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PubMed1975

Optical and magnetic resonance studies of formate binding to horse liver catalase and sperm whale myoglobin.

The binding of formate ion, a substrate for the peroxidatic reaction of catalase, has been investigated by magnetic resonance techniques. Comparative studies of formate binding to ferric myoglobin have also been performed. The nuclear magnetic relaxation (NMR) rate of formate and water protons is enhanced by the presence of ferric horse liver catalase. The enhancement is not changed significantly by the addition of cyanide, indicating that water and formate are still bound in the presence of cyanide. Formate proton to heme iron distances determined by magnetic resonance techniques indicate that formate does not directly bind to the heme iron of catalase or myoglobin but to the globin, and NMR relaxation occurs as a result of outersphere mechanisms. Evidence that water forms an innersphere complex with the iron atom of the catalase heme is presented. In similar experiments with ferric myoglobin, the addition of cyanide caused a large decrease in the enhancement of the proton relaxation rate of both formate and water, indicating the displacement of water and formate from the heme and the vicinity of the heme, respectively. Broad, high-spin, ferric ion electron paramagnetic resonance absorptions of catalase and myoglobin at room temperature obtained in the presence and absence of formate show that formate does not alter appreciably the heme environment of catalase or myoglobin or the spin state of the heme iron. Studies on the binding of formate to catalase as monitored by changes in the heme absorption spectrum in the visible region show one-to-one stoichiometry with heme concentration. However, the small changes observed in the visible region of the optical spectrum on addition of formate ion are attributed to a secondary effect of formate on the heme environment, rather than direct binding of formate to the heme moiety.

Animals↗
PubMed1997

Ligand migration in sperm whale myoglobin.

Geminate oxygen rebinding to myoglobin was followed from a few nanoseconds to a few microseconds after photolysis for more than 25 different oxymyoglobin point mutants in the presence and absence of 12 atm of xenon. In all cases, two relaxations were observed: an initial fast phase (half-time 20 ns) and a slower, smaller phase (half-time 0.5-2 micros). Generally, xenon accelerates the fast reaction but slows the slower reaction and diminishes its amplitude. The rates and proportions of the two components and the effects of xenon on them vary widely for different mutants. The locations of specific xenon binding sites [Tilton, R. F., Kuntz, I. D. Jr., and Petsko, G. A. (1984) Biochemistry 23, 2849-2857], the effects of point mutations on the geminate reactions, and molecular dynamics simulations were used to suggest locations in the protein interior occupied by ligands on the nanosecond to microsecond time scale. Photodissociated ligands may occupy xenon site 4 in the distal pocket and xenon site 1 below the plane of the heme. Rebinding from these positions corresponds to the slower geminate phase for O2 rebinding. The rapid geminate component is determined by competition between rebinding from a position closer to the iron atom and escape to solvent or more distant locations in the protein.

Animals↗
PubMed2004

Hybridization of modified-heme reconstitution and distal histidine mutation to functionalize sperm whale myoglobin.

To modulate the physiological function of a hemoprotein, most approaches have been demonstrated by site-directed mutagenesis. Replacement of the native heme with an artificial prosthetic group is another way to modify a hemoprotein. However, an alternate method, mutation or heme reconstitution, does not always demonstrate sufficient improvement compared with the native heme enzyme. In the present study, to convert a simple oxygen storage hemoprotein, myoglobin, into an active peroxidase, we applied both methods at the same time. The native heme of myoglobin was replaced with a chemically modified heme 2 having two aromatic rings at the heme-propionate termini. The constructed myoglobins were examined for 2-methoxyphenol (guaiacol) oxidation in the presence of H2O2. Compared with native myoglobin, rMb(H64D.2) showed a 430-fold higher kcat/Km value, which is significantly higher than that of cytochrome c peroxidase and only 3-fold less than that of horseradish peroxidase. In addition, myoglobin-catalyzed degradation of bisphenol A was examined by HPLC analysis. The rMb(H64D.2) showed drastic acceleration (>35-fold) of bisphenol A degradation compared with the native myoglobin. In this system, a highly oxidized heme reactive species is smoothly generated and a substrate is effectively bound in the heme pocket, while native myoglobin only reversibly binds dioxygen. The present results indicate that the combination of a modified-heme reconstitution and an amino acid mutation should offer interesting perspectives toward developing a useful biomolecule catalyst from a hemoprotein.

Animals↗