October 17, 2021, 09:31:58 PM
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Topic: Galactose —> glucose  (Read 196 times)

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Offline MurdochShewrote

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Galactose —> glucose
« on: October 08, 2021, 04:58:17 PM »
Hi guys

My tutor explained how an isolated monomer of alpha glucose can interconvert (dynamic equilibrium) between alpha and beta glucose until its isomer is formalised through covalent bonding with another monomer.  Looking at the structure of galactose, I’ve noticed that the only difference between it and glucose is between the position of the hydroxyl group on the C4.  If isolated alpha/Beta glucose can convert due to the fluid position of the hydroxyl group on C1, does this mean that galactose and glucose can convert into one and other?  Instinctively I feel as though they wouldn’t be able to – but can’t find anything to suggest so.

Has anyone got any thoughts?

Offline Babcock_Hall

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Re: Galactose —> glucose
« Reply #1 on: October 08, 2021, 05:05:47 PM »
Your instinct is correct.  Do you know the mechanism of how α-glucose and β-glucose interconvert?  Hint: the key intermediate is a free aldehyde.  Can this same mechanism occur at the 4-position of glucose?

Offline MurdochShewrote

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Re: Galactose —> glucose
« Reply #2 on: October 08, 2021, 05:11:35 PM »
Your instinct is correct.  Do you know the mechanism of how α-glucose and β-glucose interconvert?  Hint: the key intermediate is a free aldehyde.  Can this same mechanism occur at the 4-position of glucose?

I’m not familiar with the mechanism; I’m a freshman biochemistry major. 

My question was more that, whilst I know that alpha and beta glucose can interconvert dynamically, since galactose and glucose are so similar- why doesn’t galactose and glucose interconvert in the same way? The same rotation of the hydroxyl group on C4, should surely rotate on the C1. 

Any help you could offer would be appreciated

Offline Babcock_Hall

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Re: Galactose —> glucose
« Reply #3 on: October 08, 2021, 05:22:47 PM »
https://www.youtube.com/watch?v=EMEtL9Pk94U
https://www.masterorganicchemistry.com/2017/08/17/mutarotation/

The mechanisms presented in the video are slightly different from the mechanism with which I am familiar.  However, I don't believe that the exact mechanism is as important as the difference between carbon-1 and the other carbon atoms of glucose.  Only carbon-1 makes two bonds to oxygen atoms.

The mechanism by which an enzyme interconverts a derivative of glucose with a derivative of galactose is very interesting in its own right, but perhaps we should put that off for a bit.  For now suffice it to say that an enzyme is necessary.

Offline MurdochShewrote

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Re: Galactose —> glucose
« Reply #4 on: October 09, 2021, 02:15:04 AM »
https://www.youtube.com/watch?v=EMEtL9Pk94U
https://www.masterorganicchemistry.com/2017/08/17/mutarotation/

The mechanisms presented in the video are slightly different from the mechanism with which I am familiar.  However, I don't believe that the exact mechanism is as important as the difference between carbon-1 and the other carbon atoms of glucose.  Only carbon-1 makes two bonds to oxygen atoms.

The mechanism by which an enzyme interconverts a derivative of glucose with a derivative of galactose is very interesting in its own right, but perhaps we should put that off for a bit.  For now suffice it to say that an enzyme is necessary.

Ah, that’s great - thank you. Rather naively, I hadn’t realised that a mechanism was involved.  I assumed that it was just a physical rotation rotation.

With reference to and without a pictorial mechanism of galactose and glucose, why can’t/ doesn’t mutarotation occur on C1 of glucose thus converting to galactose

Thanks

Offline Babcock_Hall

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Re: Galactose —> glucose
« Reply #5 on: October 09, 2021, 01:26:58 PM »

The mechanism by which an enzyme interconverts a derivative of glucose with a derivative of galactose is very interesting in its own right, but perhaps we should put that off for a bit.  For now suffice it to say that an enzyme is necessary.

Ah, that’s great - thank you. Rather naively, I hadn’t realised that a mechanism was involved.  I assumed that it was just a physical rotation rotation.

With reference to and without a pictorial mechanism of galactose and glucose, why can’t/ doesn’t mutarotation occur on C1 of glucose thus converting to galactose

Thanks
[/quote]
With respect to the interconversions of the alpha and beta forms of glucose, two of the three mechanisms I have seen show a transient opening of the ring.  That is not possible at the C-4 position of glucose.

The compound uridine diphosphoglucose can be turned into uridine diphosphogalactose by briefly oxidizing C-4 (a hydride ion is removed).  Then the hydride ion is put back onto the opposite face of the molecule.  Bonds have to be made and broken to interconvert the two compounds.  The enzyme that does is is called UDP-galactose 4-epimerase.

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