Chapter 3: Amino Acids and the Primary Structures of Proteins

All amino acids share a common core structure centered on a chiral alpha carbon bonded to an amino group, carboxyl group, hydrogen atom, and a distinctive side chain, with glycine being the notable exception due to its hydrogen side chain that renders it achiral. The amino acids incorporated into proteins exclusively adopt the L-configuration, a characteristic inherited from the last universal common ancestor. At physiological pH, amino acids exist as zwitterions with neutral net charge due to protonation of the amino group and deprotonation of the carboxyl group. Amino acids are classified by their side chain properties into distinct functional categories: aliphatic nonpolar residues, aromatic residues that absorb ultraviolet light, sulfur-containing amino acids capable of forming stabilizing disulfide bonds, polar uncharged alcohols, basic positively charged residues critical for enzyme catalysis, and acidic negatively charged residues along with their polar amide derivatives. Beyond the standard twenty, nonstandard amino acids such as selenocysteine and pyrrolysine are occasionally incorporated, while standard amino acids also function as precursors for important signaling molecules and neurotransmitters. The ionization behavior of amino acids is characterized by distinct pKa values for different ionizable groups, defining the isoelectric point where net charge becomes zero, a concept essential for understanding protein charge distribution and three-dimensional structure. Amino acids link through peptide bonds via condensation reactions to form the primary structure of proteins, the linear sequence of residues extending from the N-terminus to the C-terminus. Modern protein analysis relies on multiple complementary techniques: purification methods exploit differences in solubility, size, charge, and binding affinity; SDS-PAGE separates proteins exclusively by mass; mass spectrometry determines precise molecular weights and enables protein identification through tryptic fingerprinting. Protein sequencing employs Edman degradation for stepwise N-terminal identification and enzymatic cleavage by trypsin, chymotrypsin, or chemical cleavage with cyanogen bromide to fragment larger proteins into analyzable segments. Comparative analysis of primary structures among homologous proteins from different organisms reveals evolutionary relationships, with sequence similarity closely reflecting evolutionary distance and providing powerful tools for constructing phylogenetic relationships that align with fossil records.