Investigating the Physiological Roles of Low-Efficiency d‑Mannonate and d‑Gluconate Dehydratases in the Enolase Superfamily: Pathways for the Catabolism of l‑Gulonate and l‑Idonate

The sequence/function space in the d-mannonate dehydratase subgroup (ManD) of the enolase superfamily was investigated to determine how enzymatic function diverges as sequence identity decreases [Wichelecki, D. J., et al. (2014) <i>Biochemistry</i> <i>53</i>, 2722–2731]. That study revealed that members of the ManD subgroup vary in substrate specificity and catalytic efficiency: high-efficiency (<i>k</i>_cat/<i>K</i>_M = 10³–10⁴ M^–1 s^–1) for dehydration of d-mannonate, low-efficiency (<i>k</i>_cat/<i>K</i>_M = 10–10² M^–1 s^–1) for dehydration of d-mannonate and/or d-gluconate, and no activity. Characterization of high-efficiency members revealed that these are ManDs in the d-glucuronate catabolic pathway {analogues of UxuA [Wichelecki, D. J., et al. (2014) <i>Biochemistry 53</i>, 4087–4089]}. However, the genomes of organisms that encode low-efficiency members of the ManDs subgroup encode UxuAs; therefore, these must have divergent physiological functions. In this study, we investigated the physiological functions of three low-efficiency members of the ManD subgroup and identified a novel physiologically relevant pathway for l-gulonate catabolism in <i>Chromohalobacter salexigens</i> DSM3043 as well as cryptic pathways for l-gulonate catabolism in <i>Escherichia coli</i> CFT073 and l-idonate catabolism in <i>Salmonella enterica</i> subsp. <i>enterica</i> serovar <i>Enteritidis</i> str. P125109. However, we could not identify physiological roles for the low-efficiency members of the ManD subgroup, allowing the suggestion that these pathways may be either evolutionary relics or the starting points for new metabolic potential