Importance of stereoisomers in a biological system Isomers are compounds that have the same molecular formula but different structural formulas. Stereoisomers are isomers that have the same sequence of bonded atoms, but they differ in their three dimensional orientation in space. [pic] shown above is an example of the two types of Stereoisomers; Enantiomers, which are stereioisomers which are non-superimposible mirror images, much the same as one’s left and right hands are the same apart from the fact they have opposite orientation.
Diastereomers, which are stereoisomers that are not Enatiomers, they occur when two or more stereoisomers of a compound have different configurations at one or more, (but not all) of the stereocenters and are not mirror images. Stereochemistry , may seem trivial at times due to the differences between stereoisomers being so subtle. However in nature, and more importantly, in a biological system such as the body, the subtle differences have wide sweeping implications. In living organisms chiral molecules are usually present in only one of their chiral forms.
For Example the amino acids that make up proteins are only found as their L iosmers whereas glucose only occurs as its D isomer. Evolution has played a large role in this fact by favouring one isomer over the other. This concept is easier to comprehend when you remember that the molecules that select an isomer to use (invariably proteins) are themselves isomers. Therefore it is not surprising that they have a ‘built in bias’. Thus, it can be said that isomer are important due to the fact that our entire biology, and that of every organism on the planet, is built on them.
Biological interations between molecules are stereospecific, The ‘fit’ in such interations must be correct. A good example of this is the ‘Lock-and-key’ and induced fit models for enzyme function. Enzymes are very specific only operating on substrate that accurately fits the shape of the binding site of the enzyme. If the binding site fits, for example the L stereoisomer of a chiral compound then it will not fit the D seteroisomer. Another good example of the importance of stereochemistry is pharmaceutical production and the break down of drugs in the body.
Most drugs are often composed of a single stereoisomer of a compound, and while one stereoisomer may have positive effects on the body the other may have negative effects. The Thalidomide crisis in the 1950’s/1960’s shows this well. Originally Prescribed as a sleeping pill it was found to lessen the effects of morning sickness in pregnant women. However, it was quickly discovered that the drug was a teratogen, causing varying birth defects in the child. As a result it was withdrawn from sale.
It was found that the drug was prescribed as a racemic mixture, meaning it contained a 50:50 mixture of mirror images (stereoisomers). Despite one stereoisomer being effective in controlling morning sickness the other was causing the deffects. Because of this fact mass amounts of research has gone into finding ways to produce compounds that are purely one stereoisomer. But not all stereoisomers are in such sharp contrast (Positive/Negative). In most case both stereoisomers are positive but one may find that one is substantially more effective than the other, such is the case with the astma drug Isoproterenol.
Although both stereoisomers are effective it is found that the D-Isoproterenol is 50-80 times more effective than L-Isoproterenol and thus can be administered at greatly reduced doses. The importance of stereoisomers in a biological system extends to more than just drugs. Our bodies can make and digest starch but not cellulose despite both being polymers of glucose, however they have differing stereochemistry. These are just a few of the numerous example of the important role stereoisomers play in a biological system and in our everyday lives.
