An automated workflow to screen alkene reductases using high-throughput thin layer chromatography.

BACKGROUND: Synthetic biology efforts often require high-throughput screening tools for enzyme engineering campaigns. While innovations in chromatographic and mass spectrometry-based techniques provide relevant structural information associated with enzyme activity, these approaches can require cost-intensive instrumentation and technical expertise not broadly available. Moreover, complex workflows and analysis time can significantly impact throughput. To this end, we develop an automated, 96-well screening platform based on thin layer chromatography (TLC) and use it to monitor in vitro activity of a geranylgeranyl reductase isolated from Sulfolobus acidocaldarius (SaGGR). RESULTS: Unreduced SaGGR products are oxidized to their corresponding epoxide and applied to thin layer silica plates by acoustic printing. These derivatives are chromatographically separated based on the extent of epoxidation and are covalently ligated to a chromophore, allowing detection of enzyme variants with unique product distributions or enhanced reductase activity. Herein, we employ this workflow to examine farnesol reduction using a codon-saturation mutagenesis library at the Leu377 site of SaGGR. We show this TLC-based screen can distinguish between fourfold differences in enzyme activity for select mutants and validated those results by GC-MS. CONCLUSIONS: With appropriate quantitation methods, this workflow can be used to screen polyprenyl reductase activity and can be readily adapted to analyze broader catalyst libraries whose products are amenable to TLC analysis.

Discovery of novel geranylgeranyl reductases and characterization of their substrate promiscuity.

BACKGROUND: Geranylgeranyl reductase (GGR) is a flavin-containing redox enzyme that hydrogenates a variety of unactivated polyprenyl substrates, which are further processed mostly for lipid biosynthesis in archaea or chlorophyll biosynthesis in plants. To date, only a few GGR genes have been confirmed to reduce polyprenyl substrates in vitro or in vivo. RESULTS: In this work, we aimed to expand the confirmed GGR activity space by searching for novel genes that function under amenable conditions for microbial mesophilic growth in conventional hosts such as Escherichia coli or Saccharomyces cerevisiae. 31 putative GGRs were selected to test for potential reductase activity in vitro on farnesyl pyrophosphate, geranylgeranyl pyrophosphate, farnesol (FOH), and geranylgeraniol (GGOH). We report the discovery of several novel GGRs exhibiting significant activity toward various polyprenyl substrates under mild conditions (i.e., pH 7.4, T = 37 °C), including the discovery of a novel bacterial GGR isolated from Streptomyces coelicolor. In addition, we uncover new mechanistic insights within several GGR variants, including GGR-mediated phosphatase activity toward polyprenyl pyrophosphates and the first demonstration of completely hydrogenated GGOH and FOH substrates. CONCLUSION: These collective results enhance the potential for metabolic engineers to manufacture a variety of isoprenoid-based biofuels, polymers, and chemical feedstocks in common microbial hosts such as E. coli or S. cerevisiae.

Probing the Flexibility of an Iterative Modular Polyketide Synthase with Non-Native Substrates in Vitro.

In the search for molecular machinery for custom biosynthesis of valuable compounds, the modular type I polyketide synthases (PKSs) offer great potential. In this study, we investigate the flexibility of BorM5, the iterative fifth module of the borrelidin synthase, with a panel of non-native priming substrates in vitro. BorM5 differentially extends various aliphatic and substituted substrates. Depending on substrate size and substitution BorM5 can exceed the three iterations it natively performs. To probe the effect of methyl branching on chain length regulation, we engineered a BorM5 variant capable of incorporating methylmalonyl- and malonyl-CoA into its intermediates. Intermediate methylation did not affect overall chain length, indicating that the enzyme does not to count methyl branches to specify the number of iterations. In addition to providing regulatory insight about BorM5, we produced dozens of novel methylated intermediates that might be used for production of various hydrocarbons or pharmaceuticals. These findings enable rational engineering and recombination of BorM5 and inform the study of other iterative modules.