Biology Reference
In-Depth Information
We'd like to know the molecular basis of the oxygenation process, not
only as a way to understand how hemoglobin functions, but also as a
model to find out how other important proteins and enzymes
regulate metabolic processes.
Chein Ho, Director, Pittsburgh NMR Center for Biomedical Research
Hemoglobin is a protein constituting more than 95% of
the solid mass of red blood cells. Its function is to carry
oxygen (O
2
) from the lungs to peripheral tissues and
return carbon dioxide (CO
2
) to the lungs. Consequently, it
is one of the most important and most studied of all
proteins. Myoglobin (sometimes called muscular
hemoglobin) is a structurally analogous protein whose
function is to bind and store O
2
in muscle tissue and
provide O
2
to the working muscle.
Chapter 7
COOPERATIVE
BINDING: HOW YOUR
BLOOD TRANSPORTS
OXYGEN
Hemoglobin is made up of four subunits or monomers
called globins. In adult hemoglobin, two of these are
alpha globin proteins and two are beta globin proteins,
expressed symbolically as
a
2
b
2
. Both alpha and beta
globins are members of gene families that resulted from
gene duplication, mutation, and selection of an ancestral
globin gene. This duplication, mutation, and selection
have allowed functional differentiation. For example,
consider epsilon (
Introduction
), the beta globin family
members found in embryonic and fetal hemoglobins.
Early in embryonic development, hemoglobin consists of
a
2
e
2
, and is followed by
e
) and gamma (
g
Binding Reactions
Mathematical Models of Hemoglobin-
Oxygen Binding
a
2
g
2
or fetal hemoglobin. These
hemoglobins have higher affinities for O
2
than adult
hemoglobin
Deriving Fractional Saturation Functions
with Binding Polynomials
a
2
b
2,
permitting more efficient uptake of O
2
from the placenta.
Appendix: Justifying Equation (7-20)
Human hemoglobin tetramers have four sites where a
molecule of O
2
may bind. A salient characteristic of
hemoglobin is that this binding is cooperative. That is,
after the first molecule of O
2
is bound, a second is more
likely to bind. This affinity increases as more O
2
molecules bind, such that the affinity for the fourth O
2
molecule is approximately 300 times that of the first.
Hemoglobin becomes saturated with O
2
in the lungs,
where the partial pressure of the O
2
is high. As
hemoglobin circulates through the body, the level of
O
2
decreases while the level of CO
2
increases.
Hemoglobin's affinity for O
2
decreases in the presence
of CO
2
. In this environment, a change in hemoglobin
structure occurs, facilitating O
2
release. CO
2
will then
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