Chapter 3: Amino Acids & Peptides: Structure & Function

While nature contains hundreds of these organic compounds, the human genetic code specifically utilizes twenty L-alpha-amino acids, along with the unique inclusion of selenocysteine, to construct essential polypeptides. These molecules serve not only as structural units but also as critical precursors for neurotransmitters, hormones, and metabolic intermediates like urea and porphyrins. The structural diversity of these units is dictated by their side chains, which are categorized based on their chemical nature—such as being water-loving or water-repelling—which ultimately guides the complex folding of mature proteins. Beyond their role in health, the text examines how certain non-protein variants found in plant seeds, like those from the sweet pea genus, can induce severe neurological conditions such as lathyrism. A significant portion of the material is dedicated to the electrochemical behavior of these molecules in aqueous environments. Concepts such as the isoelectric point (pI) and dissociation constants (pKa) are essential for understanding how these units carry electrical charges at varying pH levels, influencing their movement during laboratory techniques like electrophoresis. Furthermore, the discussion transitions into peptides, which are chains linked by a specific chemical connection known as the peptide bond. This bond possesses a partial double-bond character that creates a rigid, flat plane, significantly limiting the possible shapes a protein can assume. This structural constraint is a cornerstone of molecular biology, ensuring that proteins fold into predictable, functional three-dimensional forms. The chapter also touches upon the intriguing discovery of amino acids in meteorites, suggesting potential extraterrestrial contributions to the chemical origins of life on Earth.