Time-resolved FTIR study of CO recombination with horseradish peroxidase

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.

Photothermal studies of CO photodissociation from peroxidases from horseradish and soybean

In this work, the results of photoacoustic calorimetry (PAC) studies involving CO photodissociation from horseradish peroxidase (HRP) and soybean peroxidase (SBP) are discussed. Both proteins contain Fe-protoporphyrin IX active sites and relatively open distal heme pockets (i.e., direct solvent access). In addition, it has been shown previously that SBP binds a Tris molecule in the distal pocket near the heme group potentially regulating ligand binding to the heme iron. Results of PAC studies indicate a fast (< approximately 50 ns) relaxation for both HRP and SBP subsequent to CO photolysis in both phosphate and Tris buffers and with varying concentrations of Tris. However, the molar volume/enthalpy changes associated with CO release are distinct between the two proteins. In the case of HRP, CO photolysis results in an enthalpy change of approximately 2 kcal mol(-1) and volume change of approximately -12 mL mol(-1) attributed to solvation/structural changes regardless of buffer conditions. In contrast, SBP exhibits buffer and ionic strength dependent enthalpy changes ranging from approximately -23 kcal mol(-1) in 50 mM phosphate buffer to approximately 6 kcal mol(-1) in Tris buffer with volume changes similar to those observed in HRP. The results are consistent with a model in which photodissociation of CO from ferrous HRP or SBP leads to CO migration from the distal heme pocket to the bulk solvent with a corresponding input of a water molecule all occurring in < approximately 50 ns. The differences in enthalpies are attributed to variations in hydrogen bond formation between the incoming water molecule(s) and the protein matrix in both HRP and SBP.