Poster abstracts

Poster number 6 submitted by Veer Bhatt

Altered architecture of substrate binding region defines the unique specificity of UDP-GalNac 4-epimerases

Veer S. Bhatt (Department of Biochemistry, The Ohio State University), Peng George Wang (Department of Biochemistry, Department of Chemistry, The Ohio State University)

Abstract:
UDP-hexose 4-epimerases play a pivotal role in lipopolysaccharide biosynthesis and Leloir pathway. These epimerases are classified into 3 groups based on whether they recognize non-acetylated UDP-hexoses (group 1), N-acetylated UDP-hexoses (group 3) or both N-acetylated and non-acetylated UDP-hexoses (group 2). Although the catalysis has been investigated extensively, yet a definitive model rationalizing the substrate specificity of all the three groups on a common platform is largely lacking. In this work we present the crystal structure of WbgU, a novel UDP-hexose 4-epimerase that belongs to the group 3. WbgU is involved in biosynthetic pathway of the unusual glycan 2-deoxy-L-altruronic acid that is found in the lipopolysaccharide of the pathogen Pleisomonas shigelloides. A model that defines its substrate specificity is proposed on the basis of the active site architecture. Representatives from all the 3 groups are then compared to rationalize their substrate specificity. This investigation reveals that the group 3 active site architecture is markedly different from the ‘conserved scaffold’ of the group 1 and the group 2 epimerases. Furthermore, the interactions responsible for the origin of specificity of the group 3 epimerases towards N-acetylated hexoses are highlighted. These features ascribe a conformation to the substrate binding pocket of the group 3 epimerases that considerably lowers the free energy of the complex with N-acetylated hexoses as compared to the complex with non-acetylated hexoses. The implications of this study provide a platform for further engineering of the UDP-hexose 4-epimerases and can lead to a deeper understanding of the lipopolysaccharide biosynthesis, structure based design of novel antibacterial drugs and more economic synthesis of UDP-GalNAc and downstream products.

References:
Bhatt, V.S., Guo, Chu-yueh, Zhao, G., Yi, W., Liu Zhi-ji and Wang, P. G. Altered Architecture of Substrate Binding Region Defines the Unique Specificity of UDP-GalNac 4-epimerases. Protein Science (2011) Epub ahead of print PMID: 21384454.

Keywords: lipopolysaccharide, protein-carbohydrate interaction, epimerase