Comparisons of redox kinetics of methemerythrin and mu-sulfidomethemerythrin. Implications for interactions with cytochrome b5.
We have examined the effects on redox kinetics of changing the reduction potential of the mu-oxo-bridged binuclear iron center in octameric hemerythrin (Hr) from Phascolopsis gouldii. The opportunity to examine such effects is provided by the availability of mu-sulfidomethemerythrin (mu-S2-metHr), whose [Fe(III),Fe(III)]met----[Fe(II),Fe(III)]semi-met reduction potential is approximately 200 mV higher than that of methemerythrin (metHr). We have used, as redox partners to Hr, a set of metal complexes and the heme proteins deoxymyoglobin (Mb) and cytochrome b5. The latter protein from P. gouldii is a presumed physiological redox partner of Hr. Similar kinetics at pH 8 in the presence or absence of the allosteric effector perchlorate suggest reduction of the iron atom closer to the outer surface of each subunit in the Hr octamer during the met----semi-met transformation. For all reducing agents, the experimentally determined ratio of second-order rate constants for reductions of mu-S2-metHr and metHr, k12(mu-S2-met)/k12(met), is close to the value of 40 predicted by the simple Marcus relation for "outer-sphere" electron transfer. For oxidations of (semi-met)RHr and mu-S2-semi-metHr, the predicted value of 40 for k12[(semi-met)R]/k12(mu-S2-semi-met) is closely approximated when Fe(CN)6(3-) is used as oxidant. The ionic strength dependence of the second-order rate constant suggests electrostatic interactions of opposite charges during reduction of metHr by P. gouldii cytochrome b5.(ABSTRACT TRUNCATED AT 250 WORDS)
Pearce
L L
LL
Department of Chemistry, Iowa State University, Ames 50011.
Utecht
R E
RE
Kurtz
D M
DM
D000818:Animals; D002151:Calorimetry; D002417:Cattle; D003573:Cytochrome b Group / Q000378:metabolism*; D015786:Cytochromes b5; D004578:Electron Spin Resonance Spectroscopy; D006422:Hemerythrin / Q000031:analogs & derivatives / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D046911:Macromolecular Substances; D008667:Metalloproteins / Q000378:metabolism*; D008958:Models, Molecular; D009348:Nematoda / Q000378:metabolism; D010084:Oxidation-Reduction; D011487:Protein Conformation
D003573:Cytochrome b Group; D006422:Hemerythrin; D046911:Macromolecular Substances; D008667:Metalloproteins; C019004:methemerythrin; C055414:mu-sulfidomethemerythrin; D015786:Cytochromes b5