Cholesterol 2

Question 1

How is cholesterol converted into steroid hormones?

Answer 1

Cholesterol esters are hydrolysed to cholesterol which is taken into mitochondria, this is then converted into a hormone
e.g. Pregneolone by cytochrome 450 enzyme

Question 2

What is the role of 7-Dehydroxycholecalciferol (a vitamin D precursor)

Answer 2

Controls phosphate and calcium metabolism

Question 3

What are cholesterol esters?

Answer 3

Cholesterol molecules that have been esterified with a fatty acid at the 3’ hydroxyl group by acyltransferase enzymes

Question 4

What are lipoproteins?

Answer 4

Many lipid molecules in a monolayer which is punctuated by apolipoproteins which are amphipathic with a hydrophobic interior and hydrophilic exterior

Question 5

What is a micelle?

Answer 5

A cluster of bile salts which used their amphipathic nature to surround fat molecules, these also have enzymes like pancreatic lipase attaches which breaks down the triacylglycerol within the micelle

Question 6

How is cholesterol digested?

Answer 6

Linked with fats, is emulsified to form micelles, these are then absorbed by the intestine, chylomicrons then transport triacylglycerides and cholesterol to the liver

Question 7

In order of decreasing density what are the four major classes of lipoproteins?

Answer 7

High Density lipoprotein, Low density lipoprotein, very low density lipoprotein and chylomicrons

Question 8

What is different between the chylomicron, VLDL and LDL classes of lipoproteins?

Answer 8

The lower the density the lower the proportion of protein, cholesterol esters and phospholipids and the higher the proportion of triacylglyceride

Question 9

What proteins are different between the chylomicron, VLDL and LDL classes of lipoproteins?

Answer 9

All except LDL have C-I,II and III
Chylomicrons have A-I,II and B-48
VLDL has E and B-100
LDL has only B-100

Question 10

How are chylomicrons formed?

Answer 10

From the Endoplasmic reticulum of intestinal cells where it is then moved through the secretory pathway and released via exocytosis

Question 11

How are VLDLs formed?

Answer 11

From the Endoplasmic reticulum of liver cells where it is then moved through the secretory pathway and released via exocytosis

Question 12

How are LDLs, IDLs and some HDLs formed?

Answer 12

Made extracellulary in the blood through remodelling of VLDLs and chylomicrons

Question 13

How are lipoproteins remodelled?

Answer 13

Hydrolysis of triacylglycerols and phospholipids? By lipases
Esterification of cholesterol by acyltransferase
Transfer of cholesterol esters, triaclyglycerides and phospholipids between lipoproteins by specific lipid transfer proteins
Uptake of some particles of cholesterols and phospholipids exported by some cells
Association and dis-association of apolipoproteins from particles

Question 14

How can lipoproteins be removed from the blood?

Answer 14

Via receptor mediated activities of cells

The LDL protein is taken up by endocytosis via the LDL receptor

Question 15

What can occur if there are defects in the LDL receptor?

Answer 15

This can give rise to hypercholesterolemia and coronary heart disease through increasing the levels of cholesterol in the blood and therefore the risk of atherosclerosis

Question 16

What is the life cycle of chylomicrons?

Answer 16

They are synthesized and released by intestinal cells, they are then remodelled in the circulation to give small cholesterol enriched remnants which are taken up by receptors in the liver

Question 17

What is the life cycle of VLDL?

Answer 17

Synthesized in liver cells, move through the secretory pathway and are released via exocytosis, this is then remodelled in the blood through hydrolysis of triglycerides and phospholipids leading to the formation of an IDL or a LDL both of these can use the LDL receptor to be taken up cells and broken down and recycled