How Nature deals with stereoisomers

All natural proteins are composed of t-amino acids and are inherently chiral. The properties of both l-and chemically synthesized d-amino acids are identical except in optically asymmetric interactions. Structural studies of d-l racemic mixtures of crystallographic interest are discussed. The review also gives some recent examples of stereospecificity: how l-proteins deal with l- or d-substrates and how enzymes can function as racemases. Two particular examples of stereoselectivity are then discussed