I have been taught that yeast can act only on simple sugars (monosaccharides) and breaks them down into ethanol and CO2

I also know that yeast can break down maltose (disaccharide) and maltotriose (trisaccharide) using maltase from its enzyme complex (zymase) into monosaccharides of glucose and then ferments the same.

My question is whether yeast can directly ferment maltose and maltriose WITHOUT first breaking them down into glucose? If so how is it done - enzymatically and what is the chemical equation


The statement in your first paragraph is correct: for yeast to ferment a carbohydrate it must be fed into the glycolytic pathway which, in the case of oligomers of glucose (e.g. mannose) simply means breaking the oligomer into its constituent glucose monomers using a single enzyme. This is similar to how yeast processes sucrose: the disaccharide is hydrolysed by invertase to release glucose (fed directly into glycolysis) and fructose (fed into glycolysis via fructokinase).

A separate mannose pathway would require a completely new set of enzymes that somehow processed mannose in a stepwise fashion without breaking the glycosidic bond linking the two monomers - I'm sure that you can see that this is a much less likely option in terms of the evolution of yeast metabolism.


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