Neutral polar amino acids: Ser, Thr, Cys, Asn, and Gln
- The sidechains of these amino acids are polar and consequently they are more easily accommodated
- on the surface of proteins than the nonpolar amino acids. The hydroxyl (–OH) groups in Ser and
- Thr, as well as the carboxyl (C=O) and amine (NH2) groups of Asn and Gln, are often involved in
- hydrogen bonds. By forming hydrogen bonds with other amino acid residues these amino acids
- can be accommodated in the protein interior.
Charged amino acids: Asp, Glu, Arg, Lys, His
The sidechains of these amino acids carry a charge at neutral pH. Asp and Glu have negatively
charged carboxylate groups, while His has a positively charged imidazole ring. These amino acids
are often used to chelate certain metal ions. For example, the carboxylate group of Asp is often
used to chelate Ca2+ while the imidazole nitrogen atoms of His are often involved in Zn2+ coordination
(as exemplified by the zinc finger proteins). We will see later that the imidazole ring of His has
special properties that make it one of the most common amino acid residues at enzyme active
sites. Lys has a positively charged amine group, and Arg has a positively charged guanidinium
moiety. The charged amino acids are usually found on the surface of proteins where they can
interact with water molecules (but remember that ionic interactions involving these residues will
be much stronger when they are buried in the protein interior).
