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 mechanisms of those L-heptose moieties.In this study,we deciphered the biosynthetic pathway of the L,L-gluco-heptosamine moiety in SEPs by functional characterizing four genes and proposed that SepI initiates the process by oxidizing the 4’-hydroxyl of L-glycero-α-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 bifunctional 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 epimerase family bearing vicinal oxygen chelate(VOC)architecture.