User:Ahmedgrorab111/sandbox

stereochemistry of the Protein Amino Acids With the sole exception of glycine, the α-carbon of every amino acid is chiral. Although some protein amino acids are dextrorotatory and some levorotatory, all share the absolute configuration of l-glyceraldehyde and thus are defined as l-α-amino acids. Even though almost all protein amino acids are (R), the failure to use (R) or (S) to express absolutestereochemistry is no mere historical aberration. l-Cysteine is (S) since the atomic mass of the sulfur atom on C-3 exceeds that of the amino group on C2. More significantly, in mammals the biochemical reactions of l-α-amino acids, their precursors and their catabolites are catalyzed by enzymes that act exclusively on l-isomers, irrespective of their absolute configuration