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The ability to taste, no doubt,
arose through the need for organisms to sense
nourishing molecules and avoid those which are
harmful. As we have already discussed determining the
structure of a molecule is a rather difficult undertaking
for a scientific research team and yet it is a routine
activity for everyone of us! Many
poisonous molecules taste bitter and many nutritious
foods taste sweet. Taste is a efficient mechanism for
discriminating between molecules. On the left are the
structures of cyclamate, aspartame and saccharine. Again
there is similarity in their structures. Indeed,
scientist are able to relate the taste of many molecules
to their shapes and the distribution of electronic charge
on their surfaces. The essential components of the sweet
taste is thought to be centered on an appropriately spaced
pair of electron attracting atoms such as two oxygen
atoms attached to a hydrocaron framework. Sucrose has
several promising sets of atoms as do the artificial
sweeteners. But the taste receptor, whose actual structure
is presently unknown, is obviously very successful in
discriminating between different potentially sweet
molecules. Sweet sensations are sometime not evoked when
bombarded by a molecule which simply a mirror image of a
sweet molecule. When we taste sweet food, then, these
appropriately shaped molecules trigger receptors in taste
buds which are very discriminating. The receptors that
sense sweetness are able to discriminate between the
multitude of chemical in common foods, including
molecules closely related to sweet molecules, and reserve
their appreciative response for only the correctly shaped
molecules. The next time that you taste a sweet food
remember that receptors on your tongue are responding
to a particular three dimensional arrangement of atoms
and their electrons. You will be tasting the shape of a
molecule. It is interesting to note that the Greek
philosopher Democritus around 400 BC speculated that the
taste of substances was due to the shape of their
component particles. Democritus' model was crude, he
reasoned that acidic particles would be sharp, as they
attacked, and sweet substances would be soft, but turned
out to be surprisingly accurate. Fortunately for those who
are concerned about their weight chemists have been able
to synthesize a variety of sweet shaped molecules which
trigger the appropriate taste but contain far fewer
calories than the molecules of sucrose which they mimic
and so are less likely to be converted to unfashionable
energy reserves by the resource conscious human
metabolism.
Cyclamate
Saccharin
Aspartame
Sucrose
The image on the left shows the structure of the
virus responsible for the common cold. Human rhinovirus,
as this structure is known, is assembled from a
collection of protein molecules surrounding a single
strand of nucleic acid based genetic information. The
complete virus structure is large, containing many tens
of thousands of atoms and the coat structure appears
almost spherical. A problem that the genetic material of
a virus faces is how to cloak itself with a solid
protective casing which can enclose, transport and
protect the large nucleic acid polymer molecule. It takes
four bases to encode a single amino acid, so it is
impossible for the molecule to code for a protein which
is large enough to encase the information carrier. This
is a problem which bacteria, larger viruses and higher
organisms have solved by making use of membranes
constructed from the fruit of enzyme catalysis, but again
the rhinovirus genetic material is too small to carry the
information for such an enzyme. The solution is cunningly
simple. The protein which is produced by the exploitation
of the victim cell's protein construction apparatus is
perfectly designed to assemble, like a symmetrical three
dimensional puzzle, into a large composite structure.
This superstructure has 60 protein molecules in an
icosohedral arrangement. The icosohedron is a regular
polygon, as cubes and octahedra are regular polyhedra,
containing 20 equal triangles. Each triangular component
of the viral icosohedron is made up of three protein
molecules. The protein coat molecules are docked together
as a staggeringly large collection of locks and keys to
protect their genetic information store. Each joining
surface of the structure dovetails with the next to close
the casing structure of the virus.
Just as the virus
exploits the lock and key relationship in the formation
of its coating so it uses the principle both to defeat
the immune system of its host and to attack its target
cells. As is evident from the virus image above the
coating of the virus is by no means smooth. There are
clefts, raised regions which on a molecular level are
mountains and valleys. It turns out that the raised loop
areas rapidly change their composition as the virus
progresses through successive infected individuals. It is
suspected, therefore, that this chameleon-like
variability allows the virus to avoid the immunity which
would otherwise be built up against it by an organism's
defensive systems. As anti-bodies are developed against
the virus, and the highly variable regions in particular,
so the the virus gradually changes its signature and
consistently avoids recognition. In contrast to the
variability of the raised portions of the virus coat, the
valley's amino acid sequence is remarkably constant. This
is interpreted as indicating that it is this region with
fits a molecular lock on the surface of the victim cell.
So the virus, which many molecular biologists and
pharmacologists have sought to destroy through lock and
key targeted drugs, turns out to be well versed in
precisely this principle and efficiently exploits the
knowledge in many areas, including fabricating its coat,
locating its victims and evading capture.
Viruses are highly successful predators, able to adapt to
the defenses of their prey. Many viruses are continually
changing their outer extremities and so are undetected
and yet they are able to unerringly seek out their
targets through protected binding regions. Their targets,
we might then infer, are some highly conserved region of
the cell membrane. Perhaps a region which is encoded by
more than one gene so that changes in this structure
happen extremely slowly as an organism evolves. The
virus can exploit this conservative vulnerability and at
the same time avoid falling prey to the same defect.
However, the conserved regions of the virus coat are an
inherent weakness. An anti-viral drug has been shown to
bind in this area for the the virus that causes the
common cold.
A virus
The simplicity of viruses in comparison with bacteria
might lead to their implication as stepping stone from
the first molecular replicators
to higher forms of life. However, the
extreme dependence which viruses have on existing cells
makes this unlikely. A virus needs a pre-existing cell to
invade and exploit and without such a cell it cannot
reproduce. So although viruses are simple, they have
probably evolved after and around more sophisticated life
forms.
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