Usually living systems comprise of D amino acids and L sugars. It is not that only some antibiotics have D-amino acids. The α-amino acids have an α-carbon atom to which four different groups are attached like R or substituent or variable group, amino group, carboxylic group and hydrogen. So, all the α-amino acids have an asymmetric carbon and they are optically active. But glycine is different, in glycine the R is hydrogen. Hence, glycine is not optically active and lacks an asymmetric carbon atom. As far as optical isomerism is concerned, we can safely say that if an amino acid has more than one asymmetric carbon atom it will be optically active.