Gates and binding pockets for nitric oxide with cytochrome c', according to molecular dynamics

Random-acceleration molecular-dynamics (RAMD) simulations with models of homodimeric 6-ligated distal-NO and 5-ligated proximal-NO cytochrome c' complexes, in TIP3 H2 O, showed two distinct, non-intercommunicating worlds. In the framework of a long cavity formed by four protein helices with heme at one extremity, NO was observed to follow different pathways with the two complexes to reach the solvent. With the 6-ligated complex, NO was observed to progress by exploiting protein internal channels created by thermal fluctuations, and be temporarily trapped into binding pockets before reaching the preferred gate at the heme end of the cavity. In contrast, with the 5-ligated complex, NO was observed to surface the solvent-exposed helix 7, up to a gate at the other extremity of the protein, only occasionally finding an earlier, direct way out toward the solvent. That only bulk NO gets involved in forming the 5-ligated proximal-NO complex is in agreement with previous experimental observations, while the occurrence of binding pockets suggests that also reservoir NO might play a role with the distal-NO complex.