Lipoproteins

LIPOPROTEIN FUNCTION

Solubilize cholesterol and triacylglycerol (TAG) & transport them in plasma
Deliver them to peripheral tissues

Triacylglycerol function

Energy storage
Membrane synthesis

Cholesterol function

Component of cell membranes
Precursor for steroid hormones and vitamin D

LIPOPROTEIN STRUCTURE

Apolipoprotein

On surface of lipoprotein
3 functions:

i. Structure: bind and solubilize hydrophobic lipids ii. Recognition: contain signals that target lipoproteins to cells (recognized by receptors) iii. Enzymatic: activate or act as coenzymes in lipid metabolism

Single layer of phospholipids

Free cholesterol in layer

Hydrophobic core

Esterified cholesterol and TAG

CLASSES OF LIPOPROTEINS 1. Chylomicrons (fed state only)

Size: largest
Origin: intestine (dietary)
Composition: dietary lipids (mainly TAG)
Density: least dense

2. Very low density lipoprotein (VLDL)

Size: LDL < VLDL < chylomicron
Origin: liver (endogenous lipids)
Composition: Endogenously synthesized lipids (mainly TAG & phospholipids)
Density: LDL > VLDL > chylomicron

3. Low density lipoprotein (LDL)

Size: VLDL > LDL > HDL
Origin: VLDL
Composition: Major cholesterol carrier in body (greatest relative amount)
Density: VLDL < LDL < HDL
"Bad cholesterol" (atherosclerosis)
Regulates de novo cholesterol biosynthesis

4. High density lipoprotein (HDL)

Size: smallest
Origin: intestine and liver
Composition: picks up plasma cholesterol (released due to membrane turnover or cell death; carries less cholesterol than LDL)
Density: most dense (greatest % of protein)
Delivers cholesterol to liver for excretion (bile salts) and to tissues (steroid synthesis)
"Good cholesterol"

CLINICAL CORRELATION

HDL:LDL cholesterol ratio

Diagnostic used to determine a patient's risk for developing heart disease
Healthy ratio ~ 3.5

Hypercholesterolemia (high blood cholesterol)

Common consequence of low HDL:LDL cholesterol ratio
Can lead to atherosclerosis & heart attack

Lipoproteins

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