Journal
ORGANOMETALLICS
Volume 39, Issue 9, Pages 1532-1544Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.organomet.9b00843
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Funding
- U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), Division of Chemical Sciences, Geosciences Biosciences
- U.S. DOE, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the Science Undergraduate Laboratory Internships Program (SULI)
- DOE's Office of Biological and Environmental Research
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-FG02x04ER15563, DE-AC02-76SF00515]
- DOE Office of Biological and Environmental Research
- National Institutes of Health, National Institute of General Medical Sciences [P41GM103393]
- USDA National Institute of Food and Agriculture [1015621]
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The protein scaffold around the active site of enzymes is known to influence catalytic activity, but specific scaffold features responsible for favorable influences are often not known. This study focuses on using an artificial metalloenzyme to probe one specific feature of the scaffold, the position of a positive charge in the outer coordination sphere around the active site. Previous work showed that a small molecular complex, [Rh((PNPEt2)-N-Et2-P-glycine)(2)](-), immobilized covalently within a protein scaffold was activated for CO2 hydrogenation. Here, using an iterative design where the effect of arginine, histidine, or lysine residues placed in the outer coordination sphere of the catalytic active site were evaluated, we tested the hypothesis that positively charged groups facilitate CO2 hydrogenation with seven unique constructs. Single-, double-, and triple-point mutations were introduced to directly compare catalytic activity, as monitored by turnover frequencies (TOFs) measured in real time with H-1 NMR spectroscopy, and evaluate related structural and electronic properties. Two of the seven constructs showed a 2- and 3-fold increase relative to the wild type, with overall rates ranging from 0.2 to 0.7 h(-1), and a crystal structure of the fastest of these shows the positive charge positioned next to the active site. A crystal structure of the arginine-containing complex shows that the arginines are positioned near the metal. Molecular dynamics (MD) studies also suggest that the positive charge is oriented next to the active site in the two constructs with faster rates but not in the others and that the positive charge near the active site holds the CO2 near the metal, all consistent with a positive charge appropriately positioned in the scaffold benefiting catalysis. The MD studies also suggest that changes in the water distribution around the active site may contribute to catalytic activity, while modest structural changes and movement of the complex within the scaffold do not.
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