Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Water
Seven amino acids contain uncharged polar R-groups

These Amino Acids (Fig. 5-6) are more soluble in Water—that is, they are more hydrophilic than nonpolar amino acids—because their functional groups form Hydrogen Bonds with water molecules. This Class of amino acids includes Glycine, Serine, Threonine, Cysteine, Tyrosine, asparagine, and glutamine. The polarity of serine, threonine, and tyrosine is due to their hydroxyl groups; that of asparagine and glutamine, to their amide groups; and that of cysteine, to its sulfhydryl, or thiol, group. The R group of glycine, consisting of a single hydrogen atom, is too small to compensate for the strong polarity of the α-amino and α-carboxyl groups.

Fig. 5-6. The 20 Amino Acids Commonly Found in Proteins. Their amino and carboxyl groups are shown in the ionized state, as they actually exist at pH 7.0. The PARTS OF THE molecules common to all Amino acids are shown on a red Background, with the R groups in black.

Asparagine and glutamine are the amides of two Other Amino Acids, aspartic acid and glutamic acid, respectively, which also serve as protein building blocks. Upon acid or alkaline Hydrolysis, asparagine and glutamine are readily converted into these amino acids. Cysteine and tyrosine contain R groups that dissociate to yield H⁺ ions, but at pH 7.0 both of these groups—the thiol group of cysteine and the hydroxyl group of tyrosine—are ionized only to a slight extent.

Fig. 5-7. Cysteine and cystine. The thiol (—SH) groups of two cysteine molecules readily undergo oxidation to form a disulfide bond joining the two molecules into cystine. Both cysteine and cystine are found in proteins.

Cysteine deserves special mention for another reason. It may occur in proteins in two forms: either as cysteine itself or as cystine, a molecule consisting of two cysteine residues covalently linked by a disulfide bridge formed by The oxidation of their thiol groups (Fig. 5-7). Cystine plays a crucial role in The Structure of certain proteins, such as the hormone Insulin and IMMUNOGLOBULINS (Antibodies). In these proteins, the two halves of the cystine molecule serve as building blocks for two different polypeptide chains, cross-linking them via the disulfide bond (Section 6.8). Such cross-links are generally absent in intracellular proteins, but are widespread in proteins secreted into the extracellular fluid where they carry out their Functions.