Insights into the biosynthesis of septacidin L-heptosamine moiety unveils a VOC family sugar epimerase

L-Heptopyranoses are important components of bacterial polysaccharides and biological active secondary metabolites like septacidin (SEP), which represents a group of nucleoside antibiotics with antitumor, antifungal, and pain-relief activities. However, little is known about the formation mech-anisms of those L-heptose moieties. In this study, we deciphered the biosynthetic pathway of the L,L-glu-co-heptosamine moiety in SEPs by functional characterizing four genes and proposed that SepI initiates the process by oxidizing the 4'-hydroxyl of L-glycero-a-D-manno-heptose moiety of SEP-328 (2) to a keto group. Subsequently, SepJ (C5 epimerase) and SepA (C3 epimerase) shape the 4'-keto-L-heptopyranose moiety by sequential epimerization reactions. At the last step, an aminotransferase SepG installs the 4'- amino group of the L,L-gluco-heptosamine moiety to generate SEP-327 (3). An interesting phenomenon is that the SEP intermediates with 4'-keto-L-heptopyranose moieties exist as special bicyclic sugars with hemiacetal-hemiketal structures. Notably, L-pyranose is usually converted from D-pyranose by bifunc-tional C3/C5 epimerase. SepA is an unprecedented monofunctional L-pyranose C3 epimerase. Further in silico and experimental studies revealed that it represents an overlooked metal dependent-sugar epim-erase family bearing vicinal oxygen chelate (VOC) architecture.