Kinetic isotope effect and reaction mechanism of 2-deoxy-scyllo-inosose synthase derived from butirosin-producing Bacillus circulans

The mechanism of 2-deoxy-scyllo-inosose synthase reaction, a carbocycle formation step from D-glucose-6-phosphate in the biosynthesis of the 2-deoxystreptamine aglycon of clinically important aminocyclitol antibiotics, was investigated with a partially purified enzyme from butirosin-producing Bacillus circulans SANK 72073. Nonlabeled and double-labeled D-[4-²H, 3-¹⁸O]glucose-6-phosphate were used for cross-over experiment, and the oxime-TMS ether derivative of the 2-deoxy-scyllo-inosose product was analyzed by GC-MS. The deuterium label at C-4 of the substrate appeared to be retained at C-6 of the inosose product without scrambling of the double-labeled isotopes. Since the transient reduction of NAD⁺ cofactor was proved to be essential in the 2-deoxy-scyllo-inosose reaction, the hydride abstraction and returning appeared to take place within the same glucose molecule. The observed kinetic isotope effect was estimated to be k(H)/k(D) = 2.4. These results strongly suggest that this carbocycle formation is catalyzed by a single 2-deoxy-scyllo-inosose synthase enzyme with catalytic requirement of NAD⁺, the mechanism of which appears to be resembled closely to the 2-deoxy-scyllo-inosose synthase in the Streptomyces fradiae.