MLT GlobeChemistry: Lipids – page 4
107 Chemistry MCQs on Lipids with answers and explanations.
A fat with a high saponification number contains fatty acids that are mostly:
The saponification number is the mg KOH needed to saponify 1 g of fat; more molecules per gram (shorter chains) consume more KOH. Unsaturation is measured by iodine number, free fatty acids by acid number.
Cardiolipin, the antigen used in non-treponemal syphilis tests such as VDRL, is concentrated in the:
Cardiolipin (diphosphatidylglycerol) is characteristic of the inner mitochondrial membrane, which is also low in cholesterol. The plasma membrane and myelin are cholesterol-rich.
A higher intake of which dietary component raises the vitamin E requirement?
PUFA in membranes are prone to peroxidation, and vitamin E protects them. More PUFA means more vitamin E is needed.
Beta-oxidation shortens a fatty acyl-CoA in each cycle in which way?
Each cycle releases acetyl-CoA from the thioester (carboxyl) end. One-carbon removal (alpha-oxidation) is used only for branched fatty acids such as phytanic acid.
How many carbon atoms does cholesterol have, the parent structure for bile acids and steroid hormones?
Cholesterol is C27H46O. Progestogens and corticosteroids are C21, androgens C19 and bile acids C24, all made by shortening the side chain.
Which fatty acid synthase activity joins the growing acyl chain to a malonyl group, releasing CO2?
The condensing enzyme (KS) joins acyl and malonyl units and releases CO2, which drives the reaction. The transacylases only load substrates onto ACP.
A plasma sample left overnight at 4 °C shows a creamy top layer over clear plasma. This indicates:
Chylomicrons are large and light, so they float as a cream layer on standing. A turbid infranatant would suggest excess VLDL as well.
The Friedewald equation should not be used to calculate LDL cholesterol when triglycerides exceed:
Above 400 mg/dL the TG/VLDL-cholesterol ratio is no longer about 5:1, so the estimate is unreliable; direct LDL or another equation is used. 150 mg/dL is only the desirable TG limit.
In SI units: total cholesterol 5.5 mmol/L, HDL 1.2 mmol/L, triglycerides 2.2 mmol/L. What is the Friedewald LDL cholesterol?
In mmol/L the VLDL term is TG/2.2, here 1.0. LDL = 5.5 − 1.2 − 1.0 = 3.3 mmol/L. Using TG/5 (0.44) wrongly gives about 3.9 mmol/L.
Which apolipoprotein is the main structural protein of LDL, with one molecule per particle?
Each VLDL, IDL and LDL particle carries one apo B-100, which binds the LDL receptor. Apo A-I is the main protein of HDL.
The major apolipoprotein of HDL, which also activates lecithin-cholesterol acyltransferase (LCAT), is:
Apo A-I makes up most of HDL protein and activates LCAT, which esterifies cholesterol in HDL. Apo C-III inhibits lipoprotein lipase.
Which apolipoprotein is the activator of lipoprotein lipase?
Apo C-II on chylomicrons and VLDL activates lipoprotein lipase, which releases fatty acids from triglyceride. Its deficiency causes severe chylomicronemia.
On agarose lipoprotein electrophoresis, the pre-beta band corresponds to:
Order from the anode: HDL (alpha), VLDL (pre-beta), LDL (beta); chylomicrons stay at the origin. LDL is the beta band, not pre-beta.
Lipoprotein(a) is best described as:
Lp(a) is LDL with apo(a), which resembles plasminogen, attached to apo B-100 by a disulfide bond. Its level is mainly genetic and is an independent cardiovascular risk factor.
A routine non-fasting lipid profile shows triglycerides of 520 mg/dL (5.9 mmol/L). What is the best next step?
Non-fasting samples are acceptable for routine screening, but when non-fasting triglycerides exceed 400 mg/dL a fasting profile is recommended. Friedewald is not valid at this level.
A 12-year-old has LDL cholesterol of 290 mg/dL (7.5 mmol/L), normal triglycerides and tendon xanthomas; his father had an MI at 40. The defect most likely involves the:
This is familial hypercholesterolemia, usually from LDL receptor mutations, giving very high LDL with normal triglycerides (type IIa). LPL or apo C-II defects raise triglycerides.
In HDL cholesterol precipitation methods, phosphotungstate with magnesium is added to:
Polyanions with divalent cations precipitate VLDL, LDL and Lp(a); cholesterol left in the supernatant is HDL cholesterol. HDL itself is not precipitated.
A 55-year-old woman has new hypercholesterolemia with weight gain, cold intolerance and constipation. Which test should be done before starting lipid-lowering therapy?
Hypothyroidism reduces LDL receptor activity and is a common secondary cause of raised LDL; treating it may correct the lipids. Her symptoms point to thyroid, not adrenal disease.
What does lecithin-cholesterol acyltransferase (LCAT) do in plasma?
LCAT transfers a fatty acid from lecithin to free cholesterol, forming cholesteryl ester that moves into the HDL core. This is a key step in reverse cholesterol transport. Transfer to VLDL is done by CETP.
A 6-year-old has recurrent pancreatitis, eruptive xanthomas and triglycerides of 3000 mg/dL (34 mmol/L). Refrigerated plasma shows a creamy layer over a clear infranatant. The most likely defect is:
Familial chylomicronemia is caused by loss of lipoprotein lipase (or its activator apo C-II), so chylomicrons cannot be cleared. The clear infranatant shows VLDL is not raised, unlike type V.