POSTTRANSLATIONAL AMINO-ACID EPIMERIZATION - ENZYME-CATALYZED ISOMERIZATION OF AMINO-ACID-RESIDUES IN PEPTIDE CHAINS

Since ribosomally mediated protein biosynthesis is confined to the L-a mino acid pool, the presence of D-amino acids in peptides was consider ed for many years to be restricted to proteins of prokaryotic origin. Unicellular microorganisms have been responsible for the generation of a host of D-amino acid-containing peptide antibiotics (gramicidin, ac tinomycin, bacitracin, polymyxins). Recently, a series of mu and delta opioid receptor agonists [dermorphins and deltorphins] and neuroactiv e tetrapeptides containing a D-amino acid residue have been isolated f rom amphibian (frog) skin and mollusks. Amino acid sequences obtained from the cDNA libraries coincide with the observed dermorphin and delt orphin sequences, suggesting a stereospecific posttranslational amino acid isomerization of unknown mechanism. A cofactor-independent serine isomerase found in the venom of the Agelenopsis aperta spider provide s the first major clue to explain how multicellular organisms are capa ble of incorporating single D-amino acid residues into these and other eukaryotic peptides. The enzyme is capable of isomerizing serine, cys teine, O-methylserine, and alanine residues in the middle of peptide c hains, thereby providing a biochemical capability that, until now, had not been observed. Both D- and L-amino acid residues are susceptible to isomerization. The substrates share a common Leu-Xaa-Phe-Ala recogn ition site. Early in the reaction sequence, solvent-derived deuterium resides solely with the epimerized product (not substrate) in isomeriz ations carried out in (H2O)-H-2. Significant deuterium isotope effects are obtained in these reactions in addition to isomerizations of isot opically labeled substrates (H-2 at the epimerizeable serine alpha-car bon atom). The combined kinetic and structural data suggests a two-bas e mechanism in which abstraction of a proton from one face is concomit ant with delivery from the opposite face by the conjugate acid of the second enzymic base.