Component of the DprE1-DprE2 complex that catalyzes the 2-step epimerization of decaprenyl-phospho-ribose (DPR) to decaprenyl-phospho-arabinose (DPA), a key precursor that serves as the arabinose donor required for the synthesis of cell-wall arabinans. DprE1 catalyzes the first step of epimerization, namely FAD-dependent oxidation of the C2' hydroxyl of DPR to yield the keto intermediate decaprenyl-phospho-2'-keto-D-arabinose (DPX). The intermediate DPX is then transferred to DprE2 subunit of the epimerase complex, most probably through a 'substrate channel' at the interface of DprE1-DprE2 complex. DprE2 then catalyzes the second step of epimerization, the NAD(+)-dependent reduction of DPX that leads to the formation of DPA. NAD(+) + trans,octa-cis-decaprenylphospho-beta-D-arabinofuranose = H(+) + NADH + trans,octa-cis-decaprenylphospho-beta-D-erythro-pentofuranosid-2-ulose Cell wall biogenesis; cell wall polysaccharide biosynthesis. Interacts with DprE1 to form an epimerase complex. Belongs to the short-chain dehydrogenases/reductases (SDR) family.