Experimental and theoretical studies into the structural importance of the histidine brace motif of LPMO’s

Details

Supervisors

Singleton, Michael L. ; Robiette, Raphaël

Faculty

Faculté des sciences

Degree label

Master [120] en sciences chimiques, à finalité approfondie

Abstract

Lytic polysaccharide monooxygenases or LPMO’s are a new family of metalloenzymes present in various organisms (fungi, bacteria, and plants) that can disrupt polysaccharide crystallinity. Initially discovered for the degradation of chitin, different types of LPMO’s were discovered that act on various polysaccharides including cellulose. This activity is a breakthrough for the ecological transition and processes that require cellulose degradation (bioethanol and bio-based commodity/platform chemicals production). These enzymes can cleave the glycosidic bond of cellulose by oxidizing very strong C-H bonds (100 kcal/mol). The origin of the powerful oxidant properties is proposed to come from the unique chelation of the copper center. The metal atom in the active site is chelated by a T-shaped N3 coordination, with a N-terminal histidine, a primary amine and another histidine, called the histidine brace. An enzyme being challenging to work with, models of the active were designed, synthesized and studied to better understand de mode of action of LPMOs. Itoh’s group recently reported the best model so far in terms of first coordination sphere resemblance, electrochemical properties and catalytic activity. They proposed that those great results come from the large torsion angle present between the imidazole rings. The goal of this project was thus to prepare a new series of LPMO models with some structural variations that could let us study the influence of the torsion angle on properties in a systematic fashion. The impact of odd N-methylation observed in some enzymes was also studied. A series of six imine-complexes was prepared thanks to a copper templated reaction. The crystallographic data of the complexes showed a torsion angle between 3 and 16°. The electrochemical and reactivity studies could not highlight the impact of the torsion angle. The computational studies performed on an ideal LPMO model showed a direct correlation between the redox potential of the copper centre and the torsion angle between the imidazole rings. The synthesis of new models with different torsion angle needs to be continued to better understand which structural features of the histidine brace motif are essential.