(223t) Why Pucker, Sugar? Computational Chemistry Investigations into Carbohydrate Ring Distortion for Enzymatic Action

Authors: 
Mayes, H. B., Northwestern University
Broadbelt, L. J., Northwestern University
Beckham, G. T., National Renewable Energy Laboratory

Since the first crystal structure of a lysozyme was solved in the 1960s, researchers have noted that glycoside hydrolase (GH) enzymes consistently distort the conformation of the carbohydrate ring closest to the scissile bond away from the solution-stable 4C1 chair conformation. A variety of theories have been proposed to account for this feature, the most promising of which focus on favorable stereoelectronic features of puckered rings, including accumulation of positive charge at the anomeric carbon, the site of nucleophilic attack. Quantum mechanical studies are an obvious choice for investigating such hypotheses. In this work, we present the most thorough electronic structure study to date of five biologically-paramount monosaccharides in vacuum, β-xylose, α-glucose, β-glucose, β-mannose, and β-N-acetylglucosamine. Exploiting the inherently parallel problem of comparing different monosaccharide conformations, we evaluated over 123,000 geometries for the set of molecules in the study, comprising all 38 IUPAC puckering geometries and exocyclic group orientations. We employed a step-wise screening approach of increasingly accurate methods to allow rigorous examination of the differences between their properties that lend particular puckers especially amenable to catalysis. We isolated both local minima and transition states to reveal the puckering interconversion landscape of each sugar. Comparing our findings to experimental structural studies of GH substrate distortion, we found that previously proposed correlations between enzymatically employed puckers and specific catalytically favorable electronic structure properties do not persist across all sugars in the study, mirroring important differences between the sugars. Additional factors, such as interactions between exocyclic groups, must be considered in a full picture of sugar puckering. These results demonstrate that exocyclic group conformations must be explicitly considered and reveal a complex optimization problem reflecting the complexity of glycobiology. It contributes a more nuanced understanding relationship between substrate conformation and reactivity.