Molecule of the Month: Fetal Hemoglobin
Fetal hemoglobin allows a growing fetus to receive oxygen from their mother.
Essential Efficient Exchange
Two Subunits Make One Big Difference
A Solution for Sickle Cell Anemia?
A possible treatment was discovered by looking at individuals with a rare genetic condition: Hereditary Persistence of Fetal Hemoglobin. They naturally have higher levels of fetal hemoglobin even when they are adults, and individuals that also have the sickle cell mutation have less severe forms of the disease. Studies have shown that a level of about 20% fetal hemoglobin in the blood is enough to reduce the symptoms of sickle cell disease, so researchers are using gene therapy to increase the levels of fetal hemoglobin in people that carry the sickle cell mutation. The treatment suppresses a silencer of the fetal hemoglobin gene, BCL11A (shown here from PDB ID 6ki6). This allows fetal hemoglobin to be made and has been recently shown to improve the prognosis of individuals with sickle cell disease.
Exploring the Structure
Less 2,3-BPG for Me, Mom
In the absence of oxygen, 2,3-BPG fits snugly in the cavity between the two beta subunits of adult hemoglobin, as seen on the left from PDB ID 1b86, forming interactions between the negatively-charged phosphates and positively-charged amino acids (colored purple). In contrast, the gamma chains of fetal hemoglobin (shown on the right from PDB ID 1fdh) contain several different amino acids (colored in mint green above) at critical locations that are necessary for interaction with 2,3-BPG. In addition, these amino acid alterations increase the distance between the positively-charged amino acids so that they are too far away to bind to 2,3-BPG. These differences allow 2,3-BPG to bind and regulate adult hemoglobin more strongly than fetal hemoglobin. In the placenta, where 2,3-BPG levels are high, fetal hemoglobin can bind oxygen more tightly than maternal hemoglobin. To explore these structures in more detail, click on the image for an interactive JSmol.
Topics for Further Discussion
- The enzyme 2,3-bisphosphoglycerate mutase makes 2,3-BPG. The PDB archive includes several structures of the enzyme at different stages of the reaction, for example, in entry 2h4z.
- Hemoglobin is highly regulated by other methods in addition to 2,3-BPG. For example, take a look at the Molecule of the Month article on S-Nitrosylated Hemoglobin.
May 2021, Candice Craig, Samantha Eng, Jenna Manzo, Andrew Tkacenko, David S. Goodsell, Stephen K. Burleydoi:10.2210/rcsb_pdb/mom_2021_5