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I just started learning about sugar metabolism, when I saw how other monosaccharides enter the glycolytic pathway. Mannose enters glycolysis as fructose-6-phosphate at the expense of 1 ATP, then pretty much follows the same reactions as glucose. On the other hand, Galactose enters glycolysis as Glucose-6-phosphate then would also follow the same reactions as glucose. Does this mean that both of these monosaccharides yield the same products as Glucose? Or is there something I am missing?

I apologize if the answer to this is obvious; I can't seem to find online references that give the net yields for other monosaccharides, so I just wanted to be sure.

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The answer to this question is yes. And it is obvious to anyone familiar with metabolism. I hesitated to give a formal answer, but I hate answers in comments, so let me supplement this with some ‘principles’ (in so far as there are any principles in biology) for those less familiar with metabolism (as we all were once).

  1. To a cell, fructose 6-phosphate is fructose 6-phosphate, so its metabolic fate is likely to be the same, regardless of its source. It will be dealt with according to the activity of the enzymes that can catalyse the reactions of F6-P. (These may, however, differ in different cell compartments, as enzymes and concentrations of reactants may differ there.)

  2. Central metabolic pathways such as glycolysis are complex and highly evolved to do their job well. They are found throughout all kingdoms of life†. It is economical to feed minor substrates into them, rather than duplicate a full pathway. Hence the additional pathways for dealing with minor sugars will be those converting them to intermediates of glycolysis, and it is these where metabolic diseases of sugar metabolism are found.

† This doesn’t mean that they are all present in every organism or in every cell type of a multicellular organism. Just that their distribution is such that that (supplemented by other arguments) we consider them to have appeared early in evolution.

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  • $\begingroup$ Thank you for answering sir, I actually tried to examine how other sugars aside from glucose enter glycolysis and initially thought that there should be no difference in their yields as compared to glucose, but I got confused since I saw a couple of online resources that says galactose yields 0 ATP in glycolysis. $\endgroup$ May 14 at 9:16
  • $\begingroup$ @JoshArceno — You are welcome. I imagine that you must have misunderstood the articles you mention, as galactose is half of the disaccharide, lactose. As the function of lactose is to provide nutrition for sucklings, common sense would suggest galactose oxidation would result in net generation of ATP. $\endgroup$
    – David
    May 14 at 18:32

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