the concurrent modification of all downstream KS. Recent studies of the amino acid sequence of the two types of stereospecificity in KR have determined a perfect correlation with these residues and the predicted stereochemical outcome. This is particularly useful in situations where the gene sequence of a modular PKS is known but the final product structure has not yet been elucidated. Tailoring enzyme modifications Enzymes that act on the macrolide after it has been released and cyclized by DEBS are called tailoring enzymes. Many such enzymes are involved in the production of erythromycin from the final product of unmodified DEBS, 6-deoxyerythronolide B. Such classes of enzymes include mainly oxidoreductases and glycosyl transferases and are essential for the antibiotic activity of erythromycin. Thus far, few attempts have been made to modify tailoring pathways, however, the enzymes which participate in such pathways are currently being characterized and are of great interest. Studies are facilitated by their respective genes being located adjacent to the PKS genes, and many are therefore readily identifiable. There is no doubt that in the future, alteration of tailoring enzymes could produce many new and effective antimicrobials. Structural studies As of late 2007, 8 structures have been solved for this class of enzymes, with PDB accession codes , , , , , , , and . Other names of this enzyme class is malonyl-CoA:propanoyl-CoA malonyltransferase (cyclizing). Other names in common use include erythronolide condensing enzyme, and malonyl-CoA:propionyl-CoA malonyltransferase (cyclizing). References Further reading EC 2.3.1 Enzymes of known structure