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Azurin as a Protein Scaffold for a Low-coordinate Nonheme Iron Site with a Small-molecule Binding Pocket
- McLaughlin, Matthew
P., Retegan, Marius, Bill, Eckhard, Payne, Thomas M., Shafaat, Hannah S., Peña, Salvador, Sudhamsu, Jawahar, Ensign, Amy A., Crane, Brian R., Neese, Frank, Holland, Patrick L.
- Journal of the American Chemical Society 2012 v.134 no.48 pp. 19746-19757
- Pseudomonas aeruginosa, X-ray diffraction, absorption, anions, apoproteins, cyanides, cysteine, histidine, iron, mutation, nuclear magnetic resonance spectroscopy, oxidation, scaffolding proteins
- The apoprotein of Pseudomonas aeruginosa azurin binds iron(II) to give a 1:1 complex, which has been characterized by electronic absorption, Mössbauer, and NMR spectroscopies, as well as X-ray crystallography and quantum-chemical computations. Despite potential competition by water and other coordinating residues, iron(II) binds tightly to the low-coordinate site. The iron(II) complex does not react with chemical redox agents to undergo oxidation or reduction. Spectroscopically calibrated quantum-chemical computations show that the complex has high-spin iron(II) in a pseudotetrahedral coordination environment, which features interactions with side chains of two histidines and a cysteine as well as the CO of Gly45. In the ⁵A₁ ground state, the dz₂ orbital is doubly occupied. Mutation of Met121 to Ala leaves the metal site in a similar environment but creates a pocket for reversible binding of small anions to the iron(II) center. Specifically, azide forms a high-spin iron(II) complex and cyanide forms a low-spin iron(II) complex.