Photosensitizers, which harness light energy to upgrade weak reductants to strong reductants, are pivotal components of the natural and artificial photosynthesis machineries. However, it has proved difficult to enhance and expand their functions through genetic engineering. Here we report a genetically encoded, 27 kDa photosensitizer protein (PSP), which facilitates the rational design of miniature photocatalytic CO2-reducing enzymes. Visible light drives PSP efficiently into a long-lived triplet excited state (PSP*), which reacts rapidly with reduced nicotinamide adenine dinucleotide to generate a super-reducing radical (PSP•), which is strong enough to reduce many CO2-reducing catalysts. We determined the three-dimensional structure of PSP• at 1.8 Å resolution by X-ray crystallography. Genetic engineering enabled the site-specific attachment of a nickel–terpyridine complex and the modular optimization of the photochemical properties of PSP, the chromophore/catalytic centre distance and the catalytic centre microenvironment, which culminated in a miniature photocatalytic CO2-reducing enzyme that has a CO2/CO conversion quantum efficiency of 2.6%.
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Synthetic and experimental procedures, spectroscopic and mass spectrometric data, expression, purification and crystallization for X-ray diffraction as well as additional experiments are provided in the Supplementary Information. All other data are available from the authors upon request. Protein structures have been deposited to the Protein Data Bank under accession numbers 5YR2 and 5YR3.
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We are grateful for the financial support from the National Key Research and Development Program of China under awards 2017YFA0503704, 2016YFA0501502 and 2015CB856203; the National Science Foundation of China under awards 21750003, 91527302, U1632133, 31628004, 21473237, 31628004 and U1732264; Key Research Program of Frontier Sciences, CAS, grant numbers QYZDB-SSW-SMC032 and QYZDJ-SSW-SMC018; Tianjin Science and Technology grant 15PTCYSY00020; and Sanming Project of Medicine in Shenzhen (number SZSM201811092). We thank S. S. Zang for help with NMR spectra determination; Z. Xie for protein mass spectrometry; C. X. Zhang for ESR experiments; L. Xia for CV experiments; C. Wang and D. S. Liu for TEM experiments; J. H. Li for CD experiments and J. L. Jie for help with transient absorption spectroscopy experiments.