Nature-inspired Circular-economy Recycling (NaCRe) for Proteins

Abstract

Since a few decades our planet has been loaded with billion tons of synthetic polymer-based materials, commonly named plastics. The large scale of plastic production, associated with its limited recyclability, are the driving force for the accumulation of such materials into the environment. Garbage patches, i.e. islands of plastics in the ocean, are striking evidences of this issue. A sustainable handling, that is production and disposal, of synthetic polymer-based materials is one of the greatest challenges that humanity has to face. Proteins and nucleic acids are natural polymers. Arguably, Nature produces an amount of such polymers that is higher with respect to man-made polymers. However, these materials do not accumulate into the environment, that is the approach used by Nature to handle these polymers is sustainable. The secret for its sustainaibility lies on the circularity in the materialsâ use. Indeed, proteins and nucleic acids are sequence-defined polymers that undergo depolymerization to monomers, and recycling into new materials by reassembling the so obtained monomers into arbitrarily different sequences. Organisms digest proteins into amino acids. These monomers are in turn polymerized to produce the protein of need, at the time of the protein synthesis. In this thesis we show that this process is achievable outside living organisms. Indeed, we depolymerized structurally different short peptides, and peptides mixtures into their constitutive amino acids, and recycled such monomers into biotechnologically relevant proteins (green, and red fluorescent proteins), by using an amino acid-free cell-free transcription-translation system. We further applied our methodology to recycle proteins with high relevance in materials engineering such as β-lactoglobulin films, used for water filtration, or silk fibroin solutions into green fluorescent protein. We were also successful in recycling mixtures composed of short peptides, and technologically relevant proteins into fluorescent proteins, as well as into bioactive enzymes (catechol 2,3-dioxygenase). Finally, we achieved multiple cycles of recycling (two cycles), and we demonstrated that the strategy can be expanded beyond the set of the twenty proteinogenic amino acids. Presented herein is a nature-inspired approach to recycling of soft materials, where unknown mixtures of polymers are recycled into the polymer of need, by the local community, at the time of recycling. The materials are costantly transformed into different ones, without any external feed, and there is no way to distinguish a new polymer from a recycled one. The strength of this method lies in its compatibility with the principles of circular economy.