The human nose has a normal sense of smell. A team of researchers from Johns Hopkins University School of Medicine in Baltimore has discovered that a single protein in the nose is responsible for this.
They called the molecule the odorant binding protein. The
protein has the ability to bind to odour molecules that enter the nose.
The nose can detect some odours, even when the ratio is as low
as one odour molecule for every trillion air molecules.
In some cases, scientists have pinpointed the chemical that
produces a specific, distinctive odour.
A chemical called 2-isobutyl-3-methoxypyrazine gives the
smell of green pepper.
A chemical called amyl acetaldehyde gives the smell of fruits
like bananas and guava.
While chemists have some idea of which chemicals produce
which odours, neuroscientists are not entirely sure how odorants in the air are
converted into signals sent to the brain.
It has been known for some time that the OBP protein, which
binds odorants, binds to odour molecules, but it has been difficult to establish
whether it actually binds odorants, helps them reach receptors, and then sends
odour signals to the brain.
The Baltimore researchers had previously shown that OBP is
produced only in and around the brain and that most of the OBP protein is
found in a small gland in the brain.
Using further genetic studies, they discovered that OBP is a
member of a family of proteins with similar functions.
These proteins bind to small molecules with similar physical
and chemical properties to odour molecules and transport them across membranes.
The Baltimore researchers studied molecules to see if they could pass through
membranes.
Based on the evidence available at the time, it seems certain
that the OBP protein is located in the right place and has the necessary
physical and chemical properties to bind to odorant molecules and transport
them through the membranes in the nose to receptors.
The Baltimore researchers are studying the OBP protein
molecules using X-ray crystallography to determine their three-dimensional
structure. This will help them figure out how the OBP molecule can bind
to odorant molecules.
The OBP protein has a structure similar to other proteins.
They are all shaped like cylindrical tubes closed at one end.
This is why the OBP protein seems to be able to capture a
wide variety of odorant molecules. All of them are taken up into the nose. How
the chemical signals produced by odorants are converted into nerve signals sent
to the brain remains to be studied.
It has been discovered in detail how light falling on the eye
is converted into nerve signals through biochemical reactions, and how sound
waves falling on the ear vibrate the fibre-shaped cells in the inner ear and
thereby generate nerve signals.
Perfume companies are showing serious interest in studies on
the function of the nose to perceive odours and are providing the necessary
financial support.