In the spirit of collaboration, our team is excited to be working with the University of Iowa iGEM team this year. The Iowa 2018 team is developing a biosensor for the detection and quantification of 3-hydroxypropionate (3HP), a natural plastic precursor with considerable importance in the industrial production of bioplastics.
In order to produce a co-polymer such as polyhydroxybutyrate-co-valerate PHBV, the monomer 3-hydroxyvaleryl-CoA (3HV) can be introduced via the precursor propionyl-CoA. Currently, the necessity of propionate or propionyl-CoA is a limiting factor in production of PHBV, which has potential to become a versatile polymer in the commercial setting today. With our metabolic engineering strategy inspired by that of Srirangan et al. (2016), the E. coli can be modified to produce PHBV from substrates such as glucose or glycerol, exempting the need for direct feed with propionic acid. This requires the activation of a cluster of genes called the Sleeping Beauty Mutase (SBM) operon, which has been inactivated through multiple evolutionary selections, that encodes for the net conversion of succinyl-CoA into propionyl-CoA (Figure 1).
Figure 1. Proposed mechanism for propionate synthesis via the evolutionarily dormant Sleeping Beauty mutase operon (SBM) and a complementary methylmalonyl-CoA epimerase (MCE). Succinyl-CoA is converted into (R)-methylmalongyl-CoA by the methylmalonyl-CoA mutase (encoded by scpA). The compound is then converted into its stereoisomer (S)-methylmalonyl-CoA via MCE. The isomer is the form required for the stereospecific conversion into propionyl-CoA by methylmalonyl-CoA carboxylase (scpB). The CoA from propionyl-CoA is transferred onto succinate from the citric acid cycle by the propionyl-CoA: succinate CoA transferase (scpC), culminating in the biosynthesis of propionate and succinyl-CoA.
However, even if the conversion is successful it is difficult to verify a successful trial due to limitations in the capacity to detect production of the precursors culminating in the 3HV monomer. An assay that our team has developed to measure the quantity of propionate produced informed by the work of Phechkrajang & Yooyong (2017) was ultimately unsuccessful.
To our joy, we were happy to discover that the Iowa team may have just the answer to our prayers! As can be seen in Figure 2, the 3HP biosensor they have developed may be applicable in the direct detection of the propionyl-CoA synthesized via the SBM pathway. Motivated by a 2016 paper by Rogers & Church, we surmised that a 3HP biosensor that relies on the conversion of 3HP via pcs and prpC into a quantifiable fluorescence readout may be modified to directly quantify the biogenesis of propionyl-CoA, one of the intermediate compounds in this pathway!
Figure 2. The designated pathway through which our team expects to detect the production of propionyl-CoA from our sbm+ (woke AF) E. coli. This is taken from one of the potential pathways that a 3HP biosensor can be developed: by converting 3HP to 2-methylcitrate, fluorescence output can be quantified. Given this possibility, a modified biosensor (comprising prpC) may be used to detect the production of propionyl-CoA directly through a similar means of fluorescence readout.
Over the course of the summer we had the pleasure to meet and talk with teams from around UK. During the UK Meetup we crossed with an amazing project developed by the Westminster iGEM team. And we have one component in common: we are targeting the plastic problem.
Although from different approaches but inspired by the same aim, we talked in a way of collaborate when our Sustainability team thought that the Life Cycle Assessment (link to LCA page) tool would be a good model to see the environmental hot spots of their process. Because we experienced that LCA was being really helpful in the design of our PHBV production process and we wanted to share the experience and knowledge gained during our summer.
Phechkrajang, C.M. & Yooyong, S., 2017. Fast and simple method for semiquantitative determination of calcium propionate in bread samples. Journal of Food and Drug Analysis, 25(2), pp.254–259. Available at: http://www.sciencedirect.com/science/article/pii/S1021949816300552.
Rogers, J.K. & Church, G.M., 2016. Genetically encoded sensors enable real-time observation of metabolite production. Proceedings of the National Academy of Sciences, 113(9), pp.2388–2393. Available at: http://www.pnas.org/lookup/doi/10.1073/pnas.1600375113.
Srirangan, K. et al., 2016. Engineering of Escherichia coli for direct and modulated biosynthesis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer using unrelated carbon sources. Scientific Reports, 6(October), pp.1–11. Available at: http://dx.doi.org/10.1038/srep36470.
