The principal function of the 20 common amino acids is to be incorporated into proteins. However, beyond this, amino acids play many different roles; indeed they are the most versatile of nutrients. This is a consequence of their very different side chains that permit a wide variety of chemical modifications and reactions, much more than is the case with other nutrients. Of course, these different side chains play specific and critical roles in proteins. The branched-chain amino acids provide opportunities for hydrophobic interactions in the cores of globular proteins. Glycine residues introduce flexibility into the conformation of peptide chains, whereas proline introduces a kink. Residues with alcohol groups, such as serine, threonine and tyrosine, provide a locus for the introduction of phosphoryl groups. There are many other examples. This versatility of function of different amino acid residues in proteins is mirrored in the nonprotein functions of amino acids. Many of these ‘ancilliary’ functions concern signaling, either directly or by serving as substrates for the synthesis of key signaling molecules. It is hardly a coincidence that three of the four known gaseous signaling molecules are produced from amino acids or their derivatives: ethylene from S-adenosylmethionine, nitric oxide from arginine and hydrogen sulfide from S-containing amino acids. Other critical functions of free amino acids include the roles of amino acids and their derivatives as neurotransmitters (e.g. glutamate and GABA), their roles as signaling agents (e.g. activation of mTOR), and, via S-adenosylmethionine, their role in a wide variety of methylation reactions. Perhaps the single most remarkable advance in amino acid physiology in the last decade or so has been the emergence of roles for a number of D-amino acids. Except for glycine, all of the amino acids are chiral molecules and one of the key questions that face researchers is whether only a handful of D-amino acids play functional roles, or whether there is a much broader, although unrecognized, physiology of D-amino acids.
These themes, of amino acids as substrates for protein synthesis and as precursors for key regulatory molecules, are well represented in the studies in this section of the journal. Two reviews explore protein synthesis in pathological situations, heart failure and type 2 diabetes. Two reviews concern themselves with methionine metabolism, one on the altered metabolism of this amino acid in alcoholic liver injury and another on the nutritional and metabolic burden that attends methylation reactions. Two reviews specifically explore the synthesis and function of key signaling molecules (nitric oxide and D-serine). Finally, two reviews deal with different aspects of signaling: one introduces new concepts on the roles played by amino acid transport in signaling; this review also introduces novel concepts regarding the role of lysosomes in the amino acid activation of mTOR. The other explores the integration of nutritional, molecular and neurophysiological approaches that have so greatly advanced our knowledge of animals’ adaptation to amino acid deprivation. Amino acids have always provided new vistas and concepts; they continue to do.
Conflicts of interest
There are no conflicts of interest.