There are 20 standard amino acids
that make up the proteins found in all living organisms. From these, how many different
dipeptides can be made?
Proteins are biological
macromolecules composed of amino acid monomers. They can be enzymes, structural
molecules, signaling molecules, or antibodies, just to name a few of their many
forms and functions.
An amino acid is built around a
central carbon atom called the 𝛼-carbon. You may remember that carbon atoms
are able to form four chemical bonds. The 𝛼-carbon in an amino acid is
bonded with an amino group, a carboxyl group, an atom of hydrogen, and a molecular
structure referred to as an R group, or side chain. The 𝛼-carbon, amino group,
carboxyl group, and hydrogen are virtually the same for every amino acid. But the R group will vary, giving
every amino acid its specific chemical properties. The R group can make an amino acid
polar, nonpolar, aromatic, acidic, or basic. Amino acids form bonds called
peptide bonds. A chain of multiple amino acids
bonded together is called a polypeptide, which we can remember because the prefix
poly- means many.
Now let’s take a look at how a
peptide bond is formed. Amino acids form peptide bonds
during a chemical reaction called a condensation reaction. During a condensation reaction, the
carbon in the carboxyl group of one amino acid bonds with the nitrogen in the amino
group of another amino acid. And a water molecule is released in
The question is asking us about
dipeptides. You likely already know that the
prefix di- means two, so a dipeptide contains two amino acids, just like the
diagram. We need to determine how many
different dipeptides we can make with the 20 standard amino acids. A dipeptide can be made from any
two amino acids, even two molecules of the same amino acid. Therefore, there are 20
possibilities for what the first amino acid could be. There are also 20 possibilities for
what the second amino acid could be. So, if we multiply 20 by 20, we can
calculate the number of possible combinations of dipeptides.
Therefore, the number of different
dipeptides that can be made out of 20 amino acids is 400.