Versatile Product Detection via Coupled Assays for Ultra-high-throughput Screening of Carbohydrate-Active-Enzymes in Microfluidic Droplets

Abstract

ABSTRACTEnzyme discovery and directed evolution are the two major contemporary approaches for the improvement of industrial processes by biocatalysis in various fields. Customization of catalysts for improvement of single enzyme reactions or de novo reaction development is often complex and tedious. The success of screening campaigns relies on the fraction of sequence space that can be sampled, whether for evolving a particular enzyme or screening metagenomes. Ultrahigh-throughput screening (uHTS) based on in-vitro compartmentalization in water-in-oil emulsion of picolitre droplets generated in microfluidic systems allows screening rates >1 kHz (or >107 per day). Screening for Carbohydrate Active Enzymes (CAZymes) catalysing biotechnologically valuable reactions in this format presents an additional challenge, because the released carbohydrates are difficult to monitor in high throughput. Activated substrates with large optically active hydrophobic leaving groups provide a generic optical readout, but the molecular recognition properties of sugars will be altered by incorporation of such fluoro- or chromophores and their typically higher reactivity, as leaving groups with lowered pKa values compared to native substrates make observation of promiscuous reactions more likely. To overcome these issues, we designed microdroplet assays in which optically inactive carbohydrate products are made visible by specific cascades: the primary reaction of an unlabelled substrate leads to an optical signal downstream. Successfully implementing such assays at the picoliter droplet scale allowed us to detect glucose, xylose, glucuronic acid and arabinose as final products of complex oligosaccharide degradation by glycoside hydrolases by absorbance measurements. Enabling the use of uHTS for screening CAZyme reactions that have been thus far elusive will chart a route towards faster and easier development of specific and efficient biocatalysts for biovalorisation, directing enzyme discovery towards catalysts for their natural rather than model substrates.Graphical abstract / TOC