Questions and Answers
Chylomicrons are the largest lipoprotein particles.
True
VLDLs have a higher density than HDLs.
False
LDLs are formed from the breakdown of VLDLs.
True
Chylomicrons are primarily responsible for transporting cholesterol.
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The density of VLDLs is between 0.95 and 1.006.
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HDLs have a smaller diameter than VLDLs.
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During starvation, ketone bodies become the major source of fuel for the brain.
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Acetoacetate, being a β-keto acid, undergoes decarboxylation to acetone and CO2.
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Cholesterol is a highly hydrophilic compound.
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The steroid nucleus of cholesterol consists of 3 fused rings.
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Cholesterol has an 8-carbon, branched hydrocarbon chain attached to C-17 of the D ring.
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Ring A of cholesterol has a hydroxyl group at C-4.
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Cholesterol is the precursor of bile acids, steroid hormones, and vitamin D in specialized tissues.
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The liver plays a significant role in maintaining cholesterol homeostasis in the body.
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Trioleoylglycerol is composed of 3 oleic acid residues.
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Mixed TGs contain only one type of fatty acid residues.
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Glycerophospholipids are the major lipid components of biological membranes.
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Phosphatidic acid is the most complex type of phosphoglyceride.
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Shingophospholipids have glycerol as their backbone.
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Phosphatidylserine is formed from phosphatidic acid and ethanolamine.
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Sphingomyelin is not a significant shingophospholipid in humans.
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Lipoproteins are soluble in water.
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The nonpolar core of lipoproteins is composed mainly of phospholipids.
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There are three main types of lipoproteins based on their density.
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Apolipoprotein refers to the protein component of lipoproteins.
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The polar groups of lipoproteins face inward towards the nonpolar core.
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Lipids are soluble in water when they are in their free form.
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Lipoprotein particles vary in density, size, and lipid composition.
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Cholesterol is essential for human life but not required in diets.
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The biosynthetic process of cholesterol involves a simple pathway requiring only one precursor.
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Mevalonate is formed from the condensation of three molecules of acetyl-CoA.
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NADPH acts as the reducing equivalent in cholesterol biosynthesis.
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The first stage of cholesterol biosynthesis involves the reduction of HMG-CoA.
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3 phosphate groups from ATP are not required for forming mevalonate.
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∆3-isopentenyl pyrophosphate is the second activated isoprene produced during cholesterol biosynthesis.
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The cholesterol biosynthesis pathway begins with the synthesis of acetoacetyl-CoA.
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Study Notes
Triacylglycerols
- Trioleoylglycerol or triolein has 3 oleic acid residues
- Mixed triacylglycerols (TGs) contain 2 or 3 different types of fatty acid residues and are named according to their placement on the glycerol moiety
Phospholipids
- There are 2 classes of phospholipids: those with glycerol as a backbone (glycerophospholipids or phosphoglycerides) and those that contain sphingosine as a backbone (sphingophospholipids)
- Glycerophospholipids are the major lipid components of biological membranes
- Phosphatidic acid is the simplest phosphoglyceride and is the precursor of other members of this group
- Other phosphoglycerides are formed from phosphatidic acid (PA) and an alcohol, such as:
- PA + serine = phosphatidylserine
- PA + ethanolamine = phosphatidylethanolamine (cephalin)
- PA + choline = phosphatidylcholine (lecithin)
- PA + inositol = phosphatidylinositol
- PA + glycerol = phosphatidylglycerol
Sphingophospholipids: Sphingomyelin
- The backbone of sphingophospholipids is the amino alcohol sphingosine, rather than glycerol
- A long-chain fatty acid is attached to the amino group of sphingosine through an amide linkage, producing a ceramide
- The alcohol group of sphingosine is esterified to phosphorylcholine, producing sphingomyelin, the only significant sphingophospholipid in humans
- Sphingomyelin is an important constituent of the myelin of nerve fibers
Lipoproteins
- Lipids that are absorbed from the diet and synthesized by the liver and adipose tissue must be transported between cells and organs for effective utilization and storage
- Lipids are insoluble in water, so they are transported in the aqueous plasma (blood) by associating with amphipathic lipids (phospholipids and cholesterol) and proteins to form water-miscible lipoproteins
- Lipoproteins are spherical macromolecular complexes of lipids and proteins, consisting of a nonpolar core and a single surface layer of amphipathic lipids
- The nonpolar core consists mainly of triacylglycerol and cholesteryl ester, surrounded by a single surface layer of amphipathic phospholipid and cholesterol molecules
- The protein moiety of a lipoprotein is known as an apolipoprotein or apoprotein
Classification of Lipoproteins
- Lipoproteins differ in density, size, lipid and protein composition, and site of origin
- There are 4 lipoprotein particles:
- Chylomicrons
- Very-low-density lipoproteins (VLDL)
- Low-density lipoproteins (LDL)
- High-density lipoproteins (HDL)
- Chylomicrons are the largest lipoprotein particles, with the lowest density and highest percentage of lipid
- VLDLs and LDLs are successively denser, with higher ratios of protein to lipid
- HDL particles have the highest density
Cholesterol
- Cholesterol is a very hydrophobic compound consisting of 4 fused rings (A, B, C, and D) and an 8-carbon, branched hydrocarbon chain attached to C-17 of the D ring
- Ring A has a hydroxyl group at C-3, and ring B has a double bond between C-5 and C-6
- Cholesterol is a structural component of all cell membranes, where it modulates fluidity
- In specialized tissues, cholesterol is the precursor of bile acids, steroid hormones, and vitamin D
- The liver plays a central role in the regulation of the body's cholesterol homeostasis
Cholesterol Metabolism
- Cholesterol is essential to life, yet its deposition in arteries has been associated with cardiovascular disease and stroke
- In healthy individuals, a balance is maintained between cholesterol biosynthesis, utilization, and transport, ensuring that deposition in arteries is kept to a minimal level
- Cholesterol is not essential in human diets, because all cells can synthesize it from simple precursors
- Cholesterol biosynthesis involves the synthesis of mevalonate from acetate, followed by the conversion of mevalonate to 2 activated isoprenes
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Description
This quiz covers the different types of lipids, including triolein, mixed triglycerides, phospholipids, and their classifications. Learn about the glycerol and sphingosine backbones in lipids.
