Biology Reference
In-Depth Information
bind to the hemoglobin. When the hemoglobin returns to the lungs,
where CO
2
levels are lower and CO
2
is released, its affinity for
O
2
increases. Thus, hemoglobin again binds O
2
, and the cycle repeats.
This chapter focuses on creating mathematical models describing
hemoglobin-oxygen binding. We begin by describing the structures
and functions of myoglobin and hemoglobin to help conceptualize
the models. Similar models could be used to study drugs binding to
receptors. In the next chapter, we present numerical methods for
appropriately testing the models.
I. INTRODUCTION
To produce the large amounts of adenosine triphosphate (ATP) required
for cells to maintain organization and perform useful work, the cells of
higher organisms must have access to sufficient O
2
to support aerobic
respiration. Anaerobic respiration (fermentation) only yields two ATP
per glucose, which is simply not sufficient to support the complicated
organization of a higher eukaryote. Aerobic respiration allows the
cells to convert the energy in each glucose molecule into the equivalent
of 36 molecules of ATP, most of which is generated by the electron
transport chain. Simple unicellular organisms, and those multicellular
organisms one or two cell layers thick, may obtain sufficient O
2
via
diffusion from the environment. More complex animals require
a circulatory system, with specific molecules to bind and transport O
2
.
In vertebrates, this molecule is hemoglobin.
COO
−
Deciphering the secrets of hemoglobin has challenged generations of
organic chemists, biophysicists, biochemists, and physiologists. Until the
late 1880s, it was unclear whether hemoglobin was a low-molecular
weight compound or a macromolecule. Emil Fischer (1852-1919; Nobel
laureate, 1902) was the first to establish that hemoglobin and all other
proteins are biopolymers, called polypeptides, built of 20 different alpha-
amino acids.
1
Another major obstacle in deciphering the structure of
hemoglobin was overcome by Hans Fischer (1881-1945; Nobel laureate
1930), who established the presence of an iron-containing complex
called heme in the macromolecule (Figure 7-1). Both myoglobin and
hemoglobin contain heme.
H
2
C
H
3
C
CH
2
COO
−
H
2
C
N
H
CH
2
H
2
C
++
Fe
N
N
H
3
C
CH
3
N
CH
3
HC
CH
2
FIGURE 7-1.
Structure of the heme group, with iron (Fe
þþ
).
Heme consists of a planar, ring-like structure with resonating double
bonds, bound to an iron atom. The iron is bound by four nitrogen atoms
at the center of the ring. Because the coordination number of iron is 6,
1. For those with some chemistry background: The amino acids are linked with
one another by means of peptide bonds:
¼
C(
O)NH
. The general form of
the amino acids is R
CH(NH
2
)COOH, where different amino acids are
represented by different radicals R. Peptides consisting of more than 50 amino
acids are classified as proteins.
