C. Giulivi et E. Cadenas, HEME PROTEIN RADICALS - FORMATION, FATE, AND BIOLOGICAL CONSEQUENCES, Free radical biology & medicine, 24(2), 1998, pp. 269-279
The oxidation of myoglobin by H2O2 yields ferrylmyoglobin, which conta
ins two oxidizing equivalents: the oxoferryl complex and an amino acid
radical. This study examines the electron paramagnetic resonance (EPR
) properties of the resulting amino acid radicals and their inherent k
inetic features at [H2O2]/[protein] ratios close to physiological cond
itions (i.e., less than or equal to 1). The EPR spectrum obtained with
continuous flow at room temperature consisted of a composite of three
signals: a low intensity signal and two high intensity signals. The f
ormer had a g-value of 2.014, contributed 10-15% to the overall spectr
um and was ascribed to a peroxyl radical. Of the two high intensity si
gnals, one consisted of a six-line spectrum (g = 2.0048) that contribu
ted approximately 17-19% to the overall signal; hyperfine splitting co
nstants to ring protons permitted to identify this signal as a tyrosyl
radical. The other high intensity signal (with similar g-value and un
derlying that of the tyrosyl radical) was ascribed to an aromatic amin
o acid upon comparison with the EPR characteristics for radicals in ar
omatic amino acid-containing peptides. Analysis of these data in conne
ction with amino acid analysis and the EPR spectra obtained under simi
lar conditions with another hemoprotein, hemoglobin, allowed to sugges
t a mechanism for the formation of the protein radicals in myoglobin.
The aromatic amino acid radical was observed to be relatively long liv
ed in close proximity to the heme iron. Hence, it is likely that this
is the first site of protein radical; reduction of the oxoferryl compl
ex by Tyr (Fe-IV=O + Tyr-OH + H+ --> Fe-III + H2O + Tyr-O-.)-and alter
natively by other amino acids-leads to the subsequent formation of oth
er amino acid radicals within an electron-transfer process throughout
the protein. This view suggests that the protein radical(s) is highly
delocalized within the globin moiety in a dynamic process encompassing
electron tunneling through the backbone chain or H-bonds;md leading t
o the formation of secondary radicals. (C) 1998 Elsevier Science Inc.