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In glycogenesis (i.e. the synthesis of glycogen), are sugar phosphates the direct substrates for glycogen polymerization? I would certainly think so since glucose is phosphorylated and then stuck onto a glycogen chain, but my textbook seems to think no.

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    $\begingroup$ What exactly makes you say that your "textbook seems to think no?" What is the textbook? Sugar phosphates are, by definition, substrates of glycogenesis. $\endgroup$ May 15, 2016 at 22:32
  • $\begingroup$ It must be a typo then...it's a true or false questions and that is the literal question "In glycogenesis, are sugar phosphates are substrates for polymerization?" and the answer is F $\endgroup$ May 15, 2016 at 22:44
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    $\begingroup$ @user2879934 phosphorylated glucose is not the direct substrates for polymerization... for glycogen synthesis UDP-conjugated glucose molecules are used, not glucose-phosphate (maybe this is the distinction that your book is trying to make) UDP-glucose is different than glucose-phosphate (in my opinion a nit-picky question... but these are the sorts of questions found in textbooks all the time - the book is trying to be tricky) $\endgroup$ May 16, 2016 at 2:06
  • $\begingroup$ @VanceLAlbaugh so basically phospohrylated glucose + uracil, so it's false because no uracil? $\endgroup$ May 16, 2016 at 15:15
  • $\begingroup$ The books wants to say "what is the substrate for glycogen synthase?" You often end up with really vague questions in the more elementary texts, however, so they (the publisher) thinks you know what they mean by "what's the substrate for glycogenesis?" Really, and a quick fact check via Google or what have you would confirm, glycogen synthase is more formally a UDP-glucose-glycogen glucosyltransferase. The substrate for the enzyme is therefore UDP-glucose, and the function is that of a glycosyltransferase, just very specifically utilizing an activated glucose moiety. $\endgroup$
    – CKM
    May 16, 2016 at 18:38

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A pathway doesn’t have substrates, only a reaction does. And therefore there’s no such thing as a direct or indirect substrate. You can perhaps ask what the precursors are for glycogen (but do you mean immediate or distal), or what the substrates are for the first reaction in the pathway (but only if it is quite clear what the first reaction i).

That’s the trouble with trick questions. They are generally badly worded and tend to lend nothing to biology education.

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