Seminar - Christy Payne

Christina M. Payne

Assistant Professor
Department of Chemical and Materials Engineering
University of Kentucky

Understanding the Protein-Carbohydrate Recognition Mechanisms of Multimodular Enzymes

Abstract

Carbohydrates are the most abundant biological molecules on Earth and play important roles in metabolism, cell wall structure, and cellular-level processes; they also happen to be one of the most structurally diverse natural substrates by virtue of construction from a variety of chemically distinct monosaccharides and glycosidic linkages. In response to this diversity, carbohydrate active enzymes evolved structural approaches to aid in the synthesis, degradation, and modification of complex carbohydrate structures, one of which is modularity. Multimodular enzymes consist of carbohydrate binding modules (CBMs) and catalytic domains connected via peptide linkers to enable multiple functions and broaden specificity. Seemingly endless combinatorial constructs exist, where one can find a single CBM appended to a single catalytic domain or many different CBMs attached to a variety of catalytic domains. As the drivers of substrate recognition, CBMs play a significant role in modulating enzymatic function. To date, there are at least 80 known CBM families, each with unique protein folds delicately balancing the steric and electronic effects required for substrate recognition and specificity. As such, nature has provided a virtual building block set to engineer chimeric proteins tailored for industrial applications. However, to rationally implement these modular domains in biotechnological design, one must first develop an understanding of the protein-carbohydrate recognition mechanisms by which CBMs selectively bind their substrates. Here, we will present a case study examining protein-carbohydrate recognition phenomena in a set of cellulose-specific CBMs from families 4, 17, and 28. Using molecular dynamics (MD) simulations, free energy perturbation with replica exchange MD, and umbrella sampling MD, we elucidated preferential oligomeric binding orientations as well as the origins of high- and low-affinity binding to non-crystalline cellulose substrates. Our results suggest b-sandwich CBM folds have evolved to accommodate the structural symmetry of cello-oligomers by introducing redundant hydrogen bond acceptors along the cleft. The free energy calculations were in excellent agreement with isothermal titration calorimetry measurements from literature, suggesting these approaches are capable predictive tools in protein design. Overall, our study provides an unprecedented level of insight into the complex solid and soluble carbohydrate substrate recognition mechanisms of these CBMs, the findings of which hold considerable promise in both the enhancement of lignocellulosic biomass conversion technology and the development of tools for elucidating plant cell wall architectures. Finally, I will conclude by briefly describing our recent efforts toward understanding carbohydrate recognition processes in the catalytic domains of b-glucosidases and processive chitinases, generalizing our approaches to other important protein-carbohydrate systems such as hydrolytic enzymes.

Bio

Dr. Christina Payne is an assistant professor of chemical engineering at the University of Kentucky. She received her B. S. in Chemical Engineering from Tennessee Technological University in 2002 and her Ph. D. from Vanderbilt University in 2007. After completing her Ph.D., Dr. Payne briefly worked as a chemical process engineer for URS in the oil and gas industry and is a licensed professional engineer. She returned to research in 2011, joining the National Renewable Energy Laboratory as a postdoctoral researcher, where she was promoted the same year to staff scientist. In 2012, Dr. Payne accepted her current position at the University of Kentucky. She also holds the August T. Larsson Guest Researcher position in the Department of Chemistry and Biotechnology at the Swedish University of Agricultural Sciences. Dr. Payne’s awards include her recent National Science Foundation CAREER award and the Oak Ridge Associated Universities Ralph E. Powe Junior Faculty Award. Her contributions on carbohydrate active enzyme mechanisms have been published in the Proceedings of the National Academy of Sciences, the Journal of the American Chemical Society, Chemical Reviews, and many others peer-reviewed journals.