Redox reactions of myoglobin

Abstract

Significance: Failure to maintain myoglobin (Mb) in the reduced state causes the formation of metMb, ferryl Mb species, and cross-linked Mb. Dissociation of ferriprotoporphyrin IX from the globin and release of iron atoms can also occur as oxidized Mb accumulates. These modifications may contribute to various oxidative pathologies in muscle and muscle foods.

Recent advances: The mechanism of ferryl Mb-mediated oxidative damage to nearby structures has been partially elucidated. Dissociation of ferriprotoporphyrin IX from metMb occurs more readily at acidic pH values. The dissociated ferriprotoporphyrin IX (also called hemin) readily decomposes preformed lipid hydroperoxides to reactive oxygen species. Heme oxygenase as well as lipophilic free radicals can degrade the protoporphyrin IX moiety, which results in the formation of free iron.

Critical issues: The multiple pathways by which Mb can incur toxicity create difficulties in determining the major cause of oxidative damage in a particular system. Peroxides and low pH activate each of the oxidative Mb forms, ferriprotoporphyrin IX, and released iron. Determining the relative concentration of these species is technically difficult, but essential to a complete understanding of oxidative pathology in muscle tissue.

Future directions: Improved methods to assess the different pathways of Mb toxicity are needed. Although significant advances have been made in the understanding of Mb interactions with other biomolecules, further investigation is needed to understand the physical and chemical nature of these interactions.

FIG. 1.

Ribbon representation of O₂(II)Mb including the ferroprotoporphyrin IX moiety. Structural aspects of sperm whale O₂(II)Mb are shown. The distal histidine, proximal histidine, and ferroprotoporphyrin IX (heme) moiety are shown in stick representation. Each helix is labeled. The iron atom in the center of the heme ring and ligand O₂ are shown as spheres. The heme-6-propionate (H6P) group is to the left of the heme-7-propionate. The PDB structure 1MBO (51) was used to prepare the image shown using PyMOL software. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars. Mb, myoglobin; PDB, Protein Data Bank.

FIG. 2.

Amino acid substitution of Leu29 with phenylalanine increases O₂ affinity. Phenylalanine at site B10 stabilizes O₂ that is liganded to the iron atom of ferroprotoporphyrin IX in Mb. L29F is used to describe this Mb mutant because site B10 is the 29th residue in sperm whale Mb. Leucine at B10 in native Mb does not stabilize O₂ to the iron atom of ferroprotoporphyrin IX, which results in lower O₂ affinity. Image is adapted from ref. (63).

FIG. 3.

Pseudoperoxidase cycle involving Mb and H₂O₂. Deoxy(II)Mb reacts with hydrogen peroxide (H₂O₂) resulting in a Mb(IV)=O and water. Mb(IV)=O undergoes auto-reduction to met(III)Mb. Met(III)Mb reacts with an additional H₂O₂ molecule resulting in regeneration of Mb(IV)=O. *Formation of Mb(IV)=O after reaction of met(III)Mb with H₂O₂ is often described as coupled with the formation of a porphyrin or protein ferryl Mb radical. Reaction scheme is adapted from ref. (1).

FIG. 4.

Amino acid substitutions of Val68 and His97 alters hemin affinity in met(III)Mb. Hemin affinity of met(III)Mb is decreased 39-fold by substitution of the native His⁹⁷(FG3) with alanine. Substitution with Ala97 negates the electrostatic or hydrogen bonding interaction of His97 with the heme-7-propionate, which decreases hemin affinity. Hemin affinity is increased 25-fold by substitution of the native Val⁶⁸(E11) with threonine. Thr68 hydrogen bonds with liganded water in met(III)Mb, which increases hemin affinity since the native Val68 cannot hydrogen bond with the water. The O atom of the threonine side chain hydrogen bonds to the H atom of the water molecule. The PDB structures, 1MYG (47) and 1MNK (69), were used to prepare the images shown using PyMOL software.

FIG. 5.

Reaction pathways for O₂(II)Mb, deoxy (II)Mb, met (III)Mb, ferryl Mb species, and protoporphyrin IX. Various pathways lead to hypervalent states of Mb with the concomitant generation of hydrophilic and hydrophobic oxidants. Oxidant challenge results in formation of met(III)Mb, cross-linked Mb, dissociation of ferriprotoporphyrin IX, and degradation of the protoporphyrin ring, which releases iron atoms. Phagocytes can facilitate bursts of H₂O₂ that degrade the protoporphyrin. LH represents a polyunsaturated fatty acid. LOOH represents a lipid hydroperoxide. Cross linking of Mb, dissociation of ferriprotoporphyrin IX, and ferryl Mb-mediated lipid oxidation occur more readily at reduced pH values. Image is adapted from ref. (62).