High dissociation rate constant of ferrous-dioxy complex linked to the catalase-like activity in lactoperoxidase.
Heme reduction of ferric lactoperoxidase (LPO) into its ferrous form initially leads to the accumulation of the unstable form of LPO-Fe(II), which spontaneously converts to a more stable species, the two of which can be identified by Soret peaks at 440 and 434 nm, respectively. Our data demonstrate that both LPO-Fe(II) species are capable of binding O(2) at a similar rate to generate the ferrous-dioxy complex. Its formation with respect to O(2) was first order and monophasic and with rate constants of k(on) = 3.8 x 10(4) m(-1) s(-1) and k(off) = 11.2 s(-1). The dissociation rate constant for the formation of LPO-Fe(II)-O(2) is relatively high, in contrast to hemoprotein model compounds. This high dissociation rate can be attributed to a combination of effects that include the positive trans effect of the proximal ligand, the heme pocket environment, and the geometry of the Fe-O(2) linkage. Our results have also shown that the decay of the LPO-Fe(II)-O(2) complex occurs by two sequential O(2)-independent steps. The first step involves formation of a short-lived intermediate that can be characterized by its Soret absorption peak at 416 nm and may be attributed to the weakening of the Fe(II)-O(2) linkage with a rate constant of 0.5 s(-1). The second step is spontaneous conversion of this intermediate to generate the native enzyme and presumably superoxide as end products with a rate constant of 0.03 s(-1). A comprehensive kinetic model that links LPO-Fe(II)-O(2) complex formation to the LPO catalase-like activity, combined with the classic catalytic cycle, is presented here.
Galijasevic
Semira
S
Department of Obstetrics and Gynecology, Wayne State University School of Medicine, Detroit, Michigan 48201, USA.
Saed
Ghassan M
GM
Diamond
Michael P
MP
Abu-Soud
Husam M
HM
D000818:Animals; D002374:Catalase / Q000378:metabolism; D002384:Catalysis; D002417:Cattle; D005296:Ferrous Compounds / Q000378:metabolism*; D006418:Heme / Q000378:metabolism; D006861:Hydrogen Peroxide / Q000378:metabolism; D007700:Kinetics; D007784:Lactoperoxidase / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism; D013057:Spectrum Analysis
D005296:Ferrous Compounds; D006418:Heme; D006861:Hydrogen Peroxide; D007784:Lactoperoxidase; D002374:Catalase; D010100:Oxygen