The conformational landscape of β-phosphoglucomutase and its role in enzyme activity

The research delineated in this thesis concerns the conformational landscape of the phosphoryl transfer enzyme β-phosphoglucomutase (βPGM) from Lactococcus lactis and its impact on its activity. βPGM catalyses the isomerisation of β-glucose 1-phosphate to glucose 6-phosphate via a β-glucose 1,6-bisphosphate intermediate. βPGM is the gatekeeper between the metabolism of maltose and trehalose and glycolysis. Here we show that βPGM exchanges between two conformations with different activities on a multi-second time scale through cis-trans proline isomerisation. This exchange process is dependent on the phosphorylating agent present and only β-glucose 1,6-bisphosphate couples the phosphorylation step with the conformational switch, allowing βPGM to reach maximal activity. We termed this control mechanism allomorphy, to emphasise its relationship to and distinction from both allostery and allokairy mechanisms. Allomorphy allows βPGM to differentially modulate its activity depending on the carbohydrate concentrations inside the cell. Additionally, βPGM can achieve high catalytic proficiency by efficiently utilising the substrate binding energy to facilitate the adoption of a closed active conformation instead of using that energy to stabilise the transition state of the chemical step. Furthermore, our results uncover a trend whereby enzymes that catalyse intrinsically difficult chemistry exhibit a greater requirement to stabilise the closed active form. However, this catalytic proficiency mechanism comes at the expense of introducing substrate inhibition to catalysis. This thesis highlights the importance of understanding of the conformational landscape of enzymes and offers novel tools in the design of synthetic enzymes that catalyse difficult reactions not found in Nature.