Kinetics of CO binding to the haem domain of murine inducible nitric oxide synthase: differential effects of haem domain ligands

The binding of CO to the murine inducible nitric oxide synthase (iNOS) oxygenase domain has been studied by laser flash photolysis. The effect of the (6R)-5,6,7,8-tetrahydro-l-biopterin (BH4) cofactor l-arginine and several Type I l-arginine analogues/ligands on the rates of CO rebinding has been evaluated. The presence of BH4 in the iNOS active site has little effect on the rebinding of protein-caged haem–CO pairs (geminate recombination), but decreases the bimolecular association rates 2-fold. Addition of l-arginine to the BH4-bound complex completely abolishes geminate recombination and results in a further 80-fold decrease in the overall rate of bimolecular association. Three of the Type I ligands, S-ethylisothiourea, l-canavanine and 2,5-lutidine, displaced the CO from the haem iron upon addition to the iNOS oxygenase domain. The Type I ligands significantly decreased the rate of bimolecular binding of CO to the haem iron after photolysis. Most of these ligands also completely abolished geminate recombination. These results are consistent with a relatively open distal pocket that allows CO to bind unhindered in the active site of murine iNOS in the absence of l-arginine or BH4. In the presence of BH4 and l-arginine, however, the enzyme adopts a more closed structure that can greatly reduce ligand access to the haem iron. These observations are discussed in terms of the known structure of iNOS haem domain and solution studies of ligand binding in iNOS and neuronal NOS isoenzymes.