Heme proteins and oxidation in fresh and processed meats - Oxidation in Foods and Beverages and Antioxidant Applications - page 80

detectable heme pigment in whole muscle from trout while dark muscle from
mackerel contained roughly equal amounts of Hb and Mb (Richards and Hultin,
2002). Hb was the only detectable heme pigment in chicken breast muscle while
in thigh muscle there was 86% Hb and 14% Mb on a weight basis (Kranen et al.,
1999). In various pork muscles, Hb comprised 23±30% of the total heme protein
(Pisula, 1975). In pork Longissimus dorsi, Hb comprised 39±54% of the total
heme pigment (Rhee and Ziprin, 1987). In minced beef, Hb comprised 9% of the
total heme protein (Oellingrath et al., 1990). Longissimus dorsi from beef
contained 15±42% Hb while in the semimembranosus, Hb comprised 32±34% of
the total heme pigment (Rhee and Ziprin, 1987).
Hb concentrations can be underestimated relative to Mb due to the greater
tendency of Hb to release its porphyrin moiety during extraction (Gattoni et al.,
1998). Optical density of Hb and Mb between 700 and 400 nm is often used to
quantify the heme protein concentration. The extinction coefficients of Hb and
Mb containing its porphyrin moiety are much greater than the extinction coeffi-
cients of either the apoglobin or free porphyrin. In addition, multiple hemo-
globins (e.g., Hb isoforms) can be present especially in fish muscle (Rizzotti and
Gioppato, 1999) so that one Hb component can be mistakenly identified as Mb.
Hb is a tetramer consisting of four globins and four porphyrin moieties while
Mb is a monomer (one globin and one associated porphyrin group). The
porphyrin moiety is termed `heme' when the iron atom of the porphyrin is in the
reduced (+2) oxidation state and `hemin' when in the oxidized (+3) oxidation
state.
In Hb, there are two -chains (chain A and C) and two -chains (chain B and
D). The amino acid composition of the -chains is substantially different
compared to the -chains which causes functional differences between the
chains. The central part of the porphyrin is the iron atom that has six coordina-
tion sites (Fig. 4.1). Four of the sites are occupied by nitrogen atoms of the
protoporphyrin ring and one is attached to the proximal imidazole group of a
histidine in the globin portion. The sixth site forms a complex with ligands such
as O 2 . Water is weakly coordinated to the iron atom of the porphyrin in
deoxyMb and metMb. The ligand and oxidation state of the iron atom in the
porphyrin ring dictates the color of the flesh.
There is a distal histidine residue that coordinates different ligands in the
heme crevice while the proximal histidine covalently links the globin to the
porphyrin (Fig. 4.1). The heme-6-propionate and heme-7-propionate extend out
of the globin towards solvent. In Fig. 4.1 the heme-7-propionate is to the right.
The distal histidine is at site E7; this is the 7th residue along the E-helix. E7 is
the 64th residue in sperm whale Mb, the 58th residue in bovine Hb chains, and
the 62nd residue in bovine Hb chains. The proximal histidine is at site F8; this
is the 8th residue along the F-helix. Mbs and Hb chains have eight helices (A
through H) while Hb chains have 7 helices. The D-helix is missing from Hb
chains (Fig. 4.1). Thus there is a CD corner as a random coil between helix C
and D in Mb and Hb chains and a CE corner between the C and E-helix in Hb
chains. Acetylation of Rainbow trout Hb has been reported at the N-terminus