Page - (000211) - in Biomedical Chemistry: Current Trends and Developments

Nuno Vale* 3.1 Amino Acids and Peptides in Medicine: Old or New Drugs? 3.1.1 Introduction 3.1.1.1 Amino acids: biological and chemical concepts Amino acids (AA) are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. AA are an important class of cell signalling molecules, involved in the regulation of gene expression and the protein phosphorylation cascade, as well precursors of hormone synthesis and low-molecular nitrogenous substances (Wu, 2009). The 20 natural AA are commonly found in proteins and are also referred to as alpha amino acids (Fig. 3.1.1). The α-amino acids differ in the nature of the side-chain (R group) attached to their α-carbon, which can vary in size from just one hydrogen atom in glycine to a large heterocyclic group in tryptophan. α-Amino acids are typically divided into three categories, based on the properties of their side-chain. The first category contains α- amino acids with relatively nonpolar R groups (glycine, alanine, valine, leucine, isoleucine, proline, phenylalanine, tryptophan, and methionine), the second contains α-amino acids with uncharged but polar R groups (serine, threonine, cysteine, tyrosine, asparagine, glutamine), and the third contains α-amino acids with charged R groups (aspartic acid, glutamic acid, lysine, arginine and histidine) (Vig, 2013). Proteins are key players in many vital processes in living organisms. They transport substances, catalyze chemical reactions, pump ions or recognize signalling molecules. The complexity and variety of proteins is tremendous: in the human body alone, there are more than 100,000 different proteins at work. But almost all of them are made up of just twenty different AA. Only a few highly specialized proteins additionally contain selenocysteine (Sec, Fig. 3.1.2), the very rare 21st amino acid