Tuesday, March 18, 2008

Are You as Smart as a Third Year University Student? Q6

 
Question 1
Question 2
Question 3
Question 4
Question 5
Here are the names and structures of the aldohexoses. The common ones in biology are shown in blue.


Identify the monosaccharide shown below in three different views.



               a) D-allose
               b) D-altrose
               c) D-galactose
               d) D-glucose
               e) D-mannose


5 comments :

  1. Why have you drawn them in their open-chain forms? That's not the way they are in biochemical systems.

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  2. athel asks,

    Why have you drawn them in their open-chain forms? That's not the way they are in biochemical systems.

    Actually, there are quite a few examples of enzymes that bind sugars in the open chain conformation. I teach my students that there's an equilibrium between all of the various conformations. They all exist inside the cell.

    But that's not the reason for showing those conformations in this exam question. The point of the question is to determine whether my students understand the relationship between the way we depict molecules on a piece of paper and the way they look in three-dimensional space.

    Biochemistry is a three-dimensional science so this is important.

    Athel, would you like to take a guess at the correct answer? If you get it wrong we can discuss the basic concepts of Fischer projections and naming conventions.

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  3. ALL ALTRuists GLadly MAke GUm In GALlon TAnks...

    Must be D-Gal (c).

    ReplyDelete
  4. D-manose, two -oh in one side, and two -oh in the other.

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  5. Larry asked,

    Athel, would you like to take a guess at the correct answer? If you get it wrong we can discuss the basic concepts of Fischer projections and naming conventions.

    Now you want me to do some work! I think I understand the naming conventions OK, but visualizing your 3D views and converting them into Fischer projections would take all day, because I'm very poor at problems of 3D perception. It would be easy enough to do with some ball and stick models, but I haven't got any of those to hand.

    Actually, I wonder if you are really testing biochemistry in this question, or if you're testing ability to visualize things in 3D -- one can be very good at the latter without needing to know any biochemistry.

    Your answer,

    Actually, there are quite a few examples of enzymes that bind sugars in the open chain conformation...

    is, of course, true, but the question said nothing about enzymes. I think that to understand the properties of (say) glucose you need to understand that it is not an aldehyde but a hydrated hemiacetal, and so I'm not convinced that the fact that small amounts of the open-chain forms exist in living systems is a good reason for implying that they are predominant.

    Although your Fischer projections satisfy the IUPAC rules on stereochemistry ("The first of the two types of Fischer projection should be used whenever convenient" -- i.e. you should draw it as you did, with explicit carbon atoms, rather than with a cross) I think this recommendation was badly thought out, because the only way readers know that it's supposed to be a Fischer projection is because you tell them. Otherwise it could just contain topological information with no information about stereochemistry implied.

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