Alpha/Beta Linkages

[MedPR]

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How do we tell if a linkage is alpha or beta in a polysaccharide? Also, should we memorize the types of linkages in specific disaccharides?

For example, EK says maltose has an alpha-1,4 glycosidic linkage (two glucose molecules).

Here is maltose:

It looks to me like the glucose on the left is alpha while the glucose on the right is beta.

Then EK says lactose has a beta-1,4 glycosidic (galactose and glucose)

Here is lactose:

To me the glucose (the one on the left, correct?) is beta, while the galactose is alpha...

So does the alpha/beta designation just come from whatever configuration the monosaccharide donating its anomeric carbon is in?

 

[typicalindian]

How do we tell if a linkage is alpha or beta in a polysaccharide? Also, should we memorize the types of linkages in specific disaccharides?

Alpha linkages stem down from the anomeric carbon and beta stem upwards. Only the orientation from the anomeric carbon determined the type of linkage. (up an down referring to the sugar in ring form)
Yes you should memorize a couple like lactose and sucrose and maltose. It's very simple and I'm sure your brain has enough space for 3 extra facts 🙂

Edit: you could equate alpha, which is down, to D in a fischer projection an beta to L.

 

[MedPR]

Alpha linkages stem down from the anomeric carbon and beta stem upwards. Only the orientation from the anomeric carbon determined the type of linkage. (up an down referring to the sugar in ring form)
Yes you should memorize a couple like lactose and sucrose and maltose. It's very simple and I'm sure your brain has enough space for 3 extra facts 🙂

Ok. I wasn't sure if it was something that they would give a picture of the structure and ask "what type of linkage?" or if they would just ask you how sucrose is linked and not even give you an image.

Also, EK says amylopectin and glycogen have the same type of linkages. How would we be able to distinguish amylopectin from glycogen? Both are a chain of alpha-1,4 with branches connected by alpha-1,6.

 

[typicalindian]

Ok. I wasn't sure if it was something that they would give a picture of the structure and ask "what type of linkage?" or if they would just ask you how sucrose is linked and not even give you an image.

Also, EK says amylopectin and glycogen have the same type of linkages. How would we be able to distinguish amylopectin from glycogen? Both are a chain of alpha-1,4 with branches connected by alpha-1,6.

You won't need to, but just as a fun fact(and iirc), glycogen has the alpha 1,6 every 6 or so sugars apart. Amylopectin has them about 30 sugars apart.

 

[typicalindian]

Oh and in your picture of lactose. There is a beta 1,4 linkage from beta D-galactose to beta D-glucose. The beta for glucose isn't determined from the linkage though, you have to loo at the anomeric carbon( far right carbon in the picture) to find it out.

 

[MedPR]

Oh and in your picture of lactose. There is a beta 1,4 linkage from L-galactose to L-glucose

What? So galactose on the left, glucose on the right? How can you tell which one is glucose and which one is galactose?

 

[MedPR]

Oh and in your picture of lactose. There is a beta 1,4 linkage from beta L-galactose to beta L-glucose. The beta for glucose isn't determined from the linkage though, you have to loo at the anomeric carbon( far right carbon in the picture) to find it out.

Yea in that picture isn't it D beta-glucose on the left and D alpha-galactose on the right?

 

[typicalindian]

What? So galactose on the left, glucose on the right? How can you tell which one is glucose and which one is galactose?

The long way is to convert from the Haworth form (the ring form) to the Fischer form. The short way is to just know it

Edit: here's a proper haworth projection of lactose, the one you have is flipped upside down and all convoluted for teaching

 

[MedPR]

The long way is to convert from the Haworth form (the ring form) to the Fischer form. The short way is to just know it

Edit: here's a proper haworth projection of lactose, the one you have is flipped upside down and all convoluted for teaching

Ah I just turned my laptop upside down and it was easy to see that the one on the right is GLUCOSE! (oops). Thanks.

 

[typicalindian]

Yeah also, my mistake! It's D-glucose not L. L sugars aren't used for things like this! Sorry

 

[typicalindian]

Ah I just turned my laptop upside down and it was easy to see that the one on the right is galactose. Thanks.

The one on the RIGHT is glucose not galactose haha. I'm hoping you made a typo

 

[MedPR]

Yeah also, my mistake! It's D-glucose not L. L sugars aren't used for things like this! Sorry

Ok so the haworth you just added is glucose-galactose right? And you know it's glucose on the right because of the direction the hydroxyls point on each carbon?

Glucose hydroxyls are either up/down on the anomeric (up=beta, down=alpha), then down, up, down?

 

[MedPR]

The long way is to convert from the Haworth form (the ring form) to the Fischer form. The short way is to just know it

Edit: here's a proper haworth projection of lactose, the one you have is flipped upside down and all convoluted for teaching

Beta-1,4 because the galactose is beta-galactose. The glucose shown here is alpha glucose though, correct?

 

[typicalindian]

Ok so the haworth you just added is glucose-galactose right? And you know it's glucose on the right because of the direction the hydroxyls point on each carbon?

Glucose hydroxyls are either up/down on the anomeric (up=beta, down=alpha), then down, up, down?

No. The image I added is galactose-glucose. You can only technically put alpha or beta terminology to the anomeric carbon. Not the other ones. But yes for glucose it's normally. When you convert from Fischer to haworth, the L is up and D is down. Look up the Fischer projection of glucose and compare it to the Haworth. You'll see what I mean. I'm hoping this helps haha I'm in class and trying to take notes while writing these

 

[typicalindian]

Beta-1,4 because the galactose is beta-galactose. The glucose shown here is alpha glucose though, correct?

Yes! You got it 🙂

 

[MedPR]

No. The image I added is galactose-glucose. You can only technically put alpha or beta terminology to the anomeric carbon. Not the other ones. But yes for glucose it's normally. When you convert from Fischer to haworth, the L is up and D is down. Look up the Fischer projection of glucose and compare it to the Haworth. You'll see what I mean. I'm hoping this helps haha I'm in class and trying to take notes while writing these

Yea galactose-glucose, sorry that's what I meant.

I don't really know what L and D are. I assume they aren't referencing the anomeric carbon since you can have alpha-L and alpha-D glucose (same for beta).. Beta glucose is anomeric carbon has hydroxyl up, alpha is hydroxyl down.

Now really sure what I should be looking for to tell whether it is L or D though.

Beta-D

Beta-L

It looks like L and D are just mirror images..??

 

[typicalindian]

Nah if you really wanna know, D and L are enantiomers of each other. D sugars are used in biological processes because they naturally occur.

 

[MedPR]

Nah if you really wanna know, D and L are enantiomers of each other. D sugars are used in biological processes because they naturally occur.

Ok so they are just mirror images! Thanks 🙂

 

[dudewheresmymd]

No. The image I added is galactose-glucose. You can only technically put alpha or beta terminology to the anomeric carbon. Not the other ones. But yes for glucose it's normally. When you convert from Fischer to haworth, the L is up and D is down. Look up the Fischer projection of glucose and compare it to the Haworth. You'll see what I mean. I'm hoping this helps haha I'm in class and trying to take notes while writing these

so for lactose, or any disaccharide, can we always just look at the sugar on the left and examine the anomeric carbon to determine if it is alpha or beta? (In this case beta galactose)?

 

[Czarcasm]

View attachment 180389

so for lactose, or any disaccharide, can we always just look at the sugar on the left and examine the anomeric carbon to determine if it is alpha or beta? (In this case beta galactose)?

A linkage can be a-1,b-2 as in Sucrose indicating that the linkage is between two anomeric carbons and therefore a nonreducing sugar, or it can be at any other position and anomer as in an alpha-1,4 linkage. In this case, the anomeric carbon hydroxyl group is in a bond with the 4th hydroxide group of the 4th carbon which is non-anomeric and therefore you don't need to specificy alpha or beta. Moreover, this would also indicate that the anomeric carbon of the second sugar is free, not in a bond (a reducing sugar). The key to answering these types of questions is just realizing what an anomer is, how to identify them, and being able to count the carbons (in the appropriate order) as well as knowing how they would relate to the fischer projection.

 

[dudewheresmymd]

A linkage can be a-1,b-2 as in Sucrose indicating that the linkage is between two anomeric carbons and therefore a nonreducing sugar, or it can be at any other position and anomer as in an alpha-1,4 linkage. In this case, the anomeric carbon hydroxyl group is in a bond with the 4th hydroxide group of the 4th carbon which is non-anomeric and therefore you don't need to specificy alpha or beta. Moreover, this would also indicate that the anomeric carbon of the second sugar is free, not in a bond (a reducing sugar). The key to answering these types of questions is just realizing what an anomer is, how to identify them, and being able to count the carbons (in the appropriate order) as well as knowing how they would relate to the fischer projection.

so in sucrose you'd specify both alpha and beta for both positions? whereas in lactose you don't by convention? TBR says that lactose has a Beta 1-4 linkage is this b/c the galactose is Beta and the glucose doesn't matter?

also there was a crazy question regarding knowing the numbering system of steroids...is this reasonable?

 

[Czarcasm]

so in sucrose you'd specify both alpha and beta for both positions? whereas in lactose you don't by convention? TBR says that lactose has a Beta 1-4 linkage is this b/c the galactose is Beta and the glucose doesn't matter?

also there was a crazy question regarding knowing the numbering system of steroids...is this reasonable?

Right, in Lactose, only 1 anomer is in the glycosidic linkage whereas in Sucrose, both are and need to be specified. As far as cholesterol numbering is concerned, that was nothing ever covered in any undergraduate class and definitely not something we need to know. Classic TBR for ya.

 

[typicalindian]

View attachment 180389

so for lactose, or any disaccharide, can we always just look at the sugar on the left and examine the anomeric carbon to determine if it is alpha or beta? (In this case beta galactose)?

haha I have no clue anymore man. I've forgotten this stuff already.