Studies of the Redox Properties of CDP-6-deoxy-l-threo-d-glycero-4-hexulose-3-dehydrase (E1) and CDP-6-deoxy-l-threo-d-glycero-4-hexulose-3-dehydrase reductase (E3): Two Important Enzymes Involved in the Biosynthesis of Ascarylose†

Studies of the biosynthesis of ascarylose, a 3,6-dideoxyhexose found in the lipopolysaccharide of Yersinia pseudotuberculosis V, have shown that the C-3 deoxygenation is a process consisting of two enzymatic steps. The first enzyme involved in this transformation is CDP-6-deoxy-l-threo-d-glycero-4-hexulose-3-dehydrase (E1), which is a pyridoxamine 5‘-phosphate dependent iron−sulfur protein. The second catalyst, CDP-6-deoxy-l-threo-d-glycero-4-hexulose-3-dehydrase reductase, formally called CDP-6-deoxy-Δ3,4-glucoseen reductase (E3), is an NADH dependent plant type [2Fe-2S] containing flavoenzyme. To better understand the electron transfer carried out by these two enzymes, the potentials of the E1 and E3 redox cofactors were determined spectroelectrochemically. At pH 7.5, the midpoint potential of the E3 FAD was found to be −212 mV, with the FADox/FADsq couple (E1°‘) and the FADsq/FADhq couple (E2°‘) calculated to be −231 and −192 mV, respectively. However, the E1°‘ and E2°‘ of the FAD in E3(apoFeS) at pH 7...