title: Kinetic study of the effects of calcium ions on cationic artichoke (Cynara scolymus L.) peroxidase: calcium binding, steady-state kinetics and reactions with hydrogen peroxide.
abstract: The apparent catalytic constant (k(cat)) of artichoke (Cynara scolymus L.) peroxidase (AKPC) with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS) increased 130-fold in the presence of calcium ions (Ca2+) but the affinity (K(m)) of the enzyme for ABTS was 500 times lower than for Ca2+-free AKPC. AKPC is known to exhibit an equilibrium between 6-aquo hexa-coordinate and penta-coordinate forms of the haem iron that is modulated by Ca2+ and affects compound I formation. Measurements of the Ca2+ dissociation constant (K(D)) were complicated by the water-association/dissociation equilibrium yielding a global value more than 1000 times too high. The value for the Ca2+ binding step alone has now been determined to be K(D) approximately 10 nM. AKPC-Ca2+ was more resistant to inactivation by hydrogen peroxide (H(2)O(2)) and exhibited increased catalase activity. An analysis of the complex H(2)O(2) concentration dependent kinetics of Ca2+-free AKPC is presented.
authors.affiliation: GENZ: Grupo de Investigacíon de Enzimología, Departamento de Bioquímica y Biología Molecular-A, Facultad de Biología, Universidad de Murcia, Murcia, Spain.
authors.forename: Alexander N P
authors.initials: AN
authors.lastname: Hiner
authors.forename: Lara
authors.initials: L
authors.lastname: Sidrach
authors.forename: Soledad
authors.initials: S
authors.lastname: Chazarra
authors.forename: Ramón
authors.initials: R
authors.lastname: Varón
authors.forename: José
authors.initials: J
authors.lastname: Tudela
authors.forename: Francisco
authors.initials: F
authors.lastname: García-Cánovas
authors.forename: José Neptuno
authors.initials: JN
authors.lastname: Rodríguez-López
mesh_terms: D002118:Calcium / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D031943:Cynara scolymus / Q000201:enzymology*; D006861:Hydrogen Peroxide / Q000737:chemistry*; D007700:Kinetics; D010544:Peroxidases / Q000737:chemistry*
chemical_list: D002412:Cations; D006861:Hydrogen Peroxide; D010544:Peroxidases; D002118:Calcium