title: Kinetic study of the inhibition mechanism of dehaloperoxidase-hemoglobin a by 4-bromophenol.
abstract: The mechanism of dehaloperoxidase-hemoglobin (DHP) inhibition by 4-bromophenol (4-BP) was investigated using Michealis-Menten and transient-state kinetic analyses. Transient-state kinetics using the stopped-flow technique to mix DHP and H2O2 in the presence of inhibitor concentrations less than 10-fold greater than the enzyme concentration show that 4-BP does not fully impede H2O2 entering the distal pocket to activate DHP. It is not clear whether an oxoferryl intermediate is formed under these conditions and there may be alternative pathways for H2O2 reaction in the 4-BP bound form of DHP. Two new species have been identified during the reaction of 4-BP bound form of DHP in the transient-state kinetic experiment by using Singular Value Decomposition (SVD) and global-fitting analysis. Rather than forming Compound ES in the unbound form, an inhibitor bound intermediate that possesses blue-shifted Soret band and a double peaked Q-band is observed. This intermediate is subsequently converted to the end-point species that is distinguished from Compound RH formed in the uninhibited enzyme. Bench-top mixing kinetics of DHP were conducted in order to determine the inhibitor binding constant and to understand the enzyme inhibition mechanism from a thermodynamic perspective. It was found that the inhibition constant, Ki, decreased from 2.56 mM to 0.15 mM over the temperature range from 283 to 298 K, which permits determination of the enthalpy and entropy for inhibitor binding as -135.5 ± 20.9 kJ/mol and 526.1 ± 71.9 J/(mol·K), respectively, leading to the conclusion that inhibitor binding is entropically driven.
authors.affiliation: Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States.
authors.forename: Jing
authors.initials: J
authors.lastname: Zhao
authors.forename: Stefan
authors.initials: S
authors.lastname: Franzen
mesh_terms: D018360:Crystallography, X-Ray; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006454:Hemoglobins / Q000737:chemistry; D007700:Kinetics; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors / Q000737:chemistry; D008958:Models, Molecular; D010544:Peroxidases / Q000037:antagonists & inhibitors* / Q000737:chemistry; D010636:Phenols / Q000494:pharmacology*; D011485:Protein Binding / Q000187:drug effects
chemical_list: D004791:Enzyme Inhibitors; D006454:Hemoglobins; D010636:Phenols; C484322:2,4,6-trichlorophenol monooxygenase; D006899:Mixed Function Oxygenases; D010544:Peroxidases; C032037:4-bromophenol