CLS Lipid Metabolism & Physiology 2 — Questions and Answers
Question 1: Which enzyme is responsible for the rate-limiting step in cholesterol biosynthesis?
- Acetyl-CoA carboxylase
- HMG-CoA reductase (Correct answer)
- HMG-CoA synthase
- Mevalonate kinase
Correct answer: HMG-CoA reductase
HMG-CoA reductase catalyzes the conversion of HMG-CoA to mevalonate, the committed and rate-limiting step in cholesterol synthesis.
Question 2: In reverse cholesterol transport, which lipoprotein particle delivers cholesterol to the liver for excretion?
- VLDL
- Chylomicrons
- HDL (Correct answer)
- IDL
Correct answer: HDL
HDL mediates reverse cholesterol transport by accepting free cholesterol from peripheral tissues and delivering it to the liver via SR-BI receptors or by transferring cholesterol esters to apoB-containing lipoproteins.
Question 3: What is the primary function of lipoprotein lipase (LPL) in triglyceride metabolism?
- Synthesizes triglycerides in the liver
- Hydrolyzes triglycerides in circulating chylomicrons and VLDL (Correct answer)
- Transfers cholesterol esters between lipoproteins
- Converts IDL to LDL in the bloodstream
Correct answer: Hydrolyzes triglycerides in circulating chylomicrons and VLDL
LPL is anchored to capillary endothelium and hydrolyzes triglycerides within chylomicrons and VLDL, releasing fatty acids for uptake by muscle and adipose tissue.
Question 4: Familial LPL deficiency typically presents with markedly elevated levels of which lipid fraction?
- LDL-C
- HDL-C
- Chylomicrons and triglycerides (Correct answer)
- Lp(a)
Correct answer: Chylomicrons and triglycerides
LPL deficiency impairs clearance of chylomicrons, leading to severe hypertriglyceridemia (often >1000 mg/dL) and chylomicronemia syndrome with pancreatitis risk.
Question 5: Which apolipoproteins are associated with HDL particles?
- ApoB-100 and ApoC-III
- ApoA-I and ApoA-II (Correct answer)
- ApoE and ApoB-48
- ApoC-II and ApoB-100
Correct answer: ApoA-I and ApoA-II
HDL particles are primarily characterized by ApoA-I and ApoA-II, which activate LCAT and facilitate cholesterol efflux from macrophages.
Question 6: CETP (cholesteryl ester transfer protein) primarily facilitates which lipid exchange?
- Triglycerides from HDL to LDL/VLDL for cholesterol esters (Correct answer)
- Free fatty acids from adipose to liver
- Phospholipids from LDL to HDL
- Cholesterol from liver to peripheral tissues
Correct answer: Triglycerides from HDL to LDL/VLDL for cholesterol esters
CETP transfers cholesteryl esters from HDL to VLDL and LDL in exchange for triglycerides, lowering HDL-C and raising triglycerides in VLDL.
Question 7: Nascent HDL (pre-beta HDL) acquires free cholesterol from peripheral cells primarily through which transporter?
- SR-BI
- LDL receptor
- ABCA1 (Correct answer)
- PCSK9
Correct answer: ABCA1
ABCA1 (ATP-binding cassette transporter A1) mediates efflux of free cholesterol and phospholipids from cells to lipid-poor ApoA-I, forming nascent HDL particles.
CLS Lipid Metabolism & Physiology 2
Which enzyme is responsible for the rate-limiting step in cholesterol biosynthesis?