Characterized enzymes in the mannonate dehydratase family catalyze the dehydration of mannonate to 2-dehydro-3-deoxy-D-gluconate, as part of the catabolism of D-glucuronate. Generally, genomes that encode a member of the mannonate dehydratase family do not have a UxuA homolog. Though UxuA also has the TIM barrel fold, it is not a member of the enolase superfamily. Thus, the enolase superfamily mannonate dehydratase family and the UxuAs are an example of convergent evolution of function.
Rakus JF, Fedorov AA, Fedorov EV, Glasner ME, Vick JE, Babbitt PC, Almo SC, Gerlt JA
Evolution of Enzymatic Activities in the Enolase Superfamily: d-Mannonate Dehydratase from Novosphingobium aromaticivorans
▸ Abstract
The d-mannonate dehydratase (ManD) function was assigned to a group of orthologous proteins in the mechanistically diverse enolase superfamily by screening a library of acid sugars. Structures of the wild type ManD from Novosphingobium aromaticivorans were determined at pH 7.5 in the presence of Mg2+ and also in the presence of Mg2+ and the 2-keto-3-keto-d-gluconate dehydration product; the structure of the catalytically active K271E mutant was determined at pH 5.5 in the presence of the d-mannonate substrate. As previously observed in the structures of other members of the enolase superfamily, ManD contains two domains, an N-terminal alpha+beta capping domain and a (beta/alpha)7beta-barrel domain. The barrel domain contains the ligands for the essential Mg2+, Asp 210, Glu 236, and Glu 262, at the ends of the third, fourth, and fifth beta-strands of the barrel domain, respectively. However, the barrel domain lacks both the Lys acid/base catalyst at the end of the second beta-strand and the His-Asp dyad acid/base catalyst at the ends of the seventh and sixth beta-strands, respectively, that are found in many members of the superfamily. Instead, a hydrogen-bonded dyad of Tyr 159 in a loop following the second beta-strand and Arg 147 at the end of the second beta-strand are positioned to initiate the reaction by abstraction of the 2-proton. Both Tyr 159 and His 212, at the end of the third beta-strand, are positioned to facilitate both syn-dehydration and ketonization of the resulting enol intermediate to yield the 2-keto-3-keto-d-gluconate product with the observed retention of configuration. The identities and locations of these acid/base catalysts as well as of cationic amino acid residues that stabilize the enolate anion intermediate define a new structural strategy for catalysis (subgroup) in the mechanistically diverse enolase superfamily. With these differences, we provide additional evidence that the ligands for the essential Mg2+ are the only conserved residues in the enolase superfamily, establishing the primary functional importance of the Mg2+-assisted strategy for stabilizing the enolate anion intermediate.
Wichelecki DJ, Graff DC, Al-Obaidi N, Almo SC, Gerlt JA
Identification of the in vivo function of the high-efficiency D-mannonate dehydratase in Caulobacter crescentus NA1000 from the enolase superfamily
▸ Abstract
The d-mannonate dehydratase (ManD) subgroup of the enolase superfamily contains members with varying catalytic activities (high-efficiency, low-efficiency, or no activity) that dehydrate d-mannonate and/or d-gluconate to 2-keto-3-deoxy-d-gluconate [Wichelecki, D. J., et al. (2014) Biochemistry 53, 2722-2731]. Despite extensive in vitro characterization, the in vivo physiological role of a ManD has yet to be established. In this study, we report the in vivo functional characterization of a high-efficiency ManD from Caulobacter crescentus NA1000 (UniProt entry B8GZZ7) by in vivo discovery of its essential role in d-glucuronate metabolism. This in vivo functional annotation may be extended to ~50 additional proteins.
