... polar. Together with serine, threonine is one of two proteinogenic amino acids bearing an alcohol group (tyrosine ... .Threonine is one of two amino acids out of the twenty with two chiral centers, threonine can ... (2S,3S) and (2R,3R). However, the name L-threonine is used for one single enantiomer, (2S,3R ... is rarely present in nature, is called L-allo-threonine. The two stereoisomers (2R,3S)- and (2R,3R)- ...