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"Glycogen branching is essential because it allows for increased water solubility and several sites to break it down; this allows for easy and quick glycogen utilization when it is broken down" (Daghlas and Mohiuddin 2022).

According, to my textbook "the extensively branched structure of glycogen fits its function: more free ends are available for hydrolysis".

Firstly, does hydrolysis not occur between covalently bonded monomers and not require 'free ends'?

Secondly, both quotes imply that a unbranched polymer consisting of a specific number of glucose monomers, would be harder to hydrolyse compared to a branched polymer consisting of the exact same number of glucose monomers. I do not understand why.

I do believe I understand that a single glucose molecule/monomer (shown below) has more polar groups (5 C—OH groups and 1 C—O bond for a single glucose molecule, versus 4 C—OH groups and 1.5 C—O bonds in a monomer of glucose in a maltose molecule, ultimately, there are 6 versus 5.5 oxygen atoms per glucose monomer), compared to a glucose monomer in a polysaccharide (such as maltose shown below), linked to another glucose monomer via a dehydration reaction (polar H2O molecule is lost, contributed by two glucose monomers).

Glucose:

enter image description here

Maltose: enter image description here

However, the first quote stated branching increases water solubility. Would a branched glucose polymer not be as polar (soluble) as an unbranched glucose polymer of the same size? If we had two polymers, both made of three glucose molecules and one polymer had two 1-4 glyosidic linkages, like this:

enter image description here

Would it not have the same solubility as a polymer consisting of three glucose monomers, with one 1-4 and 1-6 glyosidic linkage? Similar to this:

enter image description here

Since both molecules would have the same number of oxygen atoms (3 glucose molecules - 2 H2O molecules) and hydroxyl groups and thus, both molecules would have the same polarity. This would apply to larger molecules of the same size, both with more branches and longer branches. I would expect unbranched and branched glucose polymers of the same size, to have the same number of polar groups.

I assume a more soluble molecule would attract water molecules 'better' thus, making its own hydrolysis 'easier'.

Finally, keep in mind I have only an introductory understanding of chemistry, as I am a 2nd year Biology student at Univeristy.

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    $\begingroup$ To your first question regarding the branching: visualize (or draw) a linear polymer of glucose, and compare it with a highly branched polymer. The purpose of glycogen hydrolysis is to get glucose molecules. Imagine you have a lot of enzyme molecules. How many glucose molecules can be produced "at once" from a linear polymer, and how many from a branched one? In a linear polymer, you have only two ends, and only two molecules can be produced at once. However, in a branched polymer, you have a lot of "free ends" so many glucose molecules can be produced in the same time. $\endgroup$
    – Domen
    Commented Jan 30, 2023 at 15:28
  • $\begingroup$ I guess this question comes from not learning that enzymes need 'free ends' to speed up glucose hydrolysis. $\endgroup$
    – Growing6884
    Commented Jan 31, 2023 at 2:31
  • $\begingroup$ Well, not all of them. Glycoside hydrolases can be classified as either endohydrolases or exohydrolases. $\endgroup$
    – Domen
    Commented Jan 31, 2023 at 14:12
  • $\begingroup$ Ok, then I would assume an endohydrolase would be able to hydrolyse branched and unbranched sugar polymers of the same size, at the same rate. $\endgroup$
    – Growing6884
    Commented Feb 1, 2023 at 0:52
  • $\begingroup$ Well, probably not. Imagine a large, very branched polymer. It is hard for an enzyme to access monomers that are located near the center of branching, because they are buried deeply in the branched structure. $\endgroup$
    – Domen
    Commented Feb 1, 2023 at 13:23

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