Steroids and Bile Acids: Mevalonate Pathway I

by Kevin Ahern, PhD

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    00:01 Cholesterol is a molecule, that a lot of people say "Can’t live with it, can’t live without it".

    00:05 I hope in this lecture you will see that the latter is true.

    00:08 In this lecture I will go through the synthesis of ketone bodies and cholesterol and we will see how they are related to each other.

    00:14 We will talk about cholesterol in health and also talk about how the steroids themselves are synthesized and how bile acids are involved. And these are all molecules that are related to cholesterol.

    00:27 Now, in the synthesis of molecules like cholesterol, we start with a very simple thing.

    00:32 The simple thing that we start with is the same thing that we started with in fatty acids synthesis and that's acetyl-CoA.

    00:38 In cholesterol synthesis we see that it's pathway overlaps with the pathway of making ketone bodies and that's the first thing that we are gonna talk about.

    00:46 So in the synthesis of ketone bodies in cholesterol 2 acetyl-CoAs are joined together to make a 4 carbon molecule called acetoacetyl-CoA.

    00:57 This reaction is catalyzed by the reversal of the last step of beta oxidation of a fatty acid.

    01:04 Now you may recall from that presentation that the enzyme thiolase catalyzes the breakdown or the cutting off of two carbon pieces from each of the fatty acid chains.

    01:15 When the fatty acid chain has 4, there are 2 molecules of acetyl-CoA that are produced and if you run that reaction backwards, the 2 molecules of acetyl-CoA can be made into the molecule that you see here, acetoacetyl-CoA.

    01:29 In the next step of the reaction, you see that 2 more carbons are added from another acetyl-CoA to make a 6 carbon molecule called HMG-CoA.

    01:39 This molecule, as we will see, is a branch point between the synthesis of the ketone bodies and cholesterol.

    01:45 So we are gonna follow, first of all, the ketone body synthesis.

    01:49 Ketone body synthesis occurs in the mitochondrion where is the cholesterol biosynthesis at least starts in the cytoplasm.

    About the Lecture

    The lecture Steroids and Bile Acids: Mevalonate Pathway I by Kevin Ahern, PhD is from the course Lipid Metabolism.

    Included Quiz Questions

    1. The ketone bodies production occurs in the cytoplasm, whereas the cholesterol synthesis starts in the mitochondrion.
    2. The fatty acid synthesis starts with acetyl CoA.
    3. The synthesis of cholesterol overlaps with the mevalonate pathway.
    4. The formation of acetoacetyl-CoA from acetyl CoA is mediated by the thiolase enzyme.
    5. HMG-CoA acts as a branching point between the synthesis of ketone bodies and cholesterol.

    Author of lecture Steroids and Bile Acids: Mevalonate Pathway I

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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