STRUCTURAL CHANGES IN THE HEME PROXIMAL POCKET INDUCED BY NITRIC OXIDE BINDING TO SOLUBLE GUANYLATE CYCLASE

When expressed in Escherichia coli, the heme domain [β1(1−385)] of rat lung soluble guanylate cyclase (sGC) is isolated with a stoichiometric amount of bound heme [Zhao, Y., and Marletta, M. A. (1997) Biochemistry 36, 15959−15964]. Nitric oxide (NO) binding to the heme in β1(1−385) leads to cleavage of the Fe−His bond and formation of a five-coordinate NO−heme complex. Addition of imidazole to the five-coordinate NO complex shifts the Soret peak from 399 to 420 nm, which appears to result from the formation of a six-coordinate NO complex. Removal of the added imidazole by gel filtration results in formation of the five-coordinate NO complex once again. The EPR spectrum of the putative six-coordinate NO complex has nine distinct derivative-shaped lines (a triplet of triplets), which is the signature spectrum of a six-coordinate NO complex with two nitrogen atoms as the axial ligands. [15N]Imidazole simplifies the six-coordinate NO complex EPR spectrum to six distinct derivative-shaped lines (a triplet of d...