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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 CO 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.