2009_Book_FoodChemistry

574 12 Meat
oxygen to myoglobin. The sigmoidal shape of
the O 2 -binding curve for hemoglobin is due
to its quaternary structure. It consists of four
polypeptide chains, with one pigment molecule
bound to each. The binding of O 2 to the four
pigment molecules occurs cooperatively because
of allosteric effects. Therefore, the degree of saturation,
S, is expressed by the following equation
(p O2 = oxygen partial pressure; k = dissociation
constant for the O 2 -protein complex):
S = k · pn O 2
1 + k · p n O 2
(12.1)
Fig. 12.13. Octahedral environment of Fe 2+ -protoporphyrin
with the imidazole ring of a globin histidine
residue and oxygen (according to Karlsson, 1977)
Myoglobin supplies oxygen because of its ability
to bind oxygen reversibly. Comparison of the
oxygen binding curves for hemoglobin and
myoglobin (Fig. 12.14) shows that at low p o2 ,
such as exists in muscle, hemoglobin releases
Fig. 12.14. Oxygen binding curves of myoglobin and
hemoglobin
For hemoglobin, n ∼ 2.8 (sigmoidal saturation
curve), and for myoglobin, n = 1 (hyperbolic saturation
curve). The efficiency of O 2 transfer from
hemoglobin to myoglobin is further enhanced by
a decrease in pH since oxygen binding is pHdependent
(the Bohr effect).
While in the living animal approx. 10% of the total
iron is bound to myoglobin, 95% of all the iron
in well-bled beef muscle is bound to myoglobin.
Unlike myoglobin, hemoglobin contributes little
to the color of meat. The contribution of other
pigments, such as the cytochromes, is negligible.
However, attention must be paid to the fact that
the visual appearance of a cut of meat is influenced
not only by the light absorption of pigments,
i. e. primarily myoglobin, but also by light
scattering by the surface of muscle fiber. A bright
red color is obtained when the coefficient of absorption
is high and that of light scattering is low.
Myoglobin (Mb) is purple (λ max = 555 nm);
oxymyoglobin (MbO 2 ), a covalent complex of
ferrous Mb and O 2 , is bright red (λ max = 542
and 580 nm); and the oxidation product of Mb in
the ferric state, metmyoglobin (MMb + ), is brown
(λ max = 505 and 635 nm). Some other ligands,
such as electron pair donors (e. g. CO, NO,
N − 3 ,CN− ), like O 2 , bind covalently, giving rise
to low-spin complexes with similar absorption
spectra and hence to a color similar to MbO 2 .
Figure 12.15 shows several absorption spectra of
myoglobins.
Heme devoid of globin (free heme, Fe 2+ -
protoporphyrin) does not form the O 2 -adduct, but
oxidizes rapidly to hemin (Fe 3+ -protoporphyrin).
A prerequisite for reversible O 2 binding is the
presence of an effective donor ligand on the
iron’s axial site, which is bound by formation of
a quadratic-pyramidal complex. The imidazole