Figure 6. Primary structure of glycophorin A. Amino acid sequence of one polypeptide of glycophorin A. Amino acids are colored to show different chemical properties. Note that the charged amino acids all lie not in the membrane but either inside or outside the red blood cell, as do most of the other hydrophilic amino acids. Many of the hydroxylated amino acids on the outside of the cell are also charged, owing to negatively charged sugars (not shown) attached to the amino acids after the protein is made. In contrast, the portion of the protein that spans the membrane consists of amino acids that are predominantly hydrophobic (see also inset at bottom of the figure). These 19 amino acids form an alpha-helix. The regions on both sides of the membrane also have secondary and tertiary structures, not shown in this cartoon. Finally, glycophorin A has a quaternary structure: in nature (but not in this cartoon) it is a dimer consisting of two identical polypeptide chains associated with one another.