Controlling the trans effect induced by nitric oxide and carbon monoxide: H93C myoglobin versus H-NOX sensors and...

Myoglobin (Mb) has been engineered to replace the proximal histidine (His93) with a cysteine in order to investigate the trans effect induced by diatomic ligands using time-resolved electronic absorption spectroscopy. This single mutation induces a change of heme coordination state and bonding character which change carbon monoxide (CO) and nitric oxide (NO) dynamics. In H93C Mb the increased Fe²⁺-S distance weakens this bond which is replaced with a distal Fe²⁺-His64 ligation. We measured dynamics very different from wild type Mb but similar with those measured in soluble guanylate cyclase (sGC). Whereas NO induces a direct negative trans effect, the strain on His64 ligation is sufficient to counteract the positive trans effect due to CO. After photodissociation, geminate recombination of NO to the transient 4-coordinate heme of H93C occurred with a fast time constant (6.9 ps) identical to that in sGC. Remarkably, we also observed picosecond geminate rebinding of CO to H93C Mb, similarly with sGC in the simultaneous presence of CO and an allosteric stimulator. This CO rebinding dynamics to the 4c-heme in H93C Mb was never measured in other Mb mutants and demonstrates the existence of 5-coordinate heme with CO, explaining the synergistic activation of sGC in presence of CO and a stimulator.