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Stereoisomers and biological functions?

  1. Jun 27, 2012 #1
    I'm reading a book concerning cell biology and have a question. Some molecules are mirror images of one another and such molecules are called optical isomers, or stereoisomers. The different stereoisomers of a molecule usually have completely different biological activities. Now in biology, function always depends on structure. Different stereoisomers of a molecule are basically the inverted form of one another (I've assumed that since its said that they are mirror images of one another) or in other words have the same structure. Why then do they perform different biological functions?
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  3. Jun 27, 2012 #2


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    A quick explanation that a lecturer once gave me regarding chiralty; your right foot is a mirror image of your left, so can you fit it in your left shoe?
  4. Jun 27, 2012 #3


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  5. Jun 27, 2012 #4
    Many molecules performing their using "lock and key" chemical reactions. In order for the reaction to take place, one molecule has to geometrically "fit" into the other. The shape of a site on one molecule has to fit exactly into the space site provided in the other molecule before the reaction takes place.
    There are some analogies that could be useful. A "lock and key" reaction is like a key fitting into a lock. Or it could be like a foot fitting into a shoe. Reversing the chirality of one molecule and not the other could make the reaction impossible.
    Very often, reversing the chirality of one molecule can make one reaction impossible but produce likelihood of another reaction.
    For example, amino acids are used to make proteins or provide energy. The "normal" amino acids usually taste sour or bitter. Reversing the chirality of one amino acid could make it useless for making proteins or providing energy. However, the reversed amino acid tastes sweet. The reversal allows certain reactions on the tongue that produce the sweet taste. This is why a lot of artificial sweeteners are made of chiral reversed amino acids.
  6. Jun 28, 2012 #5
    Bold: Here's where you're mixed up. Mirror images do not have the same structure. Ryan's explanation is spot on. You can't put a right-handed glove on your left hand.
  7. Jun 28, 2012 #6
    It is because most enzymes work on the reaction molecules by a lock and key mechanism. If the reaction molecule doesn't fit precisely in the active site of the enzyme molecule, the enzyme can't speed up the reaction it is supposed to speed up. If the structure of the enzyme is changed, even by only reversing the chirality, the reactions effected by the enzyme are going to change.
    Here is a link to article explaining the lock and key model. There is a lot more to the theory, but the basic idea presented by Emil Fischer is sufficient to qualitatively explain the importance of chirality.
    “Enzymes are very specific, and it was suggested by the Nobel laureate organic chemist Emil Fischer in 1894 that this was because both the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another.[29] This is often referred to as "the lock and key" model. However, while this model explains enzyme specificity, it fails to explain the stabilization of the transition state that enzymes achieve.”
  8. Jun 28, 2012 #7


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    I wouldn't call that a 'failure' like there was something wrong with it. Specificity is one problem and mechanism of catalysis is another.

    On a second look though the two are connected in an under-appreciated way IMO.

    In a large number of biochemical reactions you will find large parts of the participating molecules seem to have nothing to do with the reaction and seem to make no chemical sense. Rather repulsive inessentials to the minds of physicists and chemists. Say the ADPribose part of NAD, NADP in dehydrogenases. They do serve as specific anchors that hold a molecule in place when it is being attacked so that the attack can go through instead of pushing the attacked group out. In this way parts of the reacting molecule that are remote from the reacting group do play a part in the reaction.

    Other parts of their explanation will be very ancient evolutionary history, and that explanation is not in yet.
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