The H-NOX (Heme-Nitric oxide/Oxygen binding) domain is highly conserved among both eukaryotes and bacteria. In human soluble Guanylyl Cyclase (sGC), the H-NOX domain functions as a sensor for the gaseous signaling molecule Nitric Oxide (NO). Located at the N-terminus of the enzyme, the heme B-binding H-NOX domain regulates the catalytic domain at the C-terminus, which catalyzes the conversion of GTP (guanosine 5'-triphosphate) to cGMP (cyclic Guanosine MonoPhosphate). The present study employs the bacterial homolog Nostoc sp. H-NOX (Ns H-NOX) C139A mutant together with the sGC inhibitor Zinc Protoporphyrin IX (ZnPPIX) as experimental tools to investigate the still poorly understood mechanisms underlying sGC activation and inhibition, and to elucidate either the chemical features of novel scaffolds associated with a new class of activators, or the distinct activities imposed by two prosthetic groups sharing the same organic framework, but differing in the chelated transition metal. Here, we present the conditions employed for the formation of the complex between the Ns H-NOX C139A mutant and the sGC inhibitor ZnPPIX, together with the UV-Vis absorption spectra of the heme-bound and ZnPPIX-bound protein forms. Furthermore, we report the NMR backbone and side-chain resonance assignments (