In my last post “Can vitamin C prevent cardiovascular disease?” I alluded to the role of lipoproteins in covering lesions on blood vessel walls and in the development of atherosclerosis. In this and the next few posts I want to talk about the structures and functions of lipoproteins in some detail, before getting back to cardiovascular disease.

What are lipoproteins? They are particles that transport lipids to and from the tissues.

Lipids are fatty acids and their glycerol esters, cholesterol and its fatty acid esters, and the lipid-soluble vitamin families A, D, E and K. Lipids are hydrophobic, i.e. water-insoluble, and special water-miscible particles are required to shuttle them through the bloodstream to the cells that need them. Free fatty acids travel non-covalently bound to the protein albumin. The rest of the lipids are transported in lipoproteins.

The basic structure of lipoproteins is best explained by comparison to cells. The contents of a cell are enclosed by a plasma membrane called a lipid bilayer. The two layers consist of phospholipids and unesterified cholesterol (plus proteins, which we’ll get to in a moment). The hydrophobic ends of the lipids in the two layers point toward each other, and the hydrophilic ends face the inside and outside of the cell, respectively. Both the inside and outside surfaces are hydrophilic, i.e. cells are water-miscible enclosures for hydrophilic content.

Of course the lipid bilayer is only the enclosure. Proteins on the cell surface and in the plasma membrane are needed to control entry of nutrients and exit of waste, for signaling purposes etc. It is the protein components of the plasma membrane that distinguish different cell types.

What happens if you eliminate the inner shell of the lipid bilayer? What is left is a container that is still hydrophilic on the outside, but the inner surface consists of the now exposed hydrophobic ends of the phospholipids and cholesterol molecules. In other words, we now have a water-miscible container for hydrophobic content - for lipids. A particle enclosed by a monolayer membrane is called a micelle, and it is the basic structure of lipoprotein particles.

Again, proteins have to be added to this structure to direct these particles to their destinations and to load and unload their lipid cargo.

In future posts we’ll look at
· The different types of lipoproteins and their functions,
· The surface proteins that determine the behaviour of these lipoproteins, and
· The enzymes that are needed to make the whole system work

To be continued…