Alpha helix capping and the conformation of threonine

The amino acid threonine contains two chiral carbons and thus can exist in four possible conformations. However, only a single conformation (2S, 3R) o f threonine is incorporated in proteins by the cellular translation materia l. The conformation at the alpha carbon (carbon 2) is the same as that of t he other amino acids in biological proteins, and there is no explanation fo r why this enantiomer was selected. Further, there is no explanation for th e choice of conformation at the 3 carbon in threonine. Here, I suggest that the preferential ability of (2S, 3R) threonine over the (2S, 3S) enantiome r to participate in alpha helix capping interactions may have led to the se lection of the (2S, 3R) conformation. (C) 2001 Harcourt Publishers Ltd.