MLT GlobeChemistry: Lipids – page 2
107 Chemistry MCQs on Lipids with answers and explanations.
Acetyl-CoA carboxylase makes malonyl-CoA, the first committed step of fatty acid synthesis. Which cofactor does it need?
Like other ATP-dependent carboxylases, acetyl-CoA carboxylase carries CO2 on biotin. Acyl carrier protein is used later, by fatty acid synthase.
Which group of eicosanoids comes from arachidonic acid by the lipoxygenase pathway?
5-Lipoxygenase converts arachidonic acid to leukotrienes. Prostaglandins, thromboxanes and prostacyclin come from the cyclooxygenase pathway.
After lipoprotein lipase removes most of their triglyceride, chylomicron remnants are cleared by which organ?
Hepatocytes take up remnants through apoE binding the LDL receptor and LRP1. Defective apoE (E2/E2) leads to remnant accumulation in type III hyperlipoproteinaemia.
A lipaemic sample has a creamy top layer after standing overnight at 4 °C. Which lipid makes up most of this lipoprotein?
The floating layer is chylomicrons, which are about 85–90% dietary triglyceride. They are the least dense lipoproteins.
Low-dose aspirin prevents arterial thrombosis because it irreversibly inhibits which enzyme in platelets?
Aspirin acetylates a serine in COX-1, blocking thromboxane A2 for the life of the platelet (7–10 days). This is why aspirin affects platelet function assays.
How many cis double bonds does arachidonic acid (the precursor of prostaglandins) contain?
Arachidonic acid is 20:4 (omega-6), with double bonds at carbons 5, 8, 11 and 14. Docosahexaenoic acid has six.
Total cholesterol 220 mg/dL, HDL cholesterol 50 mg/dL, triglycerides 150 mg/dL. What is the LDL cholesterol by the Friedewald equation?
LDL = TC − HDL − TG/5 = 220 − 50 − 30 = 140 mg/dL. 170 mg/dL is non-HDL cholesterol, which forgets to subtract VLDL.
Apo B-48 is found only in which lipoprotein class?
Apo B-48 is made in the intestine and is carried by chylomicrons and chylomicron remnants. Liver-derived VLDL and LDL carry apo B-100.
Which lipoprotein has the highest density?
HDL has the most protein and least triglyceride, so it is the densest. Chylomicrons are the largest and least dense.
In the enzymatic cholesterol method, the role of cholesterol esterase is to:
About 70% of plasma cholesterol is esterified, so esterase frees it before cholesterol oxidase acts. The oxidase, not the esterase, produces hydrogen peroxide.
Total cholesterol is 6.2 mmol/L, HDL 1.4 mmol/L and triglycerides 1.8 mmol/L. What is the non-HDL cholesterol?
Non-HDL cholesterol = total cholesterol - HDL = 6.2 - 1.4 = 4.8 mmol/L. Triglycerides are not used; also subtracting the VLDL term (TG/2.2 = 0.8) gives the Friedewald LDL of about 4.0 mmol/L, not non-HDL cholesterol.
Which lipoprotein is the largest, least dense and richest in triglyceride?
Chylomicrons carry dietary triglyceride from the intestine; about 85–90% of their mass is triglyceride, making them the largest and lightest particles. VLDL carries mainly endogenous (liver-made) triglyceride.
Which lipid result is most affected if the patient ate a meal shortly before blood collection?
Dietary fat appears as chylomicron triglyceride within hours of a meal. Total cholesterol, HDL cholesterol and apo B change only slightly after eating.
Statins lower plasma cholesterol by inhibiting which enzyme?
HMG-CoA reductase is the rate-limiting enzyme of cholesterol synthesis. Less liver cholesterol increases LDL receptor expression, which lowers plasma LDL.
On agarose lipoprotein electrophoresis, HDL migrates in which position?
HDL moves fastest toward the anode, in the alpha position. LDL is in the beta band, VLDL in pre-beta, and chylomicrons stay at the origin.
A grossly lipemic serum must be tested for electrolytes and enzymes. What is the best way to reduce lipemic interference?
Ultracentrifugation or high-speed centrifugation floats the lipoproteins to the top so the clear lower layer can be tested. Heparin clears lipemia only in vivo by releasing lipoprotein lipase.
Which lipoprotein is often called 'bad cholesterol' because it carries cholesterol to peripheral tissues?
LDL is the main carrier of cholesterol to tissues, and high LDL promotes atherosclerosis. HDL removes cholesterol from tissues and is called 'good cholesterol'.
The main function of HDL is to:
HDL performs reverse cholesterol transport, taking cholesterol from peripheral tissues to the liver. Chylomicrons carry dietary triglyceride and LDL delivers cholesterol to tissues.
For a routine fasting lipid profile, the patient is usually asked to fast for about:
A 10–12 hour (9–12 h) fast lets chylomicrons from the last meal clear, so triglycerides and calculated LDL are reliable.
A triglyceride molecule is made of:
Triglycerides (triacylglycerols) are three fatty acids esterified to one glycerol backbone. Sphingosine is the backbone of sphingolipids.