(28b) Carbon Fixation By Rubisco-Nanostructure Complex to Produce 3-Phosphoglyceric Acid: A Life Cycle Assessment

As an effort to eliminate greenhouse gases (GHGs), biochemists, microbiologists, and engineers are working toward systems that mimic the plant cellular environment’s ability to convert GHGs into higher value products [1]. In the Calvin cycle of photosynthesis, ribulose-1,5-bisphosphate carboxylase oxygenase (RubisCO), which is an enzyme present in chloroplasts, catalyzes the conversion of ribulose 1,5-bisphosphate (RuBP) to 3-phosphoglyceric acid (3-PGA) while incorporating atmospheric CO₂ into the organic molecule [1,2]. It has been said that RubisCO is the most abundant protein in the world and accounts for most of the biological CO₂ fixed on earth [3]. Satagopan et al. have developed multienzyme-nanostructure complexes that catalyze cascade reactions in the Calvin cycle to enhance the catalytic properties of enzymes [1]. The complexes are made of three enzymes that include Rubisco and a nanostructure to support the enzymes.

In this work, a carbon footprint and other environmental impacts of 3-PGA produced through carbon fixation by the multienzyme-nanostructure complexes are evaluated using the Life Cycle Assessment (LCA) approach [4]. Two types of nanostructures for the complex are investigated as follows: Camptothecin (CPT)-dipeptide nanotube [1,5] and fluorenylmethyloxycarbonyl (Fmoc) tetrapeptide nanofiber. To examine the effectiveness of carbon fixation by those emerging technologies, another route that doesn’t fix CO₂ to prepare 3-PGA is investigated [6] and the carbon footprint of each route is compared. Also, the potential use of 3-PGA and its impacts are examined to account for a cradle-to-grave life cycle boundary. From the results of life cycle impact assessment, processes that show the highest contribution to the footprint are identified.

Many challenges exist in applying the LCA to emerging technologies due to the nascent nature of these technologies [7]. If any life cycle inventory (LCI) data are not available from existing database [8,9], the data are estimated from laboratory experiments or obtained from literatures and patents. To account for the uncertainty of those data, a sensitivity analysis is performed. Three cases are considered based on the amount of fugitive emissions and the recyclability of unreacted reactants and solvents. The reusability of the complexes is also taken into consideration through the sensitivity analysis because they function as catalysts for the carbon fixation. Also, most of energy flows are excluded from the inventory analysis because most of biological processes take place around room temperature. If product yields are unknown, they are determined by a stoichiometry calculation.

CPT is extracted from the stem of a specific tree species, Camptotheca acuminate [10]. Since its extraction yield from the tree and the subsequent product yield of the CPT-dipeptide nanotube are low, the total carbon footprint is affected significantly depending on whether biogenic CO₂ emissions are included in the analysis or not. In this work, therefore, the effects of the biogenic CO₂ emissions on the footprint are examined and discussed.

Even though this work accounts for some data uncertainty via sensitivity analysis, other sources of uncertainty are still left for a more robust analysis. Some data, such as labor, equipment, and other unavailable data, are excluded from the analysis boundary due to the lack of information. Also, inventories from different LCI database [8,9] often base on different location. Moreover, since many unavailable data from the LCI database are obtained from literatures and patents, there may be errors in the life cycle network as well as the inventory data. Those potential uncertainty and errors are mostly attributed to the nature of emerging technologies. If a more complete analysis that accounts for those issues is developed, it would present significant benefits for evaluating the impacts of emerging technologies.

References

[1] Sriram Satagopan, Yuan Sun, Jon R. Parquette, and F. Robert Tabita. Self-assembled nanostructures for capturing atmospheric CO₂. Unpublished manuscript.

[2] Spencer M. Whitney, Robert L. Houtz, and Hernan Alonso. Advancing our understanding and capacity to engineer nature’s CO₂-sequestering enzyme, Rubisco. Plant Physiology. 155(1): 27-35, 2011.

[3] John A. Raven. Rubisco: still the most abundant protein of Earth? New Phytologist. 198(1): 1-3, 2013.

[4] ISO. Environmental management—Life cycle assessment—Principles and framework (ISO 14040:2006). ISO. Geneva, Switzerland, 2016.

[5] Se Hye Kim et al. The self-assembly of anticancer camptothecin–dipeptide nanotubes: A minimalistic and high drug loading approach to increased efficacy. Chemistry - A European Journal. 21(1): 101-105, 2015.

[6] Ines Mandl and Carl Neuberg. Preparation of D(-)-3-phosphoglyceric acid. Methods in Enzymology. 3:208-214, 1957.

[7] Marcelle C. McManus et al. Challenge clusters facing LCA in environmental decision-making–what we can learn from biofuels. The International Journal of Life Cycle Assessment. 20(10):1399-1414, 2015.

[8] U.S. Life Cycle Inventory Database. National Renewable Energy Laboratory, 2012.

[9] Rolf Frischknecht et al. The ecoinvent database: Overview and methodological framework. International Journal of Life Cycle Assessment, 10(1):3–9, 2005.

[10] J. George Buta and Michael J. Novak, Isolation of camptothecin and 10-methoxycamptothecin from camptotheca acuminate by gel permeation chromatography. Industrial and Engineering Chemistry Product Research and Development. 17(2): 160-161, 1978.