Kinetics of oxygen binding to ferrous myeloperoxidase.
Myeloperoxidase (MPO), which is involved in host defence and inflammation, is a unique peroxidase in having a globin-like standard reduction potential of the ferric/ferrous couple. Intravacuolar and exogenous MPO released from stimulated neutrophils has been shown to exist in the oxyferrous form, called compound III. To investigate the reactivity of ferrous MPO with molecular oxygen, a stopped-flow kinetic analysis was performed. In the absence of dioxygen, ferrous MPO decays to ferric MPO (0.04 s(-1) at pH 8 versus 1.4 s(-1) at pH 5). At pH 7.0 and 25 degrees C, compound III formation (i.e., binding of dioxygen to ferrous MPO) occurs with a rate constant of (1.1+/-0.1) x 10(4)M(-1)s(-1). The rate doubles at pH 5.0 and oxygen binding is reversible. At pH 7.0, the dissociation equilibrium constant of the oxyferrous form is (173+/-12)microM. The rate constant of dioxygen dissociation from compound III is much higher than conversion of compound III to ferric MPO (which is not affected by the oxygen concentration). This allows an efficient transition of compound III to redox intermediates which actually participate in the peroxidase or halogenation cycle of MPO.
Jantschko
Walter
W
Metalloprotein Research Group, Division of Biochemistry, Department of Chemistry, BOKU-University of Natural Resources and Applied Life Sciences, Muthgasse 18, A-1190 Vienna, Austria.
Furtmüller
Paul Georg
PG
Zederbauer
Martina
M
Jakopitsch
Christa
C
Obinger
Christian
C
D000693:Anaerobiosis; D004795:Enzyme Stability; D005290:Ferric Compounds / Q000378:metabolism; D005296:Ferrous Compounds / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D009195:Peroxidase / Q000737:chemistry* / Q000378:metabolism*; D013053:Spectrophotometry / Q000379:methods
D005290:Ferric Compounds; D005296:Ferrous Compounds; D009195:Peroxidase; D010100:Oxygen