Determination of the nature of the heme environment in nitrosyl indoleamine 2,3-dioxygenase using Multiple-scattering analyses of X-ray absorption fine structure

Multiple-scattering analysis of X-ray absorption fine structure data on the NO adducts of indoleamine 2,3-dioxygenase (IDO) and analysis of X-ray absorption near-edge structure (XANES) have provided the first direct structural information about the iron center for this ubiquitous mammalian metalloprotein. The IDO(II)NO adduct, which is likely to play a physiological role in the immune system, differs from similar adducts such as Mb(II)NO and Lb(II)NO in that the Fe-His bond is essentially broken. At 10 K, the Fe-N(p)(av) bond length = 2.00(2) A, Fe-NO bond length = 1.75 A, and angle = 140 degrees, which are typical of five-coordinate Fe(II)NO species. The XANES is also closer to that of five-coordinate model complexes than six-coordinate species. In addition to the Fe(II)NO species, there was a minor component of the Fe(III)NO adduct because of incomplete reduction of the Fe(II) species. This was also a five-coordinate center and consists of a linear Fe(II)NO(+) moiety with the Fe-N(p)(av) bond length = 2.00(2) A, Fe-NO bond length = 1.63(3) A, and angle = 179 degrees. The results indicate that both the blocking of the heme site to O(2) binding and conformational changes induced by breaking the Fe-N(epsilon) bond may be important mechanisms by which NO inhibits IDO in vitro and in vivo.