D-AMINO ACIDS: OCCURRENCE, STEREOCHEMISTRY AND EXCLUSION FROM TRANSLATIONAL MACHINERY

The biological macromolecular world is chiral. How homochirality at the molecular level crept into the living system is an aspect which still remains unresolved. The presence of D-amino acids in organisms apart from bacteria was not appreciated until recently. Several studies in the last three decades have established the ubiquity of D-amino acids in the living world. Some of these D-amino acids have now been implicated in several important physiological functions, including neurotransmission and hormone regulation. However, proteins are comprised of only L-amino acids; a protein containing both L-and D-enantiomers loses its functionality owing to the fact that it fails to take its proper conformation. Hence, it becomes imperative for the cell to maintain "enantiomeric fidelity" during every step of translation. To this end, the cell employs several molecular machineries which, in their own capacity, help in excluding D-amino acids from polypeptides. An interesting case is presented by D-aminoacyl-tRNA deacylase which removes a D-amino acid that is mischarged onto the corresponding tRNA. Further discrimination against D-amino acids at the level of elongation factors and ribosomes has been noted. All these cellular systems play their part in perpetuation of homochirality in proteins.