Fundamental constituents of all proteins found in the body, such as: alanine, arginine, asparagine, aspartic acid, cysteine, cystine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. Some of these amino acids can be synthesized by the body; others (known as essential amino acids) must be obtained from protein in the diet.
In skincare products, amino acids act as water-binding agents, and some have antioxidant properties. Amino acids can be combined with other ingredients that have skin-restorative ability to work in unison to fight signs of aging. 
References for this information:
European Surgical Research, January-April 2002, issues 1-2, pages 53-60
natural moisturizing factor (NMF)