『(Abstract)
　Carbon fixation is the process by which CO2 is incorporated into organic compounds. In modern agriculture in which water, light, and nutrients can be abundant, carbon fixation could become a significant growth-limiting factor. Hence, increasing the fixation rate is of major importance in the road toward sustainability in food and energy production. There have been recent attempts to improve the rate and specificity of Rubisco, the carboxylating enzyme operating in the Calvin-Benson cycle; however, they have achieved only limited success. nature employs several alternative carbon fixation pathways, which prompted us to ask whether more efficient novel synthetic cycles could be devised. Using the entire repertoire of approximately 5,000 metabolic enzymes known to occur in nature, we computationally identified alternative carbon fixation pathways that combine existing metabolic building blocks from various organisms. We compared the natural and synthetic pathways based on physicochemical criteria that include kinetics, energetics, and topology. Our study suggests that some of the proposed synthetic pathways could have significant quantitative advantages over their natural counterparts, such as the overall kinetic rate. One such cycle, which is predicted to be two to three times faster than the Calvin-Benson cycle, employs the most effective carboxylating enzyme, phosphoenolpyruvate carboxylase, using the core of the naturally evolved C4 cycle. Although implementing such alternative cycles presents daunting challenges related to expression levels, activity, stability, localization, and regulation, we believe our findings suggest exciting avenues of exploration in the grand challenge of enhancing food and renewable fuel production via metabolic engineering and synthetic biology.

(Keywords:) metabolic engineering; synthetic biology; photosynthesis; carboxylation; biological optimization』

(Introduction)
Results
　Pathway analysis metrics enable a comprehensive comparison between pathways
　　NADPH cost
　　ATP cost
　　Number of enzymes
　　Metabolic compatibility of the synthetic pathways
　A systematic method to locate synthetic carbon fixation pathways reveals the simplest carbon fixation cycles
　Kinetically efficient carbon fixing pathways using the most attractive carboxylating enzymes
Discussion
　Analysis and optimization of carbon fixation pathways
　Implementing the synthetic carbon fixation pathways
Materials and methods
　Algorithm for finding carbon fixation cycles
　Calculating pathway specific activity
　Pathway evaluation and comparison
Acknowledgments
(References)