Metalloprotein entatic control of ligand-metal bonds quantified by ultrafast x-ray spectroscopy

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Science  23 Jun 2017:
Vol. 356, Issue 6344, pp. 1276-1280
DOI: 10.1126/science.aam6203

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Sulfur's balancing act in cytochrome c

Cytochrome c enzymes have two distinct functions that depend on the position of a methionine residue. When the sulfur in the methionine side chain coordinates with iron in the enzyme's active site, the protein is optimized for electron transfer; otherwise, it is poised for peroxidase activity. Mara et al. used ultrafast x-ray absorption and emission spectroscopy to probe the energetics of this Fe-S bond (see the Perspective by Bren and Raven). By breaking the bond transiently with light and then timing its reformation, they determined that the surrounding protein environment boosts the bond strength by 4 kilocalories per mole—just enough to toggle between each functional state at a practical rate.

Science, this issue p. 1276; see also p. 1236


The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine residue. This Fe–S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe–S(Met) bond enthalpy is ~4 kcal/mol stronger than in the absence of protein constraints. The 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function.

  • * Present address: Department of Chemistry, University of California, Berkeley, Berkeley, CA 94720, USA.

  • Present Address: Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA.

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