2009_Book_FoodChemistry

12.3 Muscle Tissue: Composition and Function 575
Fig. 12.15. Absorption spectra of myoglobin - - -,
oxymyoglobin – – – and metmyoglobin . (according
to Fennema, 1976)
side chain of His 93 of myoglobin has this function.
Upon interaction with this fifth ligand, iron
is raised above the heme plane by about 0.05 nm:
Binding of the sixth ligand moves the iron to its
original position in the heme plane. Since the
Fe–N bond distance (His 93 ) remains constant,
dislocation of the fifth ligand occurs (His 93 ,
proximal His), i. e. a conformational change of
the globin takes place.
The basicity of the fifth ligand affects the binding
of the sixth ligand. The imidazole ring of His 93
is a good π-donor and, hence, stabilizes the O 2 -
adduct. A weaker base would enhance oxidation
of the iron rather than adduct formation, while
a stronger base would increase the stability of the
adduct and diminish the probability of iron oxidation.
From a biochemical viewpoint, the latter effect
is rated as (O 2 supplier) negative; while from
a food science point of view, it is desirable and
positive (stable, bright red meat color).
As mentioned above, His 93 is located in a hydrophobic
pocket of the myoglobin molecule.
The electron density and, therefore, the oxidation
state of the iron are regulated by protonation and
deprotonation of the imidazole ring. With an increase
in pH, there is an increase in basicity and,
hence, an increase in binding of O 2 (the Bohr
effect; cf. Fig. 12.14). A second histidine residue
of myoglobin, His 64 (distal His), contributes to
heme-O 2 -complex stabilization by formation
of a hydrogen bridge or ionic bond between N
and O (cf. Formula 12.2).
12.3.2.2.3 Color of Meat
The color of fresh meat is determined by the ratios
of myoglobin (Mb), oxymyoglobin (MbO 2 )
and metmyoglobin (MMb + ):
(12.3)
(12.2)
Stable MbO 2 is formed at a high partial pressure
of oxygen. Fresh cuts of meat, to a depth
of about 1 cm, acquire a bright cherry-red color
which is considered a mark of quality. A slow and
continuous oxidation to MMb + occurs at a low
partial pressure of O 2 . The change of Fe 2+ →
Fe 3+ is reflected in the change in color from red to
brown. MMb + does not form an O 2 -adduct, since
Fe 3+ appears to be a less efficient π donor than
Fe 2+ . With better donor ligands than O 2 (CN − ,