Why is glycogen synthesized from UDP?

[sameeralord]

Why is it that in the glycogen synthesis UDP glucose is added to glucose. Does UDP glucose provide energy? Or is it ATP that provides energy for this polymerisation. I don't understand why you need UDP? Thanks!

 

[Ygggdrasil]

In almost all coupling reactions, you need a source of energy to counter the loss of entropy from linking two molecules together. For example, in DNA synthesis, the loss of the pyrophosphate from the incomming nucleotide triposphate powers the reaction. In the case of glycogen synthesis, the energy from the hydrolysis of UTP is stored in UDP-glucose and the loss of UDP helps to power the reaction.

 

[sameeralord]

Hey thanks Ygggdrasil for your response. That makes a lot of sense but can a removal of UDP provide energy. I thought phosphates must be removed to get energy. My knowledge of this is poor so please help me. Thanks! Also in the first reaction where UTP is used to make UDP glucose and PPi. Is energy released or does the PPi have to get broken down by water into 2Pi for energy to be released. I'm with this idea you have to remove a phosphate to get energy.

 

[Ygggdrasil]

You don't need the removal of a phosphate to get energy. You can basically think of favorable chemical reactions as ones that break an unstable bond to form a more stable bond. For example, the bond between the second (beta) and third (gamma) phosphates in ATP is unstable because of the repulsion between the two negatively charged phosphate groups. Kicking off the gamma phosphate and replacing it with an OH group from water creates a much more stable bond.

In a similar fashion, the bond between glucose and UDP is a fairly unstable bond. Therefore, kicking off the UDP and replacing it with a bond to the OH group from another glucose molecule occurs favorably. In the language of organic chemistry, the UDP acts as a good leaving group and is required for the reaction with the growing glycogen chain.

UTP is required because it is even more unstable than UDP-glucose. The UDP-glucose bond can form because it is more stable than the bond between the beta and gamma phosphates in UTP. Note that the reaction is UTP + glucose --> UDP-glucose + phosphate (so no pyrophosphate is produced).

Kicking off the OH group from a normal glucose molecule to form a bond with the OH group of another glucose molecule would not occur favorably, which is why UDP-glucose is required for glycogen synthesis in the first place.

 

[sameeralord]

Excellent. Thanks