|University of Victoria
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A diverse array of proteins has evolved to detect and affect carbohydrate structures, thereby performing critical roles in important biological events. Carbohydrate recognition usually employs a high degree of precision, as discriminating between two carbohydrate structures can depend on a single hydrogen bond or the configuration of a hydroxyl group. My work has focused on the molecular recognition of carbohydrate antigens by two biologically important classes of carbohydrate-binding proteins: antibodies and lectins. Single crystal x-ray diffraction has been employed to study the IgG2a antibody LPT3-1 and the lectins Griffonia simplicifolia 1-A4 (GSI-A4) and Lathyrus odoratus lectin (LOdL). LPT3-1 targets the conserved inner core structure of lipooligosaccharide from Neisseria meningitidis, the leading cause of meningitis and septicaemia. Structural characterization of LPT3-1 with an inner core fragment demonstrates how this antibody achieves selective cross-reactivity to variants of the inner core and provides insight that could support the development of a broadly protective N. meningitidis vaccine. Legume lectin GSI-A4 displays specificity towards the terminal galactose and N-acetyl-D-galactosamine of carbohydrates, yet the closely related lectin GSI-B4 will only recognize a terminal galactose. The structures of GSI-A4 co-crystallized with two different carbohydrates reveals the mechanism by which GSI-A4 displays this cross-reactivity, which allows for specific recognition of two important tumour-associated carbohydrate antigens. LOdL is a member of the Mannose/Glucose legume lectin family that can recognize an array of clinically significant antigens including abnormal glycosylation patterns on gp120 of HIV. Characterization of LOdL in complex with glucose at high resolution provides a putative primary sequence and molecular level insight into the molecular recognition displayed by this lectin. Structural data indicates LOdL is cross-reactive with the related glucose epimer mannose, and would display a similar if not identical affinity for glucose and mannose, enabling cross-reactivity with oligosaccharides displaying a terminal mannose. The similarity in sequence and primary recognition between LOdL and Pisum sativum lectin (PSL) suggests that LOdL also shares oligosaccharide specificity with PSL and similarly could demonstrate anti-HIV activity. Overall, the structural characterization of these three carbohydrate-binding proteins reveals mechanisms by which antibodies and lectins can employ selective cross-reactivity to discriminate among clinically-relevant carbohydrate structures. Advisors/Committee Members: Evans, Stephen V. (supervisor).