Gluconeogenesis: Metabolism of Galactose

by Kevin Ahern, PhD

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    00:01 In a previous lecture I described the way in which glucose is broken down.

    00:05 But glucose is not the only sugar that we ourselves need to deal with.

    00:08 In this lecture I will talk about the metabolism of other sugars.

    00:12 I will discuss the process of gluconeogenesis, that's the way cells make glucose that they can use.

    00:17 And then last I will talk about the phenomenon of reciprocal regulation, the way cells control sugar metabolism.

    00:26 Now cells have a lot of different sugars that they get besides glucose. So it's important that we understand something about what they do.

    00:33 In this pathway we see that metabolism of a sugar known as galactose.

    00:38 Now we get galactose in our diet fairly regularly if we eat dairy products because galactose comes from lactose which is known as milk sugar.

    00:47 So our cells have to be able to metabolize galactose.

    00:51 To metabolize galactose, the metabolism is actually shown on the screen.

    00:54 I am gonna step you through it and it looks a little complicated but it is actually fairly simple.

    01:00 The reactions of galactose basically involve its conversion into an intermediate in glycolysis.

    01:06 Now this starts with an enzyme that has a mouthful of a name: galactose-1-phosphate uridyl transferase.

    01:13 Hopefully, the reaction is simpler then the name of the enzyme is.

    01:16 The reaction catalyzed by this enzyme takes galactose-1-phosphate on the upper left and combines it with UDP glucose on the lower left.

    01:26 And what it’s doing is essentially swapping the galactose for the glucose.

    01:31 So we see when this process happens that we start with galactose-1-phosphate and we end up with UDP galactose.

    01:39 UDP glucose releases a glucose-1-phosphate.

    01:42 So because of that we now have glucose-1-phosphate that is free.

    01:47 A glucose-1-phosphate is relatively easy to get into glycolysis because there is an easy enzymatic conversion of glucose-1-phosphate to glucose-6-phosphate and we get into glycolysis.

    01:57 This means we have converted part of the molecule shown on the screen into glycolytic intermediates.

    02:02 The galactose in the UDP, however, has to also be converted into intermediate.

    02:08 And that happens in the next reaction which is catalyzed by UDP-galactose-4-epimerase.

    02:14 Again a mouthful of a name, but the reaction is quite simple.

    02:17 The galactose at the end of the UDP is converted into a glucose.

    02:22 That reaction means that we have now got into a point where we have made our starting material.

    02:28 UDP-glucose on the right. UDP-glucose on the left.

    02:30 The UDP-glucose on the left now continues the process metabolizing more lactose.

    02:35 So in this way all galactose can ultimately be converted into glycolytic intermediates.

    02:43 Now other sugars get metabolized, I have mentioned lactose. Lactose is milk sugar and it's metabolized in a very simple way.

    02:51 There is an enzyme called lactase and lactase breaks lactose into its constituent sugar which are galactose and glucose.

    02:59 You have seen how galactose is metabolized.

    03:02 Now lactase is an important enzyme; because, in some adults the reduced quantities of this enzyme leads to a phenomenon known as lactose intolerance.

    03:12 And lactose intolerance, if you ever had to deal with, that is pretty discomforting; because, it causes the bacteria in the stomach to be able to produce a lot of gas because they are working on lactose not galactose and glucose.

    About the Lecture

    The lecture Gluconeogenesis: Metabolism of Galactose by Kevin Ahern, PhD is from the course Carbohydrate Metabolism.

    Included Quiz Questions

    1. It is converted into an intermediate of glycolysis.
    2. It is combined with UDP-lactose.
    3. It requires lactase.
    4. It produces fructose.
    5. It first must be recombined with glucose to form lactose.
    1. Galactose-1-phosphate uridyl transferase: glucose-6-phosphate + GDP-glucose
    2. Lactase: lactose → glucose and galactose
    3. Galactose-1-phosphate uridyl transferase: galactose-1-phosphate + UDP-glucose → UDP-galactose + glucose-1-phosphate
    4. UDP-galactose-4-epimerase: UDP-galactose → UDP-glucose

    Author of lecture Gluconeogenesis: Metabolism of Galactose

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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    Thank you
    By Johnson O. on 13. July 2022 for Gluconeogenesis: Metabolism of Galactose

    Really a great and elaborate presentation. Thanks Prof. Kevin. Now I no longer struggle in cramming Biochemistry but rather find the subject interesting. I wish I started using your lecture guide early in Medical school, but any way not yet late! Thank you once again.

    Delivers needed information about metabolism of Galactose
    By Mohammed A. on 22. October 2020 for Gluconeogenesis: Metabolism of Galactose

    It's unfathomable how much this lecture has saved my time. Though I would appreciate a quick-summary of the products turning into intermediates for other cycles. Like with how g-1-p turns into g-6-p