(65f) Enzymatically Powered Surface-Associated Self-Motile Protocells

Protocell engineering is an active area of research in which one tries to re-create fundamental cell processes, such as motility, secretion, and division, in synthetic structures.^{[1, 2]} Numerous examples of building synthetic motile systems in solution have been developed. A notable self-motile microstructure is the microswimmer, in which bisegmental rods made of platinum (Pt) and gold (Au) swim in hydrogen peroxide (H₂O₂) solutions.^[3-5] The Pt end spontaneously oxidizes H₂O₂ and the Au end reduces water, leading to a directional flow which induces autonomous directional motion of the rod toward the Pt end.^[5] Similar electrophoretic motions have been seen in both bimetallic structures, such as nanowires^[6] and Janus nanospheres.^{[7, 8]} Other platforms for recreating motility in solution using protocellular capsules have involved engineering a built-in asymmetry.^{[3-5, 9-11]} Wilson and co-workers have made stomatocyte-like structures using asymmetric polymersome design, in which enzymatic activity of entrapped catalytic motifs lead to the directional expulsion of products and hence propulsion.^[12] Recently, Battaglia and co-workers made synthetic asymmetric swimming nanoparticles that displayed directional motion into the glucose-rich brain by entrapping the enzyme glucose oxidase (which generates H₂O₂ from glucose) and catalase (which converts H₂O₂ to water and oxygen) within polymersomes.^[13] These systems all involve the motion of a particle in solution and an inherent asymmetry in either the geometry of the particle or the distribution of its enzymatic elements.

In this study, we demonstrate the motility of an adherent particle by harnessing an asymmetric fluctuation of enzyme activity in a protocell. This type is more akin to eukaryotic cell motion, in which traction is mediated by adhesion and in which motility requires the breakage and formation of adherent contacts.^[14] This work is different than seen with microswimmers in solution, because our vesicles do not have an inherent asymmetry and are adherent to a surface. We hypothesize that fluctuations in the force generation of enzymatic reactions on the surface of the protocell is sufficient to break adhesive contacts between polymer vesicles and the surface, and drive their autonomous motion, mimicking formation and breakage of adhesive contacts displayed by real mamalian cells. We incorporate the enzyme catalase into the lumen of biotinylated polymersomes that are adhesive to a solid surface that has been functionalized with a low density of avidin. Given that each turnover event in a typical enzyme reaction generates approiximately 10 pN of force^{[10, 15, 16]} and the force to break an biotin-avidin bond is approximately 170 pN in an observation time of 3 millisecond,^[17] a differential turnover of only @ 17 substrate molecules would be necessary to break a biotin-avidin contact and induce motion. Based on the large number of catalase molecules in the vesicle and its high turnover rate,^[11] such a fluctuation seems achievable. We show that enzymatic reactions indeed induce motion similar to a random-walk and that the motility depends on the concentration of the substrate in the solution. Our results demonstrate that enzymatic reactions can be harnessed to generate motility of surface-adherent protocells, opening up numerous possibility for making motile protocells of different activity and specificity.

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[9] K. K. Dey, X. Zhao, B. M. Tansi, W. J. Mendez-Ortiz, U. M. Cordova-Figueroa, R. Golestanian, A. Sen, A., Nano Letters 2015, 15, 8311-8315.

[10] H. S. Muddana, S. Sengupta, T.E. Mallouk, A. Sen, P. J. Butler, Journal of the American Chemical Society 2010, 132, 2110-2111.

[11] C. Riedel, R. Gabizon, C. A. M. Wilson, K. Hamadani, K. Tsekouras, S. Marqusee, S. Presse, C. Bustamante, C., Nature 2015, 517, 227-U288.

[12] D. A. Wilson, R. J. M. Nolte, J. C.M. van Hest, Nature Chemistry 2012, 4, 268-274.

[13] A. Joseph, C. Contini, D. Cecchin, S. Nyberg, L. Ruiz-Perez, J. Gaitzsch, G. Fullstone, X. H. Tian, J. Azizi, J. Preston, G. Volpe, G. Battaglia, Science Advances 2017, 3.

[14] D. A. Lauffenburger, J. J. Linderman, Receptors: Models for binding, trafficking and signaling. Oxford Press: New York, 1993.

[15] P. J. Butler, K. K. Dey, A. Sen, Cellular and Molecular Bioengineering 2015, 8, 106-118

[16] S. Sengupta, K. K. Dey, H. S. Muddana, T. Tabouillot, M. E. Ibele, P. J. Butler, A. Sen, Journal of the American Chemical Society 2013, 135, 1406-1414.

[17] R. Merkel, P. Nassoy, A. Leung, K. Ritchie, E. Evans, Nature 1999, 397, 50-53.