Overview, Isoprenoids and Sterols – Lipids

by Kevin Ahern, PhD

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    00:01 Besides the lipids involved in energy storage and membrane components, other class of lipids play very important roles in regulating processes in the body. These lipids will be the subject of this lecture. This will include molecules that are derived from cholesterol such as the steroids, the bile and bile acids, as well as other molecules that are derived from fatty acids, such as the prostaglandins, thromboxanes, the leukotrienes. And last I will talk about the fat-soluble vitamins which are derived from a combination of the two.

    00:34 Now if we look at the structure we see on the screen, this general structure is referred to as the sterol. There are many compounds that have structures related to this, one of them being for example cholesterol. Now we give a general name to the various sterols that are found in different organisms, so for example phystosterols are known as plant sterols, whereas zoosterols are known as animal sterols. The sterols are assembled from relatively simple compounds. Now the simple compounds that make up sterols are called isoprenoids and they derive their name from the fact that they have five carbon building blocks. The two five carbon building blocks that are used to make all of the sterols are known, as on the left, dimethylallylpyrophosphate and on the right, isopentenylpyrophosphate.

    01:24 These two molecules are joined together in a variety of combinations to make the higher order structures that come to be the sterols. You can see for example if you put these two molecules from the last slide together, you create a ten carbon compound called geranylpyrophosphate.

    01:41 If we add one more of the five carbon building blocks, we create farnesylpyrophosphate, now that has fifteen carbons. And if we take two of those farnesylpyrophosphates and join them together, we create a linear molecule called squalene that has thirty carbons. Now this means that squalene of course has six of those five carbon units that we started with before with the isoprenoids. Squalene is an interesting molecule because we can twist and turn its single bonds in a way to create a structure that begins to resemble the sterol, and what will happen is that enzymes will take this molecule and join the bonds together to create the sterol molecule that we've seen before. In animals the very first molecule that's made from this is called lanosterol, and lanosterol has a structure that looks very much like the two compounds that we see here. Ergosterol on the left is a plant sterol, a phytosterol and cholesterol on the right is a zoosterol. Now I haven't shown you all the steps in getting from lanosterol to these compounds and the reason for that is that there are very many steps, even though lanosterol resembles these two compounds very, very much. The modifications such as the hydroxyl group additions and so forth and the double bonds that occur in here require many enzymatic reactions.

    02:59 Now, cholesterol is a very important compound in the body. People associate cholesterol with things like atherosclerosis and problems and so forth, but our body has and uses cholesterol for a very important purpose in membranes. Cholesterol is so important to the body that we actually have three sources of cholesterol for us. First of all our diet is an important source of cholesterol, second we can synthesize cholesterol within ourselves and third, we can store cholesterol. So storing cholesterol is important when cholesterol is in short supply. Now as you can see on this slide, this is what's referred to as a cholesteryl ester meaning that the cholesterol has been linked to a fatty acid, that fatty acid is shown here with the R group and this fatty acid allows the cholesterol to be stored within the membrane or near the membrane of a cell.

    About the Lecture

    The lecture Overview, Isoprenoids and Sterols – Lipids by Kevin Ahern, PhD is from the course Biochemistry: Basics.

    Included Quiz Questions

    1. They are made from isoprenoids
    2. They are not found in plants
    3. They are most commonly found in the cytoplasm of cells
    4. They are made from ketone bodies
    1. It is ultimately made from acetyl-CoA
    2. It narrows the Tm of membranes containing it
    3. It must be in the diet of mammals
    4. It is used to make squalene

    Author of lecture Overview, Isoprenoids and Sterols – Lipids

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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