Structural Insights into Unique Substrate Selectivity of Thermoplasma acidophilumd-Aldohexose Dehydrogenase.
The d-aldohexose dehydrogenase from the thermoacidophilic archaea Thermoplasma acidophilum (AldT) belongs to the short-chain dehydrogenase/reductase ( SDR) superfamily and catalyzes the oxidation of several monosaccharides with a preference for NAD(+) rather than NADP(+) as a cofactor. It has been found that AldT is a unique enzyme that exhibits the highest dehydrogenase activity against d-mannose. Here, we describe the crystal structures of AldT in ligand-free form, in complex with NADH, and in complex with the substrate d-mannose, at 2.1 A, 1.65 A, and 1.6 A resolution, respectively. The AldT subunit forms a typical SDR fold with an unexpectedly long C-terminal tail and assembles into an intertwined tetramer. The d-mannose complex structure reveals that Glu84 interacts with the axial C2 hydroxyl group of the bound d-mannose. Structural comparison with Bacillus megaterium glucose dehydrogenase (BmGlcDH) suggests that the conformation of the glutamate side-chain is crucial for discrimination between d-mannose and its C2 epimer d-glucose, and the conformation of the glutamate side-chain depends on the spatial arrangement of nearby hydrophobic residues that do not directly interact with the substrate. Elucidation of the d-mannose recognition mechanism of AldT further provides structural insights into the unique substrate selectivity of AldT. Finally, we show that the extended C-terminal tail completely shuts the substrate-binding pocket of the neighboring subunit both in the presence and absence of substrate. The elaborate inter-subunit interactions between the C-terminal tail and the entrance of the substrate-binding pocket imply that the tail may play a pivotal role in the enzyme activity.[1]References
- Structural Insights into Unique Substrate Selectivity of Thermoplasma acidophilumd-Aldohexose Dehydrogenase. Yasutake, Y., Nishiya, Y., Tamura, N., Tamura, T. J. Mol. Biol. (2007) [Pubmed]
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