Practice Problem 2

Glucose and fructose have the same formula: C₆H₁₂O₆. Glucose is the sugar with the highest concentration in the bloodstream; fructose is found in fruit and honey. Use the Fischer projections in the figure of common monosaccharides to explain the difference between the structures of these compounds. Predict what an enzyme would have to do to convert glucose into fructose, or vice versa.

Solution

Both compounds have the same structure for the third, fourth, fifth, and sixth carbon atoms. One of them, however, is an aldehyde (glucose). while the other is a ketone (fructose). Glucose could be converted into fructose by simultaneously reducing the first carbon atom from an aldehyde to a primary alcohol and oxidizing the second carbon from a secondary alcohol to a ketone. Fructose can be converted to glucose, on the other hand, by simultaneously oxidizing the first carbon atom and reducing the second.