Genetic biosensor enables in vivo glycosyltransferase screening

Glycosylation of natural products can alter their solubility and bioavailability, among other properties, which makes glycosyltransferases useful tools for increasing the production and/or generating novel compounds in microbial cell factories. However, the discovery and screening of new enzymes and engineered variants is often a low-throughput endeavor due to the need for over-expression and purification prior to in vitro experiments, which do not necessarily represent the in vivo activities of the enzyme. Therefore, a genetic biosensor controlling GFP expression was developed based on the flavonoid responsive transcriptional-repressor QdoR and expressed in E. coli. Due to the induced fluorescent response upon feeding the flavonoids Quercetin and Kaempferol, but not to their glucosides, the activity of UDP-dependent glycosyltransferases (UGTs) could be screened in vivo. Furthermore, a variant of QdoR was generated by directed evolution that showed greater dose-responsiveness and proved to allow greater discrimination of cellular populations and was thus more useful for in vivo UGT screening. The designed biosensor-based method will greatly increase the throughput of glycosyltransferase discovery and engineering. Please click Additional Files below to see the full abstract

Small-molecule biosensors for high-throughput metabolic engineering

Allosteric transcription factors (aTFs) have proven widely applicable for biotechnology and synthetic biology as ligand-specific biosensors enabling real-time monitoring, selection and regulation of cellular metabolism. However, both the biosensor specificity and the correlation between ligand concentration and biosensor output signal, also known as the transfer function, often needs to be optimized before meeting application needs. In this presentation we outline a versatile and high-throughput method to evolve and functionalize prokaryotic aTF ligand specificity and transfer functions in a eukaryote chassis, namely baker’s yeast Saccharomyces cerevisiae. From a single round of directed evolution of the aTF ligand-binding domain coupled with various toggled selection regimes, we robustly select aTF variants evolved for change in ligand specificity, increased dynamic output range, shifts in operational range, and a complete inversion of function from activation to repression. Importantly, by targeting only the ligand-binding domain, the evolved biosensors display DNA-binding affinities similar to parental aTFs and are functional when ported back into a non-native prokaryote chassis. The developed platform technology thus leverages aTF evolvability for the development of new biosensors with user-defined small-molecule specificities and transfer functions. Finally, the presentation will highlight examples on biosensor applications for high-throughput metabolic engineering. Please click Additional Files below to see the full abstract

Oligomerization engineering of the fluorinase enzyme leads to an active trimer that supports synthesis of fluorometabolites in vitro

This work was funded by The Novo Nordisk Foundation grant to the Center for Biosustainability (NNF10CC1016517). P.I.N. was funded by grants from The Novo Nordisk Foundation (NNF20CC0035580, and LiFe, NNF18OC0034818), the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement No. 814418 (SinFonia) and the Danish Council for Independent Research (SWEET, DFF-Research Project 8021-00039B). T.K. and M.N.D. were funded by fellowships from the European Union's Horizon 2020 research and innovation program under a Marie Skłodowska Curie project under grant agreement No. 713683 (COFUNDfellowsDTU).The fluorinase enzyme represents the only biological mechanism capable of forming stable C–F bonds characterized in nature thus far, offering a biotechnological route to the biosynthesis of value-added organofluorines. The fluorinase is known to operate in a hexameric form, but the consequence(s) of the oligomerization status on the enzyme activity and its catalytic properties remain largely unknown. In this work, this aspect was explored by rationally engineering trimeric fluorinase variants that retained the same catalytic rate as the wild-type enzyme. These results ruled out hexamerization as a requisite for the fluorination activity. The Michaelis constant (KM) for S-adenosyl-l-methionine, one of the substrates of the fluorinase, increased by two orders of magnitude upon hexamer disruption. Such a shift in S-adenosyl-l-methionine affinity points to a long-range effect of hexamerization on substrate binding – likely decreasing substrate dissociation and release from the active site. A practical application of trimeric fluorinase is illustrated by establishing in vitro fluorometabolite synthesis in a bacterial cell-free system.Publisher PDFPeer reviewe

Regioselective Glycosylation of Polyphenols by Family 1 Glycosyltransferases:Experiments and Simulations

Family 1 glycosyltransferases (GT1s, UGTs) form natural product glycosides with exquisite control over regio- and stereoselectivity, representing attractive biotechnological targets. However, regioselectivity cannot be predicted and large-scale activity assessment efforts of UGTs are commonly performed via mass spectrometry or indirect assays that are blind to regioselectivity. Here, we present a large high performance liquid chromatography screening discriminating between regioisomeric products of 40 diverse UGTs (28.6% average pairwise sequence identity) against 32 polyphenols, identifying enzymes able to reach high glycosylation yields (≥90% in 24 h) in 26/32 cases. In reactions with &gt;50% yield, we observed perfect regioselectivity for 47% (75/158) on polyphenols presenting two hydroxyl groups and for 30% (43/143) on polyphenols presenting ≥3 hydroxyl groups. Moreover, we developed a nuclear magnetic resonance-based procedure to identify the site of glycosylation directly on enzymatic mixtures. We further selected seven regiospecific reactions catalyzed by four enzymes on five dihydroxycoumarins. We characterized the four enzymes, showing that temperature optima are functions of the acceptor substrate, varying by up to 20 °C for the same enzyme. Furthermore, we performed short molecular dynamics simulations of 311 ternary complexes (UGT, UDP-Glc, and glycosyl acceptor) to investigate the molecular basis for regioselectivity. Interestingly, it appeared that most UGTs can accommodate acceptors in configurations favorable to the glycosylation of either hydroxyl. In contrast, evaluation of hydroxyl nucleophilicity appeared to be a strong predictor of the hydroxyl predominantly glycosylated by most enzymes.</p

Glucuronan lyases from family PL7 use a Tyr/Tyr <i>syn </i>β-elimination catalytic mechanism for glucuronan breakdown

TpPL7A and TpPL7B, members of CAZy family PL7, act as β-glucuronan lyases. TpPL7A diverges by lacking the catalytic histidine, identified as the Brønsted base in PL7 alginate lyases. Our research, including TpPL7A's crystal structure, and mutagenesis studies, reveals a shared syn-β-elimination mechanism with a single tyrosine serving as both base and acid catalyst. This mechanism may extend to subfamily PL7_4 glucuronan lyases