The Bohr effect is important in the maternal-fetal O2 transport as well. The blood that enters the placenta on the baby’s side has a high concentration of CO2. This equilibrates with the mother’s blood rapidly. As the mother takes up more Co2, she gives up more O2. Likewise, as the baby gives up more CO2, he is able to bind more O2. This effect favors the transport of O2 from mother to baby.
Also interesting to note is that the formation of fetal red blood cells occurs first in the embryonic yolk sac at about three weeks. This organ disappears after about 2-2 ½ months and the cells are then made in the embryonic liver. This location is used from about 2-5 or 6 months. Finally, the cells are made in the bone marrow of the fetus for the duration of the pregnancy and throughout the fetus’ adult life.
Fetal Hemoglobin
During the entire nine months of fetal formation, three different hemoglobins are produced, with one switching off as the next one begins. All three types contain the same heme molecules with an iron atom in the center, but they all have different globins – the structure will be discussed in more detail later.
For the first eight weeks of prenatal life, the majority of hemoglobin is of the type called embryonic hemoglobin (Hemoglobin E). Production of Hemoglobin E soon ends and fetal hemoglobin, the most important, is created next. Fetal Hemoglobin (Hemoglobin F) is the predominant form of hemoglobin expressed in a developing fetus. Hemoglobin F appears a few weeks post-conception and exists for a few months post-birth. It is not clear whether, although fetal red blood cells have a greater affinity for oxygen than an adult’s, if the same holds true for Hemoglobin F. However, at thirty-five weeks, ninety percent of the fetal blood cells still contain Hemoglobin F. This second type of hemoglobin starts to phase out around thirty-five weeks leaving the third type, adult hemoglobin (Hemoglobin A), to begin production. Hemoglobin F does not turn into Hemoglobin A however, they are completely different hemoglobins. At birth, the production of Hemoglobin F should be close to ending, so that in a normal one-year old child, only 2% of red blood cells contain it (though it can take up to two years for a complete switch to Hemoglobin A). The other 98% of red blood cells should contain Hemoglobin A – which will be produced for the duration of adult life.
Complications
Although the switching of the hemoglobin types is controlled by certain regulatory genes, there always exists the possibility of error. Occasionally, for instance, an adult never develops any Hemoglobin A and thus, only has fetal hemoglobin. Luckily, a completely normal life can be lived because of new findings through research. However, for those who develop sickle cell disease, the story ending is not so bright.
Sickle cell disease is caused by an abnormal adult hemoglobin – Hemoglobin S. In those infected with this disease, Hemoglobin S is made instead of Hemoglobin A. As a fetus, babies make Hemoglobin F and Hemoglobin S instead of F and A. Hydroxyurea, a new drug used to treat sickle cell disease in adults, works by turning on Hemoglobin F again (since a normal life can be lived with Hemoglobin F).
Fetal hemoglobin does protect individuals in certain cases. If a baby has been born with sickle cell anemia (meaning two genes for Hemoglobin S) symptoms will not arise until after the first six months of its life – baring that there still exists some Hemoglobin F in its blood. This is a benefit as long as the anemia is detected in time.
Structure
Hemoglobin is made up of 4 proteins and a “heme” group that contains iron. Hemoglobin F is made up of 2 alpha chains and 2 delta (fetal) chains. These delta chains are polypeptide chains, referred to as gamma subunits, that are homologous to the beta chains of Hemoglobin A. Adult hemoglobin is made up of 2 alpha chains and 2 beta (adult) chains.
The primary structural differences in Hemoglobin F as compared to Hemoglobin A are found in or near the 2,3 BPG binding site. 2,3 BPG binds less tightly to deoxy (desaturated) Hemoglobin F as compared to deoxy Hemoglobin A. An additional difference to note is of the P50 values (% O2 at which hemoglobin is 50% saturated). For Hemoglobin A, the value is 26.7 mmHg, while for Hemoglobin F, the P50 is 19 mmHg.
Breakdown
Each hemoglobin molecule has a life span equal to about that of the red blood cell in which it is contained, roughly 120 days. When old red blood cells are destroyed, the heme is metabolized into iron which is reused, and to porphyrin, which is degraded to bile pigments and discharged by the liver. The four polypeptide chains of the hemoglobin protein are broken down into amino acids which may be used again in other metabolic processes including the formation of new protein.
References
Cox, Michael M. et al. Principles of Biochemistry. (Worth Publishers, 1999), pp. 7-1 – 7-36.
“Fetal Hemoglobin Fact Sheet.” (Office for Hereditary Disorders), (www.mdpublichealth.org/genetics/html/hemo_f.html)
“Hemoglobin Structure and Function.” (ntri.tamuk.edu/homepage-ntri/lectures/protein/hemoglobin/hempage.html).
Orville, Elisabet O., “Fetus to Newborn: The Perinatal Period.” (Yale-New Haven Teachers Institute, 1998), ().
Sears, Duane W. “Structures and Functions of Fetal Hemoglobin.” (November 1999), (mentor.lscf.ucsb.edu/mcdb108a/tw-hbn/hbf-outline.htm).
Voth, Brian. “Fetal Vs. Adult Hemoglobin.” (University of Wisconsin Anesthesia Topics, 1993),
(. . . diatric_Anesthesia/ftlvsadult.html).