Lipoproteins Flashcards
Lipids synthesized in the liver and the intestine has to be transported to the various tissues to accomplish their metabolic functions. Because of their insolubility, they are transported in the plasma in macromolecular complexes called______.
lipoproteins
______are spherical particles with______ (triglycerides and cholesterol esters) in their core and more______ (phospholipids and free cholesterol) oriented near the surface.
Lipoproteins
nonpolar lipids
polar lipids
Lipoproteins also contain one or more specific proteins, called______, which are located on their surfaces.
apolipoproteins
_________ are primarily located on the surface of lipoprotein particles.
They help maintain the structural integrity of lipoproteins and also serve as ligands for cell receptors and as activators and inhibitors of the various enzymes that modify lipoprotein particles.
Apolipoproteins
Apolipoproteins contain a structural motif called an ______, which accounts for the ability of these proteins to bind to lipids.
amphipathic helix
________ are protein segments arranged in coils so that the hydrophobic amino acids residues interact with lipids, whereas the part of the helix containing hydrophilic amino acids faces away from the lipids and toward the aqueous environment.
Amphipathic helices
Each class of lipoprotein has a variety of apolipoproteins in differing proportions, with the exception of LDL, which contains only_____
apo B-100
_____is the major protein in HDL.
Apo A-I
_______________ are present in various proportions in all lipoproteins except LDL.
Apo C-I, C-11, C-111, and E
Apolipoproteins collectively have three major physiological functions.
They are involved in:
1) activating important enzymes in the lipoprotein metabolic pathways;
2) maintaining the structural integrity of the lipoprotein complex; and
3) facilitating the uptake of lipoprotein into cells through their recognition by specific cell surface receptors.
_______, the major protein on HDL, is frequently used as an index of the amount of the antiatherogenic HDL present in plasma.
Apolipoprotein A-1
_____is a large protein with a molecular weight of approximately 500 kD and the principal protein on LDL. VLDL and chvlomicrons.
Apo B
_____ is found on LDL and VLDL and is a ligand for the LDL receptor, and it is, therefore, critical in the uptake of LDL by cells.
Apo B-100
________, exclusively found in chylomicrons, is essentially the first 48% or half of the apo B molecule and is produced by posttranscriptional editing of the apo B-100 mRNA.
Apo B-48
______can also be found covalently linked to apo (a); a plasminogen-like protein that is found in a proatherogenic lipoprotein particle called lipoprotein (a) (Lp(al).
Apo B-100
____, another important apolipoprotein found on many types of lipoproteins (LDL, VLDL, and HDL), also serves as a ligand for the LDL. receptor and the chylomicron remnant receptor.
Apo E
There are three major isoforms of apo E: apo E2, E3, and E4.
The apo E isoforms affect_____ metabolism because they cliffer in their ability to interact with the LDL receptor.
lipoprotein metabolism
______have different physical and chemical properties as they contain different proportions of lipids and proteins.
Historically, ______ have been categorized based on differences in their hydrated densities as determined by ___________.
Lipoproteins
ultracentrifugation
Categories of lipoprotein
1) chylomicrons (CM),
2) very-low density lipoprotein (VLDL),
3) intermediate-density lipoprotein (IDL),
4) low-density lipoprotein (LDL),
5) high-density lipoprotein (HDL), and
6) lipoprotein (a) [Lp (a)l.
are large particles produced by the intestine that transport lipids of dietary origin to the tissues of the body.
They are very rich in triglycerides, but relatively poor in free cholesterol, phospholipids, and protein.
Chylomicrons
When present at high levels,_____ result in “milky” plasma and accumulate as a floating creamy layer when left undisturbed for several hours.
chylomicrons
The apolipoproteins in chylomicrons include
apoB-48
apo A-I
apo A-IV
apo C-1
apo C-II
apo C-Ill
apo E
particles are produced by the liver and supply the tissues of the body with triglycerides of endogenous, primarily hepatic, origin and cholesterol.
Very Low-Density Lipoproteins
As compared with chylornicrons,_____ particles are smaller and produce turbid plasma when present in excessive amounts.
VLDL
By mass,_____ particles contain approximately 50% triglyceride, 40% cholesterol and phospholipid, and 10% protein, mostly apo B-100 and apo C-I, apo C-II, and apo C-IlI, but also apo E.
VLDL
______ hydrolyzes VLDL, and this produces highly atherogenic, smaller triglyceride and surface material–depleted particles called VLDL remnants and IDL.
These particles can be removed from plasma by LDL receptors or further metabolized to LDL.
Lipoprotein lipase
is produced through the metabolism of VLDL in circulation and constitutes about 50% of the total lipoprotein mass in human plasma.
Low-Density Lipoproteins
do not scatter light or alter the clarity of plasma even at greatly increased concentrations.
LDL
consists of approximately 50% cholesterol
mostly esterified
25% protein
mostly apoB-100 with traces of apo C
20% phospholipid
some triglycerides
LDL
The_____ takes up most of the LDL in circulation (approximately 75%), with apo B-100 serving as a ligand for the hepatic receptor.
The remaining LDL is delivered to other_____.
Some LDL is modified and is removed from the circulation by _____such as those found in atheromatous plaque.
liver
tissues
scavenger cells
is a small particle, consisting mostly of protein, cholesterol, and phospholipids, with only traces of triglycerides.
High-Density Lipoprotein
Produced by the liver and intestine,_____ is involved in reverse cholesterol transport.
In vitro studies suggest that ____ is involved in anti-inflammatory, antioxiclant, antithrombotic, and nitric oxide-inducing mechanisms.
HDL
The major site for the clearance of HDL cholesterol is the_____, and the best understood mechanism is the selective uptake of cholesterol esters from HDL by the SR-BI receptor expressed in the liver.
liver
The__________ is considered to be an alternative pathway for HDL metabolism.
cholesterol ester transfer protein (CETP) pathway
______transfers triglycerides from apo B lipoproteins in exchange for cholesterol esters in HDL.
The result is an HDL particle depleted of cholesterol esters but enriched with triglycerides.
Cholesterol transferred to apo B-lipoproteins is then cleared by LDL receptors.
CETP
Formed through the metabolism of VLDL in circulation, ____ can be removed from circulation quickly through interaction with the LDL receptor, or can be further metabolized to LDL.
As expected, the lipid content, size, and density of ____ is intermediate between VLDL and LDL.
Intermediate-Density Lipoprotein
_______is similar to LDL in terms of density and overall composition, and can be thought of as an LDL particle to which apo(a) has been added, linked to apoB-10C via a disulfide bond.
The electrophoretic mobility of _____ is usually pre-B but can vary between that of LDL (P) and albumin (pre-a).
Lipoprotein (a). Lp(a)
Lipoproteins are classified on the basis of buoyant density or electrophoretic mobility.
Separation of lipoproteins based on_______ depends on the ratio between lipids and proteins present in them.
Lipoproteins that contain higher amounts of lipids have lower buoyant density and are collected on top of the centrifuge tube.
buoyant density
Separations based on_______ are based on the number of charges present on lipoproteins and are independent of their size and buoyant density.
electrophoretic mobility
