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Questions about carbohydrates!!

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kennywhite:
Hi guys! I've got 3 questions I've been trying to think about and tackle. Was hoping the Ochem community could offer insight on this.

In my ochem class, we learned Lactose intolerance is a very widespread metabolic defect, while Maltose intolerance is a very rare condition. I don't understand why lactose intolerance has a higher incidence. Does it have to do with the fact that for lactose, there is a greater deficiency of beta galactosidase for lactose compared to maltose? Is maltose an alpha glycoside?

Regarding sugar in general, I was curious. So I know all D carbohydrates can't be dextrorotatory. Can a glycoside of a dextrorotatory monosaccharide be levorotatory? It can right? Thanks so much in advance!

hollytara:
You have to think both biologically and chemically to understand the intolerance issue.  Lactose intolerance is NOT a metabolic defect.  Lactose intolerance is the default condition for adult mammals, as adult mammals generally will not be using milk as a source of nutrition.  Certain human populations developed a mutation where lactase (the enzyme that breaks down lactose) activity is maintained into adulthood.  This mutation was selected for when combined with the domestication of cows, goats and sheep - it provided an additional source of nutrition to those populations that helped them to survive. 

Now lactose is an unusual disacchcarbohydrate for animals as it has a beta glycosidic linkage. That is why it can't be digested by other hydrolase enzymes that hydrolyze alpha linkages.  Maltase is one of these - maltose is a disaccharide of two glucose units with an alpha linkage.  Starch is a polymer of glucose with alpha linkages.  So the major food source for herbivorous mammals (starch) requires an alpha hydrolase - so even if there is some defect in maltase production there are other enzymes that can also do the hydrolysis. 

Cellulose has beta linkages - animals can not digest it directly.  Those animals that use cellulose as a food source have symbiotic bacteria in their digestive systems that break it down using beta hydrolases. 

Yes - the glycoside of a dextrorotatory monosaccharide could be levorotatory.  It depends on what alcohol is being used for the glycoside. 

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