM of fatty acid molecules
- it enters the cell by simple diffusion
- the intracellular concentration of free fatty acids is kept low
Why do free fatty acids (FFAs) need to travel bound to a protein?
If unbound, they will act as a detergent.
What is a lipoprotein?
It is a biochemical assembly whose purpose is to transport hydrophobic lipid molecules in water, as in blood or extracellular fluid.
Describe the structure of a lipoprotein.
The membrane consists of phospholipids and cholesterol, and large apolipoproteins.
In the centre, there are cholesterol esters and triacylglycerol.
What are the five lipoproteins?
- Chylomicrons
- Very Low-Density Lipoproteins (VLDLs)
- Low-Density Lipoproteins (LDLs)
- Intermediate Density Lipoproteins (IDLs)
- High-Density Lipoproteins (HDLs)
Describe the different lipoprotein compositions.
- CHYLOMICRONS: has the most (90-95%) triglycerides, so the least dense
- VLDLs: mostly triglycerides (53%)
- LDLs: mostly (50%) cholesterol
- IDLs: intermediate in all (highest is triglycerides, 31%)
- HDLs: mostly (49%) protein
What are apoproteins/apolipoproteins?
They are proteins that bind lipids together to form lipoproteins.
What are the functions of apoproteins?
- structural functions (the backbone of the lipoproteins)
- to solubilise lipids (lipoproteins allow lipids to travel in aqueous solution)
- act as enzymes or enzyme cofactors
- tissue targeting
Give examples of apoproteins acting as enzymes/enzyme cofactors.
- APO C2 activates LipoProtein Lipase (LPL, breaks down fat in the form of triglycerides, so a lipoprotein expressing this apoprotein will lose its triglycerides)
- APO A1 activates Lecithin-Cholesterol AcylTransferase (LCAT, converts free cholesterol into cholesteryl esters)
Give examples of apoproteins involved in tissue targeting.
- APO B100 and APO E bind to the LDL receptor
- APO E binds to the HDL receptor
List the apoproteins involved in the composition of the different lipoproteins.
- CHYLOMICRONS: B48, APO C2, C3 and E
- VLDL: B100, APO C1, C2, C3 and E
- LDL: B100
- IDL: B100, APO E
- HDL: APO A1, A2, C1, C3, D and E
Describe the synthesis of chylomicrons.
They’re formed in the cells that line the gut.
In the lumen of the gut, triglycerides are broken down to fatty acids. Those, along with monoacylglycerols are brought into the mucosal cell and reformed into triacylglycerides.
Those then combine with other lipids and proteins in the cell to form chylomicrons.
Why are chylomicrons delivered directly to the lymph system?
Chylomicrons are secreted into the lymphatics which carries them via the thoracic duct to the superior vena cava. By this pathway, dietary fats avoid the direct delivery to the liver and instead are made available to the extrahepatic tissue.
In contrast, digested proteins and carbohydrates are released into the portal vein and delivered directly to the liver.
Describe the conversion of a nascent chylomicron to a mature chylomicron, in terms of apoproteins.
When first formed, the only apoprotein chylomicrons consist of are the ApoB48.
As it circulates, it interacts with HDL and the HDL donates certain apoproteins to the nascent chylomicron, primarily Apo C2 and Apo E. This forms a mature chylomicron.
This donation occurs in the SER.
Describe chylomicrons.
- they’re solely important for transporting exogenous (dietary) lipids from gut around the circulation
- they reflect meal composition (if a fatty meal is ingested, there will be many chylomicrons of that composition)
- they have a low density due to a high number of triglycerides
- they contain fat soluble vitamins such as Vitamins A and E
- their lifetime in the circulation is about one hour (the triglyceride’s is about 5 minutes)
- their remnants are removed by the liver, with the help of Apo E
Describe Lipoprotein Lipase (LPL) and how it works.
LPLs bind to and are activated by Apo C2.
A lipoprotein binds to the LDL, and the triglycerides in it are broken down into monoacylglycerols and fatty acids.
Lipoprotein Lipases levels vary with the tissue, so you will see them in cells that utilise a lot of fats. There are different isoforms of the enzyme expressed in the different tissue. Examples of such tissue would be mammary, muscle or adipose tissue.
The Km of the LPL isoform in adipocytes is greater than in muscle.
What is Hyperlipidaemia?
Hyperlipidaemia is abnormally elevated levels of any or all lipids/lipoproteins in the blood.
What are the different types of hyperlipidaemia?
- TYPE 1: This can be caused by either a deficiency in lipoprotein lipase or Apo C2 – characterised by high plasma triglyceride
- TYPE 2: Characterised by high LDL – most caused by a genetic defect in the synthesis processing or function of the LDL receptor
- TYPE 4: Most common results in increased VLDL concentrations often due to obesity or alcohol abuse
Describe VLDLs.
- they’re responsible for transporting endogenously-synthesised (synthesised in the liver) lipids
- they’re metabolised by LPL as they circulate - TG half-life is 15-60 minutes
- their formation is enhanced by: dietary carbohydrates, circulating FFAs, alcohol, raised insulin and decreased glucagon
Describe the conversion of a nascent VLDL to a mature VLDL , in terms of apoproteins.
VLDLs are synthesised in the liver, ER and Golgi.
When released as nascent VLDLs, they only have the Apo B100 apoprotein. When they interact with HDL, the HDL donates Apo C2 and Apo E to the VLDL, making it mature.
What is the difference between Apo B100 and Apo B48?
They are both synthesised from the same gene. The only difference is that B100 is 100% of the gene, while B48 is 48% of that gene.
Their expression is differentially regulated within the cell types.
What are the two fates of VLDLs?
1) When it’s lost the majority of its triglycerides, its remnants will return to the liver (removed by Apo E).
2) It may form IDLs (via Lipase), and those IDLs may be removed by the liver or may be converted to LDLs, which will be removed by the liver and by non-hepatic tissue for steroid biosynthesis.
Describe LDLs.
- they’re the major carrier of cholesterol
- they’re metabolised slowly - 3 days
- they carry cholesterol to the periphery and regulate de novo synthesis
- they contain one Apo B100, which can bind to a specific receptor on hepatocytes
What are the three ways in which HDLs are made?
- primarily, in the liver and intestines
- by budding from VLDL and chylomicrons
- from the free Apo A1
The primary function of HDLs is to remove cholesterol from the circulation. How do they do that?
In order to do this, they contain an enzyme called Lecithin-Cholesterol AcylTransferase (LCAT) which modifies free cholesterol and then retains it within the HDL. This prevents the cholesterol from diffusing out of the HDL.
The process by which it picks up cholesterol is known as Reverse Cholesterol Transport.
This occurs in the plasma and on endothelial cells. In order to signal this, they express the ABCA1 transporter, which moves cholesterol from the extracellular surface of the membrane and interacts with the Apo A1 on the HDLs to transport the cholesterol away.
What is the significance of the HDL/LDL ratio?
HDLs are commonly referred to as ‘good cholesterol’, while LDLs are referred to as ‘bad cholesterol’.
The HDL/LDL is used diagnostically to assess the susceptibility to heart disease. Normal individuals have a ratio of 3.5.
Describe the process by which lipoproteins are removed from the circulation.
Lipoproteins are removed from the circulation by Receptor-Mediated Endocytosis. The following example is using uptake of LDLs.
1) LDLs in the circulation can bind to specific receptors expressed on endothelial cells.
2) The receptor and LDL are endocytosed, with the membrane pinching off to form a vesicle.
3) The vesicle fuses with endosomes. The endosomes contain enzymes responsible for the breakdown of protein and metabolism of lipids – receptor is recycled and re-inserted into the membrane.
4) There will be a further fusion with a lysosome to provide more enzymes (hydrolytic)
5) The cholesterol esters will be converted to cholesterol, which will then diffuse out into the cytoplasm, where they will be immediately esterified (since free cholesterol is not desirable within a cell, unless associated with a membrane).
The triglycerides will be broken down to give fatty acids, which can be further metabolised, and amino acids will also be broken down.
How is receptor-mediated endocytosis regulated?
This process is regulated by the intracellular concentration of cholesterol, which regulates the expression of new and existing receptors on the membrane.
With increased cholesterol, HMG-CoA reductase activity is inhibited. HMG-CoA reductase reduces the LDL receptor expression.
What lipoproteins is Apo B100 presented on, what does it bind to and where is it distributed, tissue-wise?
Apo B100:
- it is present on VLDLs and LDLs
- it binds to LDL receptors
- it is distributed in the liver
What lipoproteins is Apo E presented on, what does it bind to and where is it distributed, tissue-wise?
It is presented on VLDLs, HDLs and chylomicrons:
- it binds to LDL receptors and other proteins
- it is distributed in the liver
It is also presented on LDLs:
- it is distributed on endothelial cells and macrophages
What is an example of when there is a loss of LDL receptor function?
An example of that would be individuals who have Familial Hypercholesterolemia (FH).
For homozygous individuals:
- as a result of the loss of LDL receptor function, they have up to four times the normal cholesterol serum levels (800 mg/ml to 200mg/ml)
- they develop blocked arteries (atherosclerosis)
- they tend to die young from heart attacks
- the de novo synthesis is not regulated by LDL
A single amino acid substitution prevents the localisation of the LDL receptor to the coated pits.
Name the two types of receptors, and describe them.
- high-affinity LDL receptor
- low-affinity scavenger receptor
They’re present on endothelial cells, macrophages and VSMC (vascular smooth muscle cells).
The low-affinity scavenger receptor is activated when the plasma LDL levels are high, or when it is chemically modified (ie. oxidised). It is not regulated by cholesterol.
List the three ways in which lipoprotein levels are regulated.
HORMONAL REGULATION:
- insulin
- cortisol
- thyroid hormones
LDL EXPRESSION:
- oestrogen
NUTRITIONAL STATUS:
- decreased synthesis during fasting
- increased by dietary fats - unsaturated fats best
List three situations where you will find abnormalities in lipid transport.
DIABETES MELLITUS:
- increased FFA metabolism
- decreased chylomicron and VLDL utilisation
GENE DEFECTS:
- gene defects in apolipoproteins, enzymes or receptors
- leading to hypercholesterolemia, atherosclerosis, etc.
OBESITY:
- can cause hypertension, NIDDM (noninsulin-dependent diabetes mellitus), hyperlipidaemia and hyperglycaemia
