92
FRED H. WILT
4 heme prosthetic groups. The molecular weight of adult human Hb is
64,500. There are two classes of polypeptides in HbA, called a and ß;
HbA is composed of 2 «-chains, 2 ß-chains, and 4 heme groups, and is
designated a 2
A ß 2
A ', the chains dissociate reversibility at low pH under
certain conditions. The «-chain contains 141 amino acids, and the
/?-chain contains 146 amino acids. Limited or extensive amino acid
substitutions may be present in either the «- or /^-chains. For instance,
human fetal Hb (HbF) is composed of 2 «-chains identical to those
found in HbA. The ß-chains of HbF are extensively substituted, 17
differences in amino acids leading to 39 differences in sequence in the
chain (Schroeder et al., 1963). The extensively modified β-chain of HbF
is called the γ-chain, and human fetal Hb is designated « 2
F
y2
F
. The conformation of mammalian Hb is well understood, based on analogies to
the X-ray crystallographic solution of the conformation of sperm whale
myoglobin by Perutz and his associates (Perutz et al, 1960). The myoglobin molecule contains long helical portions and is bent, or kinked,
in specific nonhelical portions of the molecule; the model of myoglobin
appears like a twisted sausage, assuming an approximate overall globular
shape. Heme is situated in a shallow surface depression on the globule
and interacts with histidyl residues of the polypeptide. Similarly, Hb is
composed of 4 polypeptides, each of which is a twisted sausagelike
molecule, the 4 subunits fitting together and resulting in an approximate
globular shape. A similar conformation for Hb can exist even with
extensive changes in amino acid composition, and Perutz et al. (1965)
have recently suggested that most of the amino acid substitutions occur
along the helical portions of the molecule. They propose that the spacing
of nonpolar amino acids in the helical regions and the position of prolines in nonhelical regions are important for determining the final conformation. The nonpolar amino acids tend to associate with one another
by hydrophobic bonding to form a water poor interior, and the proline
residues allow changes in relative direction of the helical axes. This
proposal allows for great variation in the amino acid composition of Hb
polypeptides.
It is certainly true that the Hb's of vertebrates may vary greatly
in their amino acid composition and sequence. On the other hand, the
general features of Hb (molecular weight, heme to protein ratio, oxygen
binding capacity) in a wide variety of mammals, birds, and amphibia
are similar. We shall examine some aspects of differences in Hb of birds
and frogs in detail, and several reviews exist (Müller, 1961 ; Manwell,
1960).
C. Biosynthesis of Hemoglobin
The rabbit reticulocyte has served as an important basis for our
knowledge of the mechanism of protein biosynthesis in animals, and it
FRED H. WILT
4 heme prosthetic groups. The molecular weight of adult human Hb is
64,500. There are two classes of polypeptides in HbA, called a and ß;
HbA is composed of 2 «-chains, 2 ß-chains, and 4 heme groups, and is
designated a 2
A ß 2
A ', the chains dissociate reversibility at low pH under
certain conditions. The «-chain contains 141 amino acids, and the
/?-chain contains 146 amino acids. Limited or extensive amino acid
substitutions may be present in either the «- or /^-chains. For instance,
human fetal Hb (HbF) is composed of 2 «-chains identical to those
found in HbA. The ß-chains of HbF are extensively substituted, 17
differences in amino acids leading to 39 differences in sequence in the
chain (Schroeder et al., 1963). The extensively modified β-chain of HbF
is called the γ-chain, and human fetal Hb is designated « 2
F
y2
F
. The conformation of mammalian Hb is well understood, based on analogies to
the X-ray crystallographic solution of the conformation of sperm whale
myoglobin by Perutz and his associates (Perutz et al, 1960). The myoglobin molecule contains long helical portions and is bent, or kinked,
in specific nonhelical portions of the molecule; the model of myoglobin
appears like a twisted sausage, assuming an approximate overall globular
shape. Heme is situated in a shallow surface depression on the globule
and interacts with histidyl residues of the polypeptide. Similarly, Hb is
composed of 4 polypeptides, each of which is a twisted sausagelike
molecule, the 4 subunits fitting together and resulting in an approximate
globular shape. A similar conformation for Hb can exist even with
extensive changes in amino acid composition, and Perutz et al. (1965)
have recently suggested that most of the amino acid substitutions occur
along the helical portions of the molecule. They propose that the spacing
of nonpolar amino acids in the helical regions and the position of prolines in nonhelical regions are important for determining the final conformation. The nonpolar amino acids tend to associate with one another
by hydrophobic bonding to form a water poor interior, and the proline
residues allow changes in relative direction of the helical axes. This
proposal allows for great variation in the amino acid composition of Hb
polypeptides.
It is certainly true that the Hb's of vertebrates may vary greatly
in their amino acid composition and sequence. On the other hand, the
general features of Hb (molecular weight, heme to protein ratio, oxygen
binding capacity) in a wide variety of mammals, birds, and amphibia
are similar. We shall examine some aspects of differences in Hb of birds
and frogs in detail, and several reviews exist (Müller, 1961 ; Manwell,
1960).
C. Biosynthesis of Hemoglobin
The rabbit reticulocyte has served as an important basis for our
knowledge of the mechanism of protein biosynthesis in animals, and it