Re: Why are Isomers important to life?

Introduction

Chemical isomers are molecules that have the same chemical formula (the
same number and types of atoms) but have different structural formulae
(different arrangements of those atoms).  In general, isomers fall into two
broad categories:  structural isomers and stereoisomers.

Structural isomers, are molecules in which the atoms are arranged in
different patterns about the bonds.  The bonds, themselves may also be
different.  The difference in structural formulae can be simple - subtle
changes in the arrangements around common bonds, such as with iso-propanol
and n-propanol (Figure 1, I/II) – or more obvious, such as with propanol
and methyl ethyl ether (Figure 1, I/II vs. III).  


Figure 1:  n-propanol (I), isopropanol (II), methyl ethyl ether (III)

Stereoisomers isomers take several forms but have one property in common. 
They are isomers in which the pattern of bonds is the same, only the
geometric positioning of the atoms differs.  Stereoisomers will commonly
fall into two types:  those with geometric variations about a double bond
(called ‘cis-trans isomers’) and those with geometric changes in the
positioning of substituent atoms about a core atom (called enantiomers). 
There are also conformational and rotational isomers, which I won’t discuss
here.  A common biological example of cis-trans isomers are the all-trans
retinal and 11-cis retinal (Figure 2), while alpha- and beta- D-glucose are
common forms of enantiomeric isomers (Figure 3). 


Figure 2:  Retinal isomers, cis-11 retinal (a), all-trans retinal (b)


Figure 3:  Glucose enantiomers, beta-D-glucose and alpha-D-glucose.
 Note the different arrangement of the atoms around the carbon attached to
the green and red oxygens.

Furthermore, a very large molecule with a complex three dimensional shape,
may have the ability to take on multiple ‘shapes’ due to its flexibility. 
In such a case, the different forms are called ‘topoisomers’ (topological
isomers).  These are most common in polymer and protein chemistry.

And now…the answer

Isomers of all types are biologically abundant.  Their importance comes,
primarily, from two significant facts.  First, virtually every biologically
important molecule has one or more isomers.  This is due, in part, to the
necessary complexity of those molecules, which require numerous different
types of atoms (a minimun of carbon, oxygen, hydrogen and often nitrogen,
phosphorus and others) and will commonly contain a mixture of single and
double bonds.  Second, evolution, in almost all cases, has favoured the use
of one isomer for a given purpose over the use of the others that exist. 
This second fact is more easily understood if you think that the molecules
selecting the isomer to use (invariably proteins of some kind), are also
isomers themselves and therefore it is not surprising that they have a
‘built-in’ bias.

Thus, isomers are important because our entire biology, and that of every
organism on the planet, is built on them.  This dependence come in many
forms, but invades every aspect of our body as you can see from some
examples below.  

Our vision requires a protein called  rhodopsin (itself an isomer, as are
all proteins).  Rhodopsin, however, has a small helper molecule -- cis-11
retinal, which is made from Vitamin A.  Cis-11 retinal is converted to
all-trans retinal (Figure 4) when exposed to light.  Therefore, the light
energy is absorbed during the retinal ‘isomerization’ (conversion between
isomers).  It is then given off to the protein and transported through the
visual system to enable us to see. 


Figure 4:  Light-induced isomerization of cis-11 retinal  to
all-trans retinal

Another very common biological difference produced by isomers is with that
of starch versus cellulose.  Both starch and cellulose are polymers of
(1,4) D-glucose, yet one (starch) is the principle dietary component of
every culture on earth (whether from corn, wheat, rice, potato…) while
cellulose is the principle structural component of a large portion of the
world’s plant-life and is indigestible to most animals (except with the
help of microorganisms or insects).  What is the difference between the
two?  The glucose monomers used in their construction are enantiomers. 
Starch is composed of alpha-D-glucose while cellulose is composed of
beta-D-glucose (Figure 5).  Clearly, isomers play an important part in the
world!


Figure 5a:  The simplified structures of starch, an alpha-(1,4)
glucose polymer (A), and cellulose, a beta-(1,4) glucose polymer (B)

In modern times and technologies, humans desire to create drugs that will
help us overcome pain, allergies, psychological problems, infection, and
many other medical conditions.  Many, if not most, of these drugs will
interact with proteins -- either receptors on cell membranes or enzymes. 
Such proteins are highly specific in recognizing their targets and
therefore drugs must be made carefully to mimic such targets.  This
included creating the correct isomer, as only one isomer will be
functional.  Often, for technical reasons, a drug will be made in what is
called a ‘racemic’ mixture, where both isomers exist in the solution. 
Then, the two isomers will be separated, if possible.  The separation is
important because, not only is one isomer generally ineffective, but that
isomer may also be harmful.  It is occasionally the case that side-effects
from drugs come from isomeric impurities (of course, there are other
reasons for side-effects also). 

These are but a few examples of the importance of isomers.  As you can see,
without isomers, life as we know it would not exist.

References

1) Figures 1,2 4 were acquired from Wikipedia, searching for:
Isomers

2) There is a good discussion of starch and cellulose at the London South
Bank University web site (where I acquired the components of figure 5):
Starch
Cellulose

3) Frank R. Gorga has a nice, basic web course in isomers using CHIME
demonstrations (Internet Explorer only - requires the 
CHIME plugin).

4)  Elmhurst
College has a discussion of glucose (where I got Figure 3).

5) A discussion of the biological importance of ‘chirality’ and isomers can
be found in virtually all organic chemistry or biochemistry text books such as:

Biochemistry
by Donald Voet, Judith Voet.  J.Wiley&Sons, 2004, 3rd ed.

or

Lehninger Principles of
Biochemistry by Albert L. Lehninger, David L. Nelson, Michael M. Cox. 
W H Freeman & Co, 2004, 4th ed.