title: Thermodynamic and kinetic stability of penicillin acylase from Escherichia coli.
abstract: Thermal denaturation of penicillin acylase (PA) from Escherichia coli has been studied by high-sensitivity differential scanning calorimetry as a function of heating rate, pH and urea concentration. It is shown to be irreversible and kinetically controlled. Upon decrease in the heating rate from 2 to 0.1 K min(-1) the denaturation temperature of PA at pH 6.0 decreases by about 6 degrees C, while the denaturation enthalpy does not change notably giving an average value of 31.6+/-2.1 J g(-1). The denaturation temperature of PA reaches a maximum value of 64.5 degrees C at pH 6.0 and decreases by about of 15 degrees C at pH 3.0 and 9.5. The pH induced changes in the denaturation enthalpy follow changes in the denaturation temperature. Increasing the urea concentration causes a decrease in both denaturation temperature and enthalpy of PA, where denaturation temperature obeys a linear relation. The heat capacity increment of PA is not sensitive to the heating rate, nor to pH, and neither to urea. Its average value is of 0.58+/-0.02 J g(-1) K(-1). The denaturation transition of PA is approximated by the Lumry-Eyring model. The first stage of the process is assumed to be a reversible unfolding of the alpha-subunit. It activates the second stage involving dissociation of two subunits and subsequent denaturation of the beta-subunit. This stage is irreversible and kinetically controlled. Using this model the temperature, enthalpy and free energy of unfolding of the alpha-subunit, and a rate constant of the irreversible stage are determined as a function of pH and urea concentration. Structural features of the folded and unfolded conformation of the alpha-subunit as well as of the transition state of the PA denaturation in aqueous and urea solutions are discussed.
authors.affiliation: N.M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Vavilov Street 28, 119991, Moscow, Russia.
authors.forename: Valerij Ya
authors.initials: VY
authors.lastname: Grinberg
authors.forename: Tatiana V
authors.initials: TV
authors.lastname: Burova
authors.forename: Natalia V
authors.initials: NV
authors.lastname: Grinberg
authors.forename: Tatiana A
authors.initials: TA
authors.lastname: Shcherbakova
authors.forename: Dorel T
authors.initials: DT
authors.lastname: Guranda
authors.forename: Ghermes G
authors.initials: GG
authors.lastname: Chilov
authors.forename: Vytas K
authors.initials: VK
authors.lastname: Svedas
mesh_terms: D002152:Calorimetry, Differential Scanning; D004795:Enzyme Stability / Q000187:drug effects; D004926:Escherichia coli / Q000187:drug effects / Q000201:enzymology*; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010399:Penicillin Amidase / Q000378:metabolism*; D011489:Protein Denaturation / Q000187:drug effects; D017510:Protein Folding; D021122:Protein Subunits / Q000378:metabolism; D013696:Temperature; D013816:Thermodynamics; D014508:Urea / Q000494:pharmacology
chemical_list: D021122:Protein Subunits; D014508:Urea; D010399:Penicillin Amidase