title: Photothermal studies of CO photodissociation from peroxidases from horseradish and soybean.
abstract: 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.
authors: Department of Chemistry, University of South Florida, 4202 East Fowler Ave., Tampa, FL 33620, USA.
authors: Audrey
authors: A
authors: Mokdad
authors: Jaroslava
authors: J
authors: Miksovská
authors: Randy W
authors: RW
authors: Larsen
mesh_terms: D002151:Calorimetry; D002248:Carbon Monoxide / Q000378:metabolism*; D006735:Horseradish Peroxidase / Q000378:metabolism*; D008958:Models, Molecular; D009211:Myoglobin / Q000378:metabolism; D010544:Peroxidases / Q000378:metabolism*; D055668:Photochemical Processes; D013025:Glycine max / Q000201:enzymology; D013816:Thermodynamics
chemical_list: D009211:Myoglobin; D002248:Carbon Monoxide; D006735:Horseradish Peroxidase; D010544:Peroxidases