[constant thinker]

This is a random question I was thinking about...

What causes fructose and galactose to be produced instead of glucose?

Mainly I just want to know what is going on for those 2 sugars to be produced. There must be a difference compared to glucose production to cause the different isomers. I know it can't just be a random occurrence here and there when you consider that sugar is sucrose, and milk has lactose.

[Yggdrasil]

In humans, both fructose and galactose can be obtained from diet.  They can also be synthesized from glucose.  The enzyme UDP-Gal-4-epimerase can convert a nucleoside-activated glucose (UDP-Glc) to UDP-Gal.  The epimerase works by oxidizing the equatorial OH on carbon 4 to a ketone, then stereospecifically reducing the ketone to a hydroxyl in the axial position.

Fructose is made from glucose in glycolysis.  The enzyme, glucophosphate isomerase converts glucose-6-phosphate to fructose-6-phosphate.  Here, the reaction proceeds through an enol intermediate.

[constant thinker]

Thank you Yggdrasil.