Kinetics and Thermodynamics of Ligand Binding

The four nitrophorins can be divided into two groups based not only upon sequence homology (Fig. 4), but also on the basis of their rates of NO binding and release. NP2 and NP3 bind NO more tightly, giving smaller values of the equilibrium dissociation constants, K, as well as larger second-order association rate constants and smaller dissociation rate constants, than NPl and NP4. 

The pH dependence of could be due to changes in A-B loop disorder rates, perhaps the chemical exchange phenomenon observed for NPl-ImH (Section ll,E,2,b), or to changes in ligand bond strength. The change in lies in the off-rates (Tables I-Ill) consistent with the loop disorder model. Plots of vs pH display an excellent fit with the equation for a titration curve (Fig. 21), indicating that the transition 

Fast and Slow Phase Dissociation Rate Constants (kos, s ) foe NO, Measured by Histamine Displacement of NO at pH 5.0 and 8.0 (50) 

Although the association kinetics of NO binding to NPl and NP4 appear to indicate simple, one-step reactions, the kinetics of NO binding and release are biphasic for NP2 and NP3 (50). The biphasic association kinetics were analyzed as a two-step reaction in which the first step is represented by the fast phase  

The bimolecular association rate constant ki and the monomolecular reverse rate constant k- were obtained from the fast reaction phase 

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Isin EM, Guengerich FP (2006) Kinetics and thermodynamics of ligand binding by cytochrome P450 3A4. J Biol Chem 281 9127-9136... 

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