Crystal Structures and Functional Studies Clarify Substrate Selectivity and Catalytic Residues for the Unique Orphan Enzyme N-Acetyl-D-Mannosamine Dehydrogenase.Sola-Carvajal, A., Gil-Ortiz, F., Garcia-Carmona, F., Rubio, V., Sanchez-Ferrer, A.
(2014) Biochem.J. 462: 499
- PubMed: 24969681
- DOI: 10.1042/BJ20140266
- Primary Citation of Related Structures:  4CR7, 4CR8
- PubMed Abstract:
NAMDH (N-acetyl-D-mannosamine dehydrogenase), from the soil bacteroidete Flavobacterium sp. 141-8, catalyses a rare NAD+-dependent oxidation of ManNAc (N-acetyl-D-mannosamine) into N-acetylmannosamino-lactone, which spontaneously hydrolyses into N-ac ...
NAMDH (N-acetyl-D-mannosamine dehydrogenase), from the soil bacteroidete Flavobacterium sp. 141-8, catalyses a rare NAD+-dependent oxidation of ManNAc (N-acetyl-D-mannosamine) into N-acetylmannosamino-lactone, which spontaneously hydrolyses into N-acetylmannosaminic acid. NAMDH belongs to the SDR (short-chain dehydrogenase/reductase) superfamily and is the only NAMDH characterized to date. Thorough functional, stability, site-directed mutagenesis and crystallographic studies have been carried out to understand better the structural and biochemical aspects of this unique enzyme. NAMDH exhibited a remarkable alkaline pH optimum (pH 9.4) with a high thermal stability in glycine buffer (Tm=64°C) and a strict selectivity towards ManNAc and NAD+. Crystal structures of ligand-free and ManNAc- and NAD+-bound enzyme forms revealed a compact homotetramer having point 222 symmetry, formed by subunits presenting the characteristic SDR α3β7α3 sandwich fold. A highly developed C-terminal tail used as a latch connecting nearby subunits stabilizes the tetramer. A dense network of polar interactions with the substrate including the encasement of its acetamido group in a specific binding pocket and the hydrogen binding of the sugar 4OH atom ensure specificity for ManNAc. The NAMDH-substrate complexes and site-directed mutagenesis studies identify the catalytic tetrad and provide useful traits for identifying new NAMDH sequences.
*Department of Biochemistry and Molecular Biology-A, Faculty of Biology, Regional Campus of International Excellence "Campus Mare Nostrum", University of Murcia, Campus Espinardo, E-30100 Murcia, Spain.