During early development, the human fetus expresses different and hemoglobin gen

Question

During early development, the human fetus expresses different and hemoglobin genes. These
are similar but not identical to the hemoglobin genes expressed in adults. This fetal hemoglobin is
less sensitive to BPG than the adult hemoglobin. As a result, fetal hemoglobin has a higher affinity
for oxygen, allowing transfer from the maternal adult hemoglobin. The structural difference between
the fetal and adult hemoglobin is that the amino acid residue 143 of the chain in adult hemoglobin
is histidine while that of the fetal hemoglobin is serine. Discuss how this single amino acid difference
contributes to different sensitivity to BPG.

Explanation / Answer

Hemoglobin is a tetrameric protein containing two chains and two chains. BPG and oxygen binds to hemoglobin at distinct sites. 2,3-bisphosphoglycerate (BPG) regulates the oxygen binding affinity of hemoglobin as in presence of BPG, hemoglobin has reduced affinity for oxygen. In adults the amino acid residue 143 of the subunit of hemoglobin is the positively charged histidine which constitutes the binding site of negatively charged BPG. However, in fetal hemoglobin serine is present instead of histidine which makes the binding site of BPG less positive resulting in the lower affinity of fetus hemoglobin to BPG.

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