Video Transcript
The diagram shows a basic outline of an amino acid. Which two parts of amino acids join to form a peptide bond? (A) NH2 and COOH. (B) NH2 and the R group. (C) NH2 and H. (D) COOH and the R group. Or (E) COOH and H.
In order to answer this question correctly, we must recall how amino acids are joined together to form polypeptides. Remember that multiple amino acids are joined together to form peptides and polypeptides, which themselves form proteins.
Take a look at the structure of a single amino acid. This compound contains three distinct chemical groups: a carboxyl group, an amino group, and an R group. The R group is bound to the central carbon of the amino acid, which is also bound to a hydrogen, as well as the amino, carboxyl, and R groups. You may remember that the R group differs among amino acids. Some will contain polar R groups, while others will contain nonpolar R groups. In addition, some R groups are acidic, while others are basic, depending on the charge they possess.
Because the R groups are variable among different amino acids, we could not reliably expect that they would join amino acids together to form polypeptide bonds. Therefore, we are looking for groups that are consistent across all amino acids, such as the carboxyl and amino groups that are indeed present in every amino acid.
In fact, when forming peptide bonds, the nitrogen atom of the amino group will be attracted to the carboxyl carbon of a neighboring amino acid. The formation of a bond between these atoms results in the loss of a hydrogen from the amino group and a hydroxyl, or OH, from the carboxyl group. When those molecules join together, it results in a water molecule. This process of joining together two molecules with the loss of a water molecule is called a condensation reaction.
So, peptide bond formation involves a bond between the amino group of one amino acid and the carboxyl group of the next amino acid via dehydration synthesis, with the loss of water.
Now that we have discussed the process of peptide bond formation, we are able to take a look at our answer choices. The question asks, which two parts of amino acids join to form a peptide bond? We know that the amino and carboxyl groups join together during peptide bond formation, so the correct answer to our question is (A). The parts of an amino acid that join together to form a peptide bond are the NH2 and COOH.
