Hemoglobin E: What's the biochemistr-E?
Hemoglobin E (HbE) is an abnormal form of the oxygen-carrying molecule hemoglobin that can cause blood disorders. It was first discovered by Virginia Minnich in 1954 (you can learn more about Minnich in this WiSE Wednesday companion piece). In addition to being one of the most common genetic mutations (you were probably screened for it as an infant), it is a great subject around which to discuss many fundamental biochemistry, molecular biology, and genetics topics.
Hemoglobin & diseases associated with it
Hemoglobin is one of the most important molecules in your body – it’s the protein that carries oxygen throughout your bloodstream, making it essential for life. Lungs are useless if you can’t get the oxygen you breathe in to the places in your body where you need it! As you might imagine, then, problems with hemoglobin can cause system-wide problems with a range of severity depending on the nature of the problem.
There are two main types of diseases involving hemoglobin: hemoglobinopathies and thalassemias. The difference? In a hemoglobinopathy, the hemoglobin protein is structurally abnormal, whereas in a thalassemia, the hemoglobin itself can be normal but it’s present in decreased levels. Hemoglobin E fits both categories, and in this article, I will explain why.
Proteins and their classification
Proteins are made up of chains of building blocks called amino acids that fold into intricate structures. Some proteins are made from a single chain – we call these proteins monomers. Other proteins, oligomers, are made up of multiple chains stuck together. If each chain (subunit) is the same, we call the protein homomeric and if they’re different we call it heteromeric. We further classify oligomers based on how many chains are in the final protein. Hemoglobin (Hb), for instance, is a tetramer (4 chains) made up of 2 dimers (2 chains). Although the dimers are identical, each is made up of 2 different chains, so we classify it as a heteromer. So, if we want to get specific, Hb is a heterotetramer.
The dimers in the hemoglobin adults produce is made up of 1 α subunit and 1 β subunit, making the final product α2β2. When these subunits are all normal, normal adult hemoglobin (HbA) is produced.
You might wonder why I’m speaking in terms of “adult” hemoglobin – this is because fetuses actually produce a different form of hemoglobin, HbF, which has γ subunits instead of β subunits (α2y2). This form of Hb has an increased affinity for oxygen, which allows it to bind oxygen from the mother’s blood even though that blood isn’t highly oxygenated. After birth, this extra affinity is no longer needed, so the baby gradually (over a few months) switches over to producing HbA. When it comes to hemoglobin disorders, therefore, problems with the α subunits will be present at birth but problems with the β subunit won’t become apparent until these subunits start replacing the γ subunits.
The instructions for a protein are contained in a gene, written in the DNA language made up of a nucleotide alphabet consisting of 4 bases (A, T, G, & C). Each amino acid within a protein is coded for in that protein’s gene by a 3-nucleotide “word” called a codon. When the DNA of a gene gets altered, it can affect the protein output (the levels produced [as in hemoglobinopathies] and/or the structure of the protein itself [as in thalassemias]).
The genetic code is redundant, meaning that multiple 3-base combinations code for the same amino acid. For example, GGG and GGA in a gene both lead to the production of a glutamic acid molecule, so a mutation from GGG to GGA or vice versa won’t affect the protein structure. We call this type of mutation, in which the amino acid is unchanged, “synonymous.”
“Non-synonymous” or “missense” DNA mutations, on the other hand, do change which amino acid gets produced at a certain location on a protein, and this is the type of mutation that causes HbE. The causal HbE mutation is a single point mutation at amino acid position 26 in the β-chain that changes the amino acid from glutamic acid (Glu or E) to lysine (Lys or K). In biochemistry shorthand, we write this as GluE26Lys or E26K.
Non-synonymous mutations aren’t necessarily harmful. Some mutations can actually be beneficial and can even serve as the basis for evolution of new functions. Other mutations can be disastrous, but many are merely “neutral.” Sometimes, a mutation may be good for some of the protein’s functions but not for others. The effects of the mutation largely depend on where on the protein the change occurs (some regions of the protein are more important than others) and how “different” the swapped amino acid is from the one that was there before (each amino acid has unique biochemical properties, some being quite similar and others being drastically different).
In the case of the HbE mutation, a glutamic acid is replaced by a lysine, causing mild biochemical differences that weaken the interaction between subunits and makes the hemoglobin slightly less stable under certain conditions. Therefore, HbE can be characterized as a hemoglobinopathy.
When it comes to causing symptoms, however, the effects of these structural differences are thought to be minor compared to the mutation’s effects on hemoglobin production. When a cell wants to make a protein, it first must copy (transcribe) the DNA instructions into messenger RNA (mRNA) that is then translated into protein. The DNA form of the gene has “extra” segments of information (introns) between the parts that contain the amino acid instructions (exons) and these intervening regions are removed in a process called splicing. The splicing machinery knows where on the unedited mRNA to cut because of “splice sites” encoded in the genes.
In addition to changing the 26th amino acid in the chain, the HbE mutation also reveals a hidden (cryptic) splice site that “confuses” the cell when it’s trying to edit the RNA copy of the gene. Less functional mRNA is therefore produced, and less β-globin made, so we classify HbE as a thalassemia.
Different versions of the same gene are called alleles and a person inherits one allele of each gene from each parent. If these alleles are the same, the person is said to be homozygous for that gene and if they’re different the person is said to be heterozygous for that gene.
The allele responsible for HbE (containing the E26K mutation) is referred to as βE. If a person inherits a βE allele from one parent and a normal β allele from the other parent, they are said to carry the HbE trait (this heterozygous state is frequently written EA). The normal copy of the β chain they have is able to compensate for any reduced production from the abnormal copy and the person is healthy (though they have a 50% chance of passing the abnormal allele onto their children).
If a person is homozygous for βE (they inherit one abnormal copy from each parent), they are said to have HbE disease (EE). Although the name has “disease” in it, most people with the condition don’t have any symptoms (though their blood does look slightly different under the microscope) because β globin is still produced, even if at reduced levels, and it functions fairly normally. However, because a person with HbE disease doesn’t have any normal copies, they have a 100% chance of passing on the βE allele (their children will, at a minimum, carry the HbE trait).
More serious problems arise if a person inherits a βE allele from one parent but instead of also inheriting a normal β allele from the other parent, they inherit a β allele with a different, more compromising, mutation. This type of situation is referred to as compound heterozygosity and, in this case, it leads to HbE/β-thalassemia. This second mutation could be one of many mutant β alleles, including ones that prevents any β production from that allele (β0) or one that causes severe structural changes such as βS (which causes sickle cell anemia in the homozygous state). People with the HbE trait don’t show symptoms because their normal copy can compensate, but in the case of compound heterozygosity, the second copy can’t compensate and, depending on the severity of this other mutation, the person may suffer from mild to severe thalassemia with symptoms including poor growth, organ damage and anemia (low red blood cell count).
Modifying roles of α-globin
HbE/β-thalassemia is, as the name suggests, a type of β-thalassemia, because the reduced production is of the β-globin chain, but there are also conditions called α-thalassemias, in which insufficient α-globin chain is produced. Unlike β-globin, which is coded for by a single gene (HBB), α-globin is coded for by 2 genes (HBA1 and HBA2), so a person normally inherits 2 α-globin genes from each parent. Sometimes, however, these genes get deleted, so a person can have 1-3 copies (0 copies is fatal before or shortly after birth). If a person only has 1 copy, they have HbH disease (symptoms include anemia, liver and spleen problems, and bone abnormalities) and if they have 2 copies, they have HbH trait (which may cause mild anemia).
Interestingly, decreased α-globin production can actually lessen the severity of symptoms associated with HbE. This is because it is not only the amount of α and β-globin produced, but the ratio between them, that matters. α and β-globin are much more stable in dimers than alone; if the amounts of one are much greater than that of the other (globin-chain imbalance), not all of them will be able to form dimers, so they may aggregate (clump up) and cause problems.
I hope this article gave you some insight into the wonders and complexities of life on the molecular level that contribute to life on the human level!