초록
<P><B>Optimizing designer metabolisms in vitro</B></P><P>Biological carbon fixation requires several enzymes to turn CO<SUB>2</SUB> into biomass. Although this pathway evolved in plants, algae, and microorganisms over billions of years, many reactions and enzymes could aid in the production of desired chemical products instead of biomass. Schwander <I>et al.</I> constructed an optimized synthetic carbon fixation pathway in vitro by using 17 enzymes—including three engineered enzymes—from nine different organisms across all three domains of life (see the Perspective by Gong and Li). The pathway is up to five times more efficient than the in vivo rates of the most common natural carbon fixation pathway. Further optimization of this and other metabolic pathways by using similar approaches may lead to a host of biotechnological applications.</P><P><I>Science</I>, this issue p. 900; see also p. 830</P><P>Carbon dioxide (CO<SUB>2</SUB>) is an important carbon feedstock for a future green economy. This requires the development of efficient strategies for its conversion into multicarbon compounds. We describe a synthetic cycle for the continuous fixation of CO<SUB>2</SUB> in vitro. The crotonyl–coenzyme A (CoA)/ethylmalonyl-CoA/hydroxybutyryl-CoA (CETCH) cycle is a reaction network of 17 enzymes that converts CO<SUB>2</SUB> into organic molecules at a rate of 5 nanomoles of CO<SUB>2</SUB> per minute per milligram of protein. The CETCH cycle was drafted by metabolic retrosynthesis, established with enzymes originating from nine different organisms of all three domains of life, and optimized in several rounds by enzyme engineering and metabolic proofreading. The CETCH cycle adds a seventh, synthetic alternative to the six naturally evolved CO<SUB>2</SUB> fixation pathways, thereby opening the way for in vitro and in vivo applications.</P>