Page 34 - The Miracle of Protein

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Why Proteins Constituted Only 20 of the 200
Amino Acids
More than 200 amino acids are used in various biochemi-

cal reactions in living beings. In theory, one would expect the
number of amino acids to be far more than 200. Even in the hu-
man body, many amino acids not used in proteins are used in
the body's metabolic functions. Why do proteins use only 20
amino acids when so many are more available?
We can answer this question by examining proteins' func-
tions and structures. In order to perform their functions, the

proteins essential to life must possess specific features, and
amino acids are one of the main elements that give them those
properties. For instance, it is essential that some amino acids
possess hydrophobic (or water-repellent) side chains. Further-
more, these side chains cannot be very large, or it will be impos-

sible to pack and install them inside the proteins.
The side chains of some amino acids must also possess two
features known as helix and layered formations. It is only as a
result of these features, that a protein can assume its three-di-
mensional form, which is crucial for the protein to work prop-
erly.

Research has shown that of the 20 amino acids used in pro-
teins, most have hydrophobic side chains and half possess a-he-
lix properties and the other half, b-layer properties.
Examining the properties of these 20 amino acids one by
one will make us understand why they have been specially se-

lected for proteins. For instance, even glycine—the smallest and
simplest amino acid—has a very important role to play in colla-
gen, one of the most important proteins. In the three amino
acids that comprise collagen, one is glycine. Its small dimension
plays an important role in the structure of collagen by permit-