Promiscuous phosphoketolase and metabolic rewiring enables novel non-oxidative glycolysis in yeast for high-yield production of acetyl-CoA derived products.

Carbon-conserving pathways have the potential of increasing product yields in biotechnological processes. The aim of this project was to investigate the functionality of a novel carbon-conserving pathway that produces 3 mol of acetyl-CoA from fructose-6-phosphate without carbon loss in the yeast Saccharomyces cerevisiae. This cyclic pathway relies on a generalist phosphoketolase (Xfspk), which can convert xylulose-5-phosphate, fructose-6-phosphate and sedoheptulose-7-phosphate (S7P) to acetyl phosphate. This cycle is proposed to overcome bottlenecks from the previously reported non-oxidative glycolysis (NOG) cycle. Here, in silico simulations showed accumulation of S7P in the NOG cycle, which was resolved by blocking the non-oxidative pentose phosphate pathway and introducing Xfspk and part of the riboneogenesis pathway. To implement this, a transketolase and transaldolase deficient S. cerevisiae was generated and a cyclic pathway, the Glycolysis AlTernative High Carbon Yield Cycle (GATHCYC), was enabled through xfspk expression and sedoheptulose bisphosphatase (SHB17) overexpression. Flux through the GATHCYC was demonstrated in vitro with a phosphoketolase assay on crude cell free extracts, and in vivo by constructing a strain that was dependent on a functional pathway to survive. Finally, we showed that introducing the GATHCYC as a carbon-conserving route for 3-hydroxypropionic acid (3-HP) production resulted in a 109% increase in 3-HP titers when the glucose was exhausted compared to the phosphoketolase route only.

High-throughput single-cell activity-based screening and sequencing of antibodies using droplet microfluidics.

Mining the antibody repertoire of plasma cells and plasmablasts could enable the discovery of useful antibodies for therapeutic or research purposes1. We present a method for high-throughput, single-cell screening of IgG-secreting primary cells to characterize antibody binding to soluble and membrane-bound antigens. CelliGO is a droplet microfluidics system that combines high-throughput screening for IgG activity, using fluorescence-based in-droplet single-cell bioassays2, with sequencing of paired antibody V genes, using in-droplet single-cell barcoded reverse transcription. We analyzed IgG repertoire diversity, clonal expansion and somatic hypermutation in cells from mice immunized with a vaccine target, a multifunctional enzyme or a membrane-bound cancer target. Immunization with these antigens yielded 100-1,000 IgG sequences per mouse. We generated 77 recombinant antibodies from the identified sequences and found that 93% recognized the soluble antigen and 14% the membrane antigen. The platform also allowed recovery of ~450-900 IgG sequences from ~2,200 IgG-secreting activated human memory B cells, activated ex vivo, demonstrating its versatility.

Author Correction: High-throughput single-cell activity-based screening and sequencing of antibodies using droplet microfluidics.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Emergence of a catalytic tetrad during evolution of a highly active artificial aldolase.

Designing catalysts that achieve the rates and selectivities of natural enzymes is a long-standing goal in protein chemistry. Here, we show that an ultrahigh-throughput droplet-based microfluidic screening platform can be used to improve a previously optimized artificial aldolase by an additional factor of 30 to give a >109 rate enhancement that rivals the efficiency of class I aldolases. The resulting enzyme catalyses a reversible aldol reaction with high stereoselectivity and tolerates a broad range of substrates. Biochemical and structural studies show that catalysis depends on a Lys-Tyr-Asn-Tyr tetrad that emerged adjacent to a computationally designed hydrophobic pocket during directed evolution. This constellation of residues is poised to activate the substrate by Schiff base formation, promote mechanistically important proton transfers and stabilize multiple transition states along a complex reaction coordinate. The emergence of such a sophisticated catalytic centre shows that there is nothing magical about the catalytic activities or mechanisms of naturally occurring enzymes, or the evolutionary process that gave rise to them.

Single-cell deep phenotyping of IgG-secreting cells for high-resolution immune monitoring.

Studies of the dynamics of the antibody-mediated immune response have been hampered by the absence of quantitative, high-throughput systems to analyze individual antibody-secreting cells. Here we describe a simple microfluidic system, DropMap, in which single cells are compartmentalized in tens of thousands of 40-pL droplets and analyzed in two-dimensional droplet arrays using a fluorescence relocation-based immunoassay. Using DropMap, we characterized antibody-secreting cells in mice immunized with tetanus toxoid (TT) over a 7-week protocol, simultaneously analyzing the secretion rate and affinity of IgG from over 0.5 million individual cells enriched from spleen and bone marrow. Immunization resulted in dramatic increases in the range of both single-cell secretion rates and affinities, which spanned at maximum 3 and 4 logs, respectively. We observed differences over time in dynamics of secretion rate and affinity within and between anatomical compartments. This system will not only enable immune monitoring and optimization of immunization and vaccination protocols but also potentiate antibody screening.