Amino acids with uncharged polar side chains

These amino acids have zero net charge at physiologic pH, although the side chains of cysteine and tyrosine can lose a proton at an alkaline pH . Serine, threonine, and tyrosine each contain a polar hydroxyl group that can participate in hydrogen bond formation (Fig. 1). The side chains of asparagine and glutamine each contain a carbonyl group and an amide group, both of which can also participate in hydrogen bonds.

Figure 1. Hydrogen bond between the phenolic hydroxyl group of tyrosine and another molecule containing a carbonyl group.

1. Disulfide bond: The side chain of cysteine contains a sulfhydryl (thiol) group (−SH), which is an important component of the active site of many enzymes. In proteins, the –SH groups of two cysteines can be oxidized to form a covalent cross-link called a disulfide bond (−S–S–). Two disulfide-linked cysteines are referred to as cystine. 

2. Side chains as attachment sites for other compounds: The polar hydroxyl group of serine, threonine, and (rarely) tyrosine can serve as a site of attachment for structures such as a phosphate group. In addition, the amide group of asparagine, as well as the hydroxyl group of serine or threonine, can serve as a site of attachment for oligosaccharide chains in glycoproteins .