Core Concepts
In this article we learn about hemoglobin, one of the most important proteins in animals, including its structure and oxygen binding dynamics.
Topics Covered in Other Articles
- Protein Denaturation
- The Haber Process
- Citric Acid Cycle
- Polymerase Chain Reaction
- Molecular Cloning
- Colloids
Hemoglobin and Transportation Proteins
In biology, proteins provide the most common way of shuttling materials to where they need to go. Within the cell, motor proteins like kinesin transport lipid vesicles around the cell along cytoskeletal filaments. Outside the cell, ABC transporters deliver important nutrients, like vitamins and minerals, while lipoproteins carry cholesterol to the liver to be metabolized. Additionally, hemoglobin and myoglobin have the crucial responsibility of circulating oxygen to all tissues of the body.
For its part, hemoglobin has attracted the attention of many different bioscience disciplines. Geneticists find interest in the protein due to its link to serious genetic conditions, namely sickle cell anemia. Evolutionary biologists find interest in the protein due to its wide structural variance across all the organisms that use it. Biochemists also find interest in hemoglobin, due to the importance of equilibrium and tautomeric concepts in its biological function.
In this article, we will primarily focus on the biochemistry of hemoglobin. To start, let’s take a moment to explore the protein’s structure.
Hemoglobin Structure
As mentioned previously, hemoglobin is a protein, which means that it is primarily composed of amino acids. The entire structure of human hemoglobin includes 574 total amino acids. However, those 574 allocate between four peptide subunits: two Ɑ subunits of 141 amino acids and two ß subunits of 146 amino acids. Thus, hemoglobin counts as a tetrameric protein, due to its 4 peptide quaternary structure. Interestingly, the protein has a significant cavity in its center, between each of the subunits.
In addition to its peptide components, each subunit covalently links to a heme group. This special group includes an organic porphyrin ring with a central iron (Fe2+) atom. Importantly, this iron is responsible for binding to diatomic oxygen for transport. Four nitrogen atoms within the porphyrin ring coordinately bind to the iron, which also binds to a histidine from the hemoglobin protein. In its deoxygenated form, the porphyrin ring has a slight pucker (4.0Å) away from the histidine (see image in Hemoglobin Cooperative Binding).
Biochemists often call hemes “prosthetic groups”, like the enzyme cofactor class, since they intrinsically link to the hemoglobin protein. Additionally, due to the presence and function of iron, hemoglobin is often described as a “metalloprotein”.
Hemoglobin Cooperative Oxygen Binding
As mentioned before, hemoglobin carries oxygen using its heme groups. When oxygen binds to the iron, the heme group’s pucker flattens into a plane, as another histidine stabilizes the oxygen from above.
Importantly, the binding of oxygen also stimulates a cascade of structural changes in hemoglobin structure. These changes ultimately result in one Ɑ-ß pair rotating 15° with respect to the other. As a result, the central cavity in the protein closes.
This oxygenated conformation of hemoglobin (or R state) better exposes its heme groups than its more constrained conformation (or T state). Thus, after the first oxygen binds to a heme group, all the other heme groups have a much stronger affinity to bind to their own oxygen molecules. Biochemists use the term “cooperative binding” to describe this phenomenon of one binding increasing the likelihood of subsequent bindings.
Additionally, when the protein becomes fully saturated with oxygen, and thus reaches the complete R state, the structural changes reverse after the first oxygen dissociates. All other heme groups then slightly lose affinity for their oxygens, making a second dissociation more likely, and a third more likely after that.
Ultimately, hemoglobin’s capability of cooperative binding results in easy oxygen binding in oxygen-rich conditions and easy dissociation in oxygen-poor conditions. This proves incredibly beneficial in oxygen transport, as hemoglobin quickly saturates in the oxygen-rich lungs, but automatically unloads its oxygen once it reaches oxygen-poor tissue.
Hemoglobin Mutation: Sickle Cell Anemia
Like all proteins, DNA sequences ultimately dictate the sequence of amino acids in hemoglobin, which informs its final three-dimensional structure. However, DNA tends to occasionally mutate into a slightly different sequence. One such mutation in the gene coding for human hemoglobin results in a substitution of glutamic acid (Glu) for valine (Val) in both ß subunits. Notably, Glu has a polar structure, fit for interacting with the solvent outside the protein, while Val is non-polar.
This mutation then creates a new hydrophobic pocket on the surface of the protein, which sticks to hydrophobic groups on other hemoglobins. Ultimately, the aggregation of mutated hemoglobin deforms the red blood cells that carry them into sickle shapes when deoxygenated. These deformed cells tend to clog and carry less oxygen than wild-type blood cells. As a result, patients who have this genetic condition tend to have a higher likelihood of pain and blood-related illness. Interestingly, sickle cell anemia also tends to have a higher resistance to malaria than wild-type blood, suggesting a possible reason for its persistence in the human gene pool.
