Lipids & lipoproteins

Question 1

Cholesterol

Answer 1

Synthesised in the liver from acetyl-CoA

A characteristic ring structure connected to a hydrocarbon chain

Weakly polar

Functions = formation of steroid hormones, bile acids

Question 2

Cholesteryl ester

Answer 2

The storage molecule of cholesterol

Found in cells and the blood

Completely hydrophobic

Question 3

Triglyceride

Answer 3

A fat storage molecule

Glycerol backbone attached to 3 fatty acid chains

Completely hydrophobic

Question 4

Phospholipid

Answer 4

Found in cell membranes

Has a hydrophilic head and hydrophobic tail and hence is amphipathic

Question 5

Fat digestion

Answer 5

Ingested fats are emulsified by bile acids and digested by pancreatic lipse

Their digestion releases fatty acids and glycerol at the enterocyte membrane

Inside the enterocyte, the fat is re-formed into a chylomicron, which enters the lymphatic system

Question 6

Lipoproteins

Answer 6

There are many types of lipoprotein

Typically, they have a core of triacylglycerols and cholesterol esters

The core is then surrounded by surface components such as apolipoproteins, cholesterol and phospholipid

Question 7

Chylomicrons

Answer 7

Formed by enterocytes in the gut wall

Triglyceride-rich and covered with proteins ApoB48, ApoE and ApoA

The main lipid involved in the exogenous pathway of lipid metabolism

Question 8

VLDL

Answer 8

Very-low density lipoprotein

Triglyceride-rich and covered with proteins ApoB100, ApoE and ApoC

The main lipid involved in the endogenous pathway of lipid metabolism

Question 9

LDL

Answer 9

Low-density lipoprotein

The end point of the endogenous pathway of fat digestion

Cholesterol-rich and covered with ApoB100

Pro-atherogenic [bad]

Question 10

HDL

Answer 10

High-density lipoprotein

Cholesterol rich and covered by a wide range of proteins

Anti-atherogenic [good] and involved in reverse cholesterol transport

Question 11

Lipoprotein lipase

Answer 11

An enzyme found in the blood vessel walls attached to endothelial cells

Hydrolyses triglycerides

Activated by ApoC-II

Question 12

Hormone sensitive lipase

Answer 12

An enzyme found in adipose tissue stores

Mobilises fat stores

Only active during the fasting state when insulin levels are low

Question 13

ACAT & LCAT

Answer 13

Enzymes that synthesise cholesterol esters

Question 14

Exogenous cycle of lipoprotein metabolism

Answer 14

Deals with ingested fats and active during the fed state

1. Fat is digested and chylomicrons reach the systemic circulation via the lymphatic system
2. Circulating chylomicrons are gradually digested with lipoprotein lipase in the blood vessel walls
3. This leaves behind a chylomicron remnant which is taken up by the liver

Question 15

Endogenous cycle of lipoprotein metabolism

Answer 15

Deals with synthesised fats and is active during the fasting state

1. The liver synthesises VLDLs that enter the systemic circulation
2. The VLDLs are hydrolysed by lipoprotein lipase into intermediate-density lipoproteins, these are either taken up by the liver or further hydrolysed into LDLs

Question 16

Cholesterol uptake

Answer 16

LDL delivers cholesterol to cells

This cholesterol is then taken into the cell via LDL receptors in endocytosis

Once inside the cell, the cholesterol inhibits HMG-CoA reductase and LDL receptors whilst activating ACAT

Question 17

Hyperlipidaemia

Answer 17

High blood fat

Classified in several ways

• Primary (genetic)
• Fredrickson (phenotypic)
• Descriptive
• Secondary (non-genetic)

Question 18

Familial hypercholesterolaemia

Answer 18

A defect of the LDL receptor limits LDL uptake, causing it to persist in the blood

Autosomal dominant

Clinical features = hypercholesterolaemia, ischaemic heart disease

Management = diet/lifestyle changes, lipid lowering medications

Question 19

Familial defective ApoB

Answer 19

A mutation of the ApoB gene leads to hypercholesterolaemia

Clinical features = hypercholesterolaemia, ischaemic heart disease

Management = diet/lifestyle changes, lipid lowering medications

Question 20

Familial dysbetalipoproteinaemia

Answer 20

A defect of the ApoE gene results in an excess of remnant chylomicrons in the blood

Clinical features = high cholesterol/ triglycerides, ischaemic heart disease

Treatment = diet/lifestyle changes, lipid lowering medications