Synthesis and Evaluation of Peptidic Probes for Tissue Transglutaminase and Factor XIIIa
Creators: name: Mulani, Amina
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2014
Subjects: Enzymes
Transglutaminases
Inhibition
Transition state analogues
Fluorescent probes
High-affinity peptides

description: Transglutaminases (TGases) are a group of enzymes that catalyze the formation of an amide bond between the γ-carboxamide group of a glutamine residue and an amine donor, usually an ε-amino group of the lysine residue, leading to the formation of ε-(γ-glutamyl)lysine crosslinks. Owing to the roles that transglutaminases such as tissue transglutaminase (TG2) and Factor XIIIa (FXIIIa) have been found to play in a wide range of disease states, efforts have been directed towards the study of these proteins. The study of enzymes to better understand their function and mode of action is facilitated through the use of tools such as protein labelling, enzyme inhibition, and substrate analogue kinetic studies among others. Transition state analogues have been effective inhibitors in the study of enzyme activity. Sulfoxide inhibitors can efficiently mimic transition states leading to the tetrahedral intermediate of an acyl transfer reaction and we discuss the synthesis towards sulfoxide transition state analogue inhibitors of TG2 in chapter 2. Novel sulfoxide compounds were synthesized, though the desired target compounds proved difficult to isolate due to their instability. Fluorescent probes are effective in protein labelling as a means of discerning activity. This technique was applied in order to elucidate intracellular TG2 activity, which is a topic of controversy. To that end, the synthesis of a fluorescent, TG2-specific, cell permeable probe is discussed in chapter 3. However, preliminary in vivo results show that while the probe is cell permeable and fluorescent, it was not TG2-specific. Molecular modelling suggests that the hexa-arginine tag, designed to improve cell permeability, decreases the affinity of the probe for its intended target. Finally, FXIIIa has become a new addition to the study of transglutaminases in the Keillor group. Given our interest in this enzyme, we had three goals for this work as explained in chapter 4. Firstly, owing to the anticipated high demand for FXIIIa required for later experiments, our primary aim was the development of an optimized method for the expression and purification of recombinant FXIIIA. After evaluating different conditions for FXIIIA expression, the Studier auto-induction ZYP media1 at 20 °C for 24 h was found to provide the optimal conditions for the expression of recombinant GST-tagged FXIIIA, typically giving a total of 1.5 mg of protein/L of culture. Secondly, a variety of different peptides were synthesized and tested using a glutamate dehydrogenase (GDH)-based assay to identify a high affinity sequence for a substrate of FXIIIa. The two peptides with the highest affinity for FXIIIa were Ac-DQMMMAF-OH and Ac-DQMML-OH. Testing with TG2 displayed negligible reactivity, confirming their use as orthogonal peptides, results reinforced by modelling studies of the peptides with both FXIIIa and TG2. This discovery represents the first time peptides orthogonal to TG2 with affinity for FXIIIa have been kinetically characterized with both transglutaminase enzymes. Lastly, our work towards a fluorogenic activity assay by incorporating a coumarin ester through attachment to a glutamic acid residue into a peptide sequence recognized by FXIIIa, will be discussed.