Heme proteins and oxidation in fresh and processed meats - Oxidation in Foods and Beverages and Antioxidant Applications - page 92

peroxide can have varying effects depending on whether Mb or Hb is examined.
More studies are needed to compare the relative ability of dissociated hemin and
cross-linked Mb (and Hb) to promote lipid oxidation.
4.6.4 Ability of mammalian and fish Hbs to promote lipid oxidation
Perch Hb was found to promote lipid oxidation in washed cod muscle rapidly
compared to trout Hb at pH 6.3 (Richards and Dettmann, 2003). Bovine Hb was
a remarkably poor promoter of lipid oxidation in washed cod compared to trout
Hb at pH 6.3 (Richards et al., 2002). These findings can be partly attributed to
the rapid autooxidation of the fish Hbs compared to the bovine Hb (Aranda et
al., 2009). The hemin affinity of the met forms of fish and mammalian Hbs
should also be considered based on the ability of released hemin to promote lipid
oxidation (Tappel, 1955).
Perch Hb and trout IV Hb released hemin 55-fold and 26-fold faster com-
pared to bovine Hb, respectively at pH 6.3 (Aranda et al., 2009). These dramatic
differences can be attributed to steric and amino acid differences around the
heme moiety when comparing fish and mammalian hemoglobins. The gap for
solvent entry (water and protons) into the heme crevice at CD3 was 8 Ê in perch
Hb, around 6 Ê in trout IV Hb, and around 4 Ê in bovine Hb (Aranda et al.,
2009). Hydration of the proximal histidine will decrease hemin affinity
(Hargrove et al., 1996). Lysine at site E10 in bovine Hb formed favorable
electrostatic and hydrogen bond interactions with the heme-7-propionate group
while the smaller threonine at site E10 in the fish Hbs did not (Aranda et al.,
2009). The interaction of Lys(E10) with the heme-7-propionate group increases
hemin affinity in the bovine Hb. Perch Hb and trout IV Hb have glycine at site
E14 while bovine Hb has the larger alanine ( chains) and serine ( chains) at
this site. We have found that the Ala(E14)Gly Mb mutant rapidly autooxidized
and had lower hemin affinity compared to WT Mb (unpublished observation).
Gly(E14) creates a channel for solvent entry into the heme crevice and may
affect stability of the E-helix.
4.6.5 Effect of pH on Mb and Hb-mediated lipid oxidation
There are numerous reasons that decreasing pH increases the ability of Hb and
Mb to promote lipid oxidation. First, protonation of the heme propionates at low
pH will decrease hydrogen bonding and electrostatic interactions of the heme
propionates with neighboring amino acids of the globin. Loss of these inter-
actions decreases hemin affinity which promotes lipid oxidation. Second,
protonation of the proximal histidine at low pH weakens the covalent linkage
between the proximal histidine and the iron atom of the porphyrin. Hemin
affinity of sperm whale Mb decreased 200-fold when decreasing the pH from 6.0
to 5.0 (Hargrove et al., 1994). Third, protonation of the distal histidine prevents
hydrogen bonding with liganded water in metMb (Fig. 4.5) which decreases
hemin affinity (Hargrove et al., 1996). Fourth, formation of hydrogen peroxide