Chapter 1 of MCAT Biochemistry Review.Aside from the zwitterionic properties common to every amino acid, each one has properties
defined by its R group, or side chain. The 20 eukaryotic proteogenic amino acids are grouped into
five categories: nonpolar nonaromatic, aromatic, polar, negatively charged (acidic), and
positively charged (basic). Nonpolar nonaromatic amino acids tend to have side chains that are
saturated hydrocarbons, like alanine, valine, leucine, and isoleucine; they also include glycine,
proline (which is cyclic, with a secondary amine), and methionine (which contains sulfur). Aromatic
amino acids include tryptophan, phenylalanine, and tyrosine. Nonpolar amino acids—both
nonaromatic and aromatic—are also hydrophobic and tend to be sequestered in the interior of
proteins. Polar amino acids tend to have terminal groups containing oxygen, nitrogen, or sulfur.
These include serine, threonine, asparagine, glutamine, and cysteine. Negatively charged (acidic)
amino acids include aspartic acid and glutamic acid. These amino acids have terminal carboxylate
anions in their R groups. Finally, positively charged (basic) amino acids, including arginine, lysine,
and histidine, have a protonated amino group in their R groups. Polar, acidic, and basic amino acids
are all hydrophilic and tend to form hydrogen bonds with water in aqueous solution.
Amino acids undergo condensation reactions to form peptide bonds. The molecules these bonds
form, called polypeptides, are the base unit of proteins. The reverse reaction, hydrolysis of the
peptide bond, is catalyzed by a strong acid or base. Both of these reactions are shown in Figure 10.4.
Figure 10.4. Peptide Bond Formation and CleavageLike other carbonyl-containing functional groups, amides have two resonance structures, as shown
in Figure 10.5. The true structure of the amide bond is therefore a hybrid of these two structures,
with partial double-bond character between the nitrogen atom and the carbonyl carbon. This
double-bond character limits rotation about the C–N bond, which adds to the rigidity and stability