Why on earth are five-carbon sugars unfermentable?

Last week we received the following query from Arthur Black of National Wine & Spirits in Indianapolis:
 
“Can anyone tell me why yeasts can only metabolize the six-carbon sugars glucose and fructose, while five-carbon sugars like ribose and xylose are unfermentable? I know the latter two make up an exceptionally small amount of overall dissolvable solids in grapes, but I am still curious!”

Now I know that most of you can live without knowing the answer to this burning question, but for the sake of advanced wine students everywhere, I’m publishing the following response from research scientist Forbes...