DEMONSTRATION OF THE ROLE OF SCISSION OF THE PROXIMAL HISTIDINE-IRON BOND IN THE ACTIVATION OF SOLUBLE GUANYLYL CYCLASE THROUGH METALLOPORPHYRIN SUBSTITUTION STUDIES

Activation of soluble guanylyl cyclase (sGC) by NO correlates with sci ssion of the proximal iron-histidine bond, as demonstrated by the appl ication of electronic absorption and resonance Raman spectroscopy to t he study of metalloporphyrin-substituted enzymes. The non-native metal loporphyrins, Mn(II)PPIX and Co(II)PPIX, can be introduced into heme-d eficient sGC forming five-coordinate complexes. The similarity among M n(II)sGC, Co(II)sGC, and the corresponding metalloporphyrin-substitute d derivatives of Mb and Hb provides confirming evidence for the presen ce of an axial histidine ligand in sGC. Upon addition of NO, Mn(II)sGC forms a six-coordinate species with the histidine ligand still bound to the Mn, and the enzyme is not activated. In contrast, the Co(II)sGC (NO) adduct is five-coordinate and the enzyme is activated. These data imply that the activated state of sGC is attained when the proximal h istidine-metal bond is broken.