12.3 Muscle Tissue: Composition and Function 575Fig. 12.15.Absorption spectra of myoglobin - - -,
oxymyoglobin – – – and metmyoglobin. (accord-
ing toFennema, 1976)
side chain of His^93 of myoglobin has this func-
tion. Upon interaction with this fifth ligand, iron
is raised above the heme plane by about 0.05 nm:
(12.2)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 goodπ-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 oxida-
tion. From a biochemical viewpoint, the latter ef-
fect 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 hy-
drophobic 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 in-
crease in pH, there is an increase in basicity and,
hence, an increase in binding of O 2 (theBohr
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 MeatThe color of fresh meat is determined by the ra-
tios of myoglobin (Mb), oxymyoglobin (MbO 2 )
and metmyoglobin (MMb+):(12.3)Stable MbO 2 is formed at a high partial pres-
sure 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 efficientπdonor than
Fe^2 +. With better donor ligands than O 2 (CN−,