Gluconeogenesis: Metabolism of Galactose

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.