STEREOCHEMICAL EVIDENCE FOR A CIS-ENEDIOL INTERMEDIATE IN MN-DEPENDENT ALDOSE ISOMERASES

Three Mn2+-dependent bacterial aldose-ketose isomerases were examined to determine the stereochemistry of the C-1 proton of the ketose that is abstracted in the formation of the aldehyde. In the cases of d-xylose isomerase (d-xylose ketol-isomerase, EC 5.3.1.5) and l-arabinose isomerase (l-arabinose ketol-isomerase, EC 5.3.1.4) it is the pro-R position of the d-xylulose and l-ribulose that is activated, whereas in the l-fucose isomerase reaction it is the pro-S position of l-fuculose that is labeled in tritiated water. Little proton exchange occurs in these reactions, especially the xylose isomerase, where retention of the substrate proton is complete. These stereochemical results conform to those of four other sugar isomerases: triose-P, d-ribose-5-P, d-mannose-6-P, and d-glucose-6-P isomerase. Protonation at C-1 or C-2 from the same side of the plane of a cis-enediol gives the correct stereochemical result for all seven isomerase. © 1969.