CO binding and ligand discrimination in human myeloperoxidase

Despite the fact that ferrous myeloperoxidase (MPO) can bind both O(2) and NO, its ability to bind CO has been questioned. UV/visible spectroscopy was used to confirm that CO induces small spectral shifts in ferrous MPO, and Fourier transform infrared difference spectroscopy showed definitively that these arose from formation of a heme ferrous-CO compound. Recombination rates after CO photolysis were monitored at 618 and 645 nm as a function of CO concentration and pH. At pH 6.3, k(on) and k(off) were 0.14 mM(-1) x s(-1) and 0.23 s(-1), respectively, yielding an unusually high K(D) of 1.6 mM. This affinity of MPO for CO is 10 times weaker than its affinity for O(2). The observed rate constant for CO binding increased with increasing pH and was governed by a single protonatable group with a pK(a) of 7.8. Fourier transform infrared spectroscopy revealed two different conformations of bound CO with frequencies at 1927 and 1942 cm(-1). Their recombination rate constants were identical, indicative of two forms of bound CO that are in rapid thermal equilibrium rather than two distinct protein populations with different binding sites. The ratio of bound states was pH-dependent (pK(a) approximately 7.4) with the 1927 cm(-1) form favored at high pH. Structural factors that account for the ligand-binding properties of MPO are identified by comparisons with published data on a range of other ligand-binding heme proteins, and support is given to the recent suggestion that the proximal His336 in MPO is in a true imidazolate state.