Elucidating the Formation of 6-Deoxyheptose: Biochemical Characterization of the GDP-d-glycero-d-manno-heptose C6 Dehydratase, DmhA, and Its Associated C4 Reductase, DmhB

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Abstract
6-Deoxyheptose is found within the surface polysaccharides of several bacterial pathogens. In Yersinia pseudotuberculosis, it is important for the barrier function of the O-antigen in vitro and for bacterial dissemination in vivo. The putative C6 dehydratase DmhA and C4 reductase DmhB, that were identified as responsible for 6-deoxyheptose synthesis based on genetics data, represent potential therapeutical targets. Their detailed biochemical characterization is presented herein. The substrate, GDP-d-glycero-d-manno-heptose, was synthesized enzymatically from sedoheptulose 7-phosphate using overexpressed and purified GmhA/B/C/D enzymes from Aneurinibacillus thermoaerophilus. Overexpressed and purified DmhA used this substrate with high efficiency, as indicated by its Km of 0.23 mM and kcat of 1.1 s−1. The mass spectrometry (MS) analysis of the reaction product was consistent with a C6 dehydration reaction. DmhB could readily reduce this compound in the presence of NAD(P)H to produce GDP-6-deoxy-d-manno-heptose, as indicated by MS and NMR analyses. DmhA also used GDP-mannose as a substrate with a Km of 0.32 mM and a kcat of 0.25 min−1. This kinetic analysis indicates that although the Km values for GDP-mannose and GDP-manno-heptose were similar, the genuine substrate for DmhA is GDP-manno-heptose. DmhB was also able to reduce the GDP-4-keto-6-deoxymannose produced by DmhA, although with poor efficiency and exclusively in the presence of NADPH. This study is the first complete biochemical characterization of the 6-deoxyheptose biosynthesis pathway. Also, it allows the screening for inhibitors, the elucidation of substrate specificity determinants, and the synthesis of carbohydrate antigens of therapeutic relevance.

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