ID #1606

Is a dissacharide broken off of a starch molecule, known as cellobiose, and a dissacharide broken from a glycogen is known as maltose. If this is true, then why is a beta 1,4 glycosidic linkage present in cellobiose, and how can the second glucose in maltose be a beta glucose?

Maltose is in fact a disaccharide of glucose joined by an alpha-1,4 glycosidic linkage. And in the figure I showed in class, that glucose molecule donating its #4 carbon to the linkage happens to be in the beta configuration. Since starch and glycogen are all alpha-1,4 linkages, if this beta glucose molecule stayed in its current configuration, it wouldn't be glycogen or starch if the "next" glucose was added. But remember 2 very important things. First, the answer to your first question was that alpha and beta configurations readily interconvert. Second, enzyme are exquisitely specific about the reactions they catalyze. So the enzyme responsible for synthesizing starch, for example, wouldn't recognize that disaccharide as a substrate for lengthening that chain on the "right" as it's been drawn. But... that beta glucose will most likely become alpha glucose very soon, and then it would be an appropriate substrate. That maltose molecule could just as easily have been drawn with two alpha glucoses, but was probably drawn with the "second" glucose in the beta configuration to hammer home the point that for naming this linkage, we just don't care what configuration it's in. That is, until it becomes the molecule donating its #1 carbon to the next glycosidic linkage, and then it means everything!

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