Azurin as a Protein Scaffold for a Low-coordinate Nonheme Iron Site with a Small-molecule Binding Pocket

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.