The initial goal was the conversion of Bifidobacterium adolescentis Sucrose Phosphorylase (BaSP) into a polyphenol glucosidase by structure based enzyme engineering. BaSP was chosen because of its ability to utilize sucrose, an economically viable and sustainable donor substrate, and transfer the glucosyl moiety to various acceptor substrates. The introduction of aromatic residues into the active site was considered a viable way to render it more suitable for aromatic acceptor compounds by reducing its polarity and potentially introducing π-π-interactions with the polyphenols. An investigation of the active site revealed Gln345 as a suitable mutagenesis target. As a proof of concept BaSP Q345F was employed in the glycosylation of (+)-catechin, (-)-epicatechin and resveratrol. The variant was selective for the aromatic acceptor substrates and the glucose disaccharide side reaction was only observed after almost quantitative conversion of the aromatic substrates. A crystal structure of BaSP Q345F in complex with glucose was obtained and it displayed an unexpected shift of an entire domain by 3.3 Å. A crystal structure of BaSP D192N-Q345F, an inactive variant in complex with resveratrol-3-α-D-glucosid, the glucosylation product of resveratrol, synthesized by BaSP Q345F was solved. It proved that the domain shift is in fact responsible for the ability of the variant to glycosylate aromatic compounds. Simultaneously a ligand free crystal structure of BaSP Q345F disproved an induced fit effect as the cause of the domain shift. The missing link, a crystal structure of BaSP Q345F in the F-conformation is obtained. This does not feature the domain shift, but is in outstanding agreement with the wildtype structure. The domain shift is therefore not static but rather a step in a dynamic process. It is further conceivable that the domain shifted conformation of BaSP Q345F resembles the open conformation of the wild type and that an adjustment of a conformational equilibrium as a result of the Q345F point mutation is observed. An investigation into the background reaction, the formation of glucose-glucose disaccharides of BaSP Q345F and three further variants that addressed the same region (L341C, D316C-L341C and D316C-N340C) revealed the formation of nigerose by BaSP Q345F. / Saccharose Phosphorylase aus Bifidobacterium adolescentis (BaSP) sollte durch strukturbasiertes Enzym-Engineering in die Lage versetzt werden Polyphenole zu glukosylieren. In die katalytische Tasche sollten aromatische Seitenketten eingeführt werden um die Polarität an jene der gewünschten Akzeptorsubstrate anzupassen und eine weitere Stabilisierung durch π-π-Wechselwirkungen zwischen Enzym und Substrat zu erlauben. BaSP Q345F war in der Lage die Zielsubstrate zu glukosylieren und behielt gleichzeitig ausreichen Aktivität bei. Die weitere Untersuchung dieses Enzyms ist in vier Studien beschrieben. Die Kristallstruktur einer inaktiven Variante, BaSP D192Q-Q345F in komplex mit dem Glukosylierungsprodukt Resveratrol-3-α-D-Glukosid wurde gelöst. Dadurch konnte gezeigt werden, dass einer Verschiebung einer Domäne für die Fähigkeit der Variante Glukose auf aromatische Substrate zu übertragen, verantwortlich ist. Die Orientierung des π-Systems von Resveratrol erlaubt weiterhin T-förmige π-π-Wechselwirkungen mit Phe156 und Phe345.Die detaillierte kinetische Untersuchung von BaSP Q345F mit acht Akzeptorsubstraten ergab eine starke Affinität der Variante zu den aromatischen Substraten (KM 0.08 bis 1.55 mM). Weitere Strukturdaten zeigen, dass die Verschiebung der Domäne Teil eines dynamischen Prozesses ist. Des weiteren ist die Q345F Variante in der Lage, den seltenen Zucker Nigerose zu synthetisisern.
| Identifer | oai:union.ndltd.org:uni-wuerzburg.de/oai:opus.bibliothek.uni-wuerzburg.de:19247 |
| Date | January 2020 |
| Creators | Kraus, Michael |
| Source Sets | University of Würzburg |
| Language | English |
| Detected Language | English |
| Type | doctoralthesis, doc-type:doctoralThesis |
| Format | application/pdf |
| Rights | https://opus.bibliothek.uni-wuerzburg.de/doku/lic_ohne_pod.php, info:eu-repo/semantics/openAccess |
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