574 12 Meat
Fig. 12.13.Octahedral environment of Fe^2 +-proto-
porphyrin with the imidazole ring of a globin histidine
residue and oxygen (according toKarlsson, 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 po 2 ,
such as exists in muscle, hemoglobin releases
Fig. 12.14.Oxygen binding curves of myoglobin and
hemoglobin
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 satu-
ration, S, is expressed by the following equation
(pO 2 =oxygen partial pressure; k=dissociation
constant for the O 2 -protein complex):S=k·pnO
2
1 +k·pnO 2(12.1)For hemoglobin, n∼ 2 .8 (sigmoidal saturation
curve), and for myoglobin, n=1 (hyperbolic sa-
turation curve). The efficiency of O 2 transfer from
hemoglobin to myoglobin is further enhanced by
a decrease in pH since oxygen binding is pH-
dependent (theBohreffect).
While in the living animal approx. 10% of the to-
tal 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 influ-
enced not only by the light absorption of pig-
ments, 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 ab-
sorption is high and that of light scattering is low.
Myoglobin (Mb) is purple (λmax=555 nm);
oxymyoglobin (MbO 2 ), a covalent complex of
ferrous Mb and O 2 , is bright red (λmax= 542
and 580 nm); and the oxidation product of Mb in
the ferric state, metmyoglobin (MMb+), is brown
(λmax=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