A Sliding Scale Rule for Selectivity among NO, CO, and O2 by Heme Protein Sensors

Selectivity among NO, CO, and O-2 is crucial for the physiological function of most heme proteins. Although there is a million-fold variation in equilibrium dissociation constants (K-D), the ratios for NO:CO:O-2 binding stay roughly the same, 1:similar to 10(3):similar to 10(6), when the proximal ligand is a histidine and the distal site is apolar. For these proteins, there is a sliding scale rule for plots of log(K-D) versus ligand type that allows predictions of K-D values if one or two are missing. The predicted K-D for binding of O-2 to Ns H-NOX coincides with the value determined experimentally at high pressures. Active site hydrogen bond donors break the rule and selectively increase O-2 affinity with little effect on CO and NO binding. Strong field proximal ligands such as thiolate, tyrosinate, and imidazolate exert a leveling effect on ligand binding affinity. The reported picomolar K-D for binding of NO to sGC deviates even more dramatically from the sliding scale rule, showing a NO:CO K-D ratio of 1:similar to 10(8). This deviation is explained by a complex, multistep process, in which an initial lowaffinity hexacoordinate NO complex with a measured K-D of approximate to 54 nM, matching that predicted from the sliding scale rule, is formed initially and then is converted to a high-affinity pentacoordinate complex. This multistep six-coordinate to five-coordinate mechanism appears to be common to all NO sensors that exclude O-2 binding to capture a lower level of cellular NO and prevent its consumption by dioxygenation.