Erythromycin biosynthesis. The 4-pro-S hydride of NADPH is utilized for ketoreduction by both module 5 and module 6 of the 6-deoxyerythronolide B synthase

Incubation of chirally deuterated NADPH with 6-deoxyerythronolide B synthase (DEBS) modules 5 and module 6 and analysis of the derived triketide lactones established that the two ketoreductase domains, KR5 and KR6, are both specific for the 4-pro-S hydride of the nicotinamide cofactor.