Biology Reference
In-Depth Information
there is far more functional diversity observed in membrane proteins than is found in
membrane lipids.
B. THE AMINO ACIDS
The first basic question is why some proteins are water-soluble while others partition into
the oily interior of a membrane. Since all proteins are a linear string of amino acids, it must be
that the types and arrangement of the amino acids ultimately determine the eventual location
of the protein. The term amino acid
[2,3]
pertains to any small molecule that possesses both
an amine and a carboxylic acid group. There are about 500 known types of amino acids of
biological origin. The first amino acid was discovered in 1806 by the French chemists Louis
Nicolas Vauquelin and Pierre Jean Robiquet from asparagus juice, and so it was named
asparagine
[4]
. Vauquelin was prolific, authoring some 376 papers covering many topics.
He is best known for his discovery of chromium (1797) and beryllium (1798) and for the
production of liquid ammonia at atmospheric pressure (
Figure 6.1
).
There are 20 common amino acids, so-called because they are coded for by the genetic
code. It is a natural tendency of scientists to categorize everything and this includes the
amino acids. Although there have been several proposed amino acid classifications, they
are all artificial and so have advantages and disadvantages. Since this topic is primarily inter-
ested in membrane proteins, a first approach would be to simply divide the amino acids into
water-soluble (polar) and water-insoluble (non-polar) based on the structure of the amino
acid side chains. While this is easy to do with most amino acids, some present a problem.
For example, what do you do with glycine that has no real side chain (only a
H), and tyro-
e
sine that has a polar
OH attached to a non-polar benzene ring?
e
FIGURE 6.1
L.N. Vauquelin (1763
e
1829).
